[542859] Extract coercion rules into IConverters

Separate the actual evaluation of the expressions, done by the
evaluate(EObject, String) method, from the conversion into the result
type expected by the multiple evaluateXXX() methods. This makes the
evaluateXXX() methods of all leaf interpreters (except the
DefaultInterpreter, which is a rather special case) trivial and same
across all implementations, something like:

  return converter.convert(evaluate(target, expr)).toXXX();

Most actual interpreters use the same coercion rules now defined in
the new DefaultConverter class, which are now explicit and can be
unit-tested (not done in this patch). The two execptions are OCL and
Acceleo/MTL for which the existing rules are kept thanks to custom
IConverter implementations, even though they do not exactly match what
all the other interpreters do.

This is a first step towards replacing all the evaluateXXX() methods
with a single, cleaner and more powerful evaluate() method coupled
with a rich evaluation result type.

Bug: 542859
Change-Id: Icd43c17264f83222c2ee3b12982a766d6b79860d
Signed-off-by: Pierre-Charles David <pierre-charles.david@obeo.fr>

5 files changed, 471 insertions, 414 deletions

diff --git a/plugins/org.eclipse.sirius.common.acceleo.mtl/src/org/eclipse/sirius/common/acceleo/mtl/business/internal/interpreter/AcceleoMTLInterpreter.java b/plugins/org.eclipse.sirius.common.acceleo.mtl/src/org/eclipse/sirius/common/acceleo/mtl/business/internal/interpreter/AcceleoMTLInterpreter.java
index b9610a17c3..f602103d01 100644
--- a/plugins/org.eclipse.sirius.common.acceleo.mtl/src/org/eclipse/sirius/common/acceleo/mtl/business/internal/interpreter/AcceleoMTLInterpreter.java
+++ b/plugins/org.eclipse.sirius.common.acceleo.mtl/src/org/eclipse/sirius/common/acceleo/mtl/business/internal/interpreter/AcceleoMTLInterpreter.java
@@ -17,7 +17,6 @@ import java.net.URL;
 import java.security.CodeSource;
 import java.text.MessageFormat;
 import java.util.ArrayList;
-import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.Enumeration;
@@ -28,6 +27,8 @@ import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.ListIterator;
 import java.util.Map;
+import java.util.Optional;
+import java.util.OptionalInt;
 import java.util.Set;
 
 import org.eclipse.acceleo.common.IAcceleoConstants;
@@ -82,6 +83,7 @@ import org.eclipse.sirius.common.tools.api.interpreter.TypeName;
 import org.eclipse.sirius.common.tools.api.interpreter.TypedValidation;
 import org.eclipse.sirius.common.tools.api.interpreter.ValidationResult;
 import org.eclipse.sirius.common.tools.api.interpreter.VariableType;
+import org.eclipse.sirius.common.tools.internal.interpreter.IConverter;
 import org.eclipse.sirius.ecore.extender.business.api.accessor.EcoreMetamodelDescriptor;
 import org.eclipse.sirius.ecore.extender.business.api.accessor.MetamodelDescriptor;
 import org.eclipse.sirius.ecore.extender.business.api.accessor.ModelAccessor;
@@ -97,20 +99,85 @@ import com.google.common.collect.Sets;
 
 /**
  * This interpreter will allow for the use of Acceleo 3 expressions.
- * 
+ *
  * @author <a href="mailto:laurent.goubet@obeo.fr">Laurent Goubet</a>
  */
 public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
+
+    /**
+     * Encapsulates coercion rules used by this interpreter.
+     *
+     * @author pcdavid
+     *
+     */
+    private static class MTLConverter implements IConverter {
+
+        @Override
+        public OptionalInt toInt(Object rawValue) {
+            Integer coerced = AcceleoMTLInterpreter.coerceValue(rawValue, Integer.class);
+            if (coerced != null) {
+                return OptionalInt.of(coerced.intValue());
+            } else {
+                return OptionalInt.empty();
+            }
+        }
+
+        @Override
+        public Optional<Boolean> toBoolean(Object rawValue) {
+            final Boolean coerced;
+            if (rawValue instanceof Boolean) {
+                coerced = (Boolean) rawValue;
+            } else {
+                coerced = AcceleoMTLInterpreter.coerceValue(rawValue, Boolean.class);
+            }
+
+            if (coerced != null) {
+                return Optional.of(coerced);
+            }
+            return Optional.empty();
+        }
+
+        @Override
+        public Optional<String> toString(Object rawValue) {
+            if (rawValue != null) {
+                return Optional.of(rawValue.toString());
+            } else {
+                return Optional.empty();
+            }
+        }
+
+        @Override
+        public Optional<EObject> toEObject(Object rawValue) {
+            return Optional.ofNullable(AcceleoMTLInterpreter.coerceValue(rawValue, EObject.class));
+        }
+
+        @Override
+        public Optional<Collection<EObject>> toEObjectCollection(Object rawValue) {
+            Collection<EObject> coercedResult = new ArrayList<>();
+            if (rawValue instanceof Collection<?>) {
+                Iterables.addAll(coercedResult, Iterables.filter((Collection<?>) rawValue, EObject.class));
+            } else if (rawValue != null && rawValue.getClass().isArray()) {
+                Iterables.addAll(coercedResult, Iterables.filter(Lists.newArrayList((Object[]) rawValue), EObject.class));
+            } else {
+                EObject coerced = AcceleoMTLInterpreter.coerceValue(rawValue, EObject.class);
+                if (coerced != null) {
+                    coercedResult.add(coerced);
+                }
+            }
+            return Optional.of(coercedResult);
+        }
+    }
+
     /**
      * This represents the prefix of an Acceleo 3 expression.
-     * 
+     *
      * @see IAcceleoConstants#DEFAULT_BEGIN
      */
     private static final String ACCELEO_EXPRESSION_PREFIX = IAcceleoConstants.DEFAULT_BEGIN;
 
     /**
      * This represents the suffix of an Acceleo 3 expression.
-     * 
+     *
      * @see IAcceleoConstants#INVOCATION_END
      */
     private static final String ACCELEO_EXPRESSION_SUFFIX = IAcceleoConstants.DEFAULT_END_BODY_CHAR + IAcceleoConstants.DEFAULT_END;
@@ -121,17 +188,14 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
     private static final String FILE_SEPARATOR_REGEX = "/|\\\\"; //$NON-NLS-1$
 
     /**
-     * This will hold all dependencies added to this interpreter through import
-     * handlers. Keys are the Java class name in the form
-     * <code>my.package.MyClass</code>, values are the Modules returned by the
-     * handlers.
+     * This will hold all dependencies added to this interpreter through import handlers. Keys are the Java class name
+     * in the form <code>my.package.MyClass</code>, values are the Modules returned by the handlers.
      */
     protected final Set<ModuleDescriptor> extendedDependencies = new LinkedHashSet<>();
 
     /**
-     * This will hold all dependencies added to this interpreter. Keys are the
-     * dependencies path in the form <code>my::package::myModule</code>, values
-     * are the actual URIs of the modules.
+     * This will hold all dependencies added to this interpreter. Keys are the dependencies path in the form
+     * <code>my::package::myModule</code>, values are the actual URIs of the modules.
      */
     protected final Multimap<String, URI> mtlDependencies = LinkedHashMultimap.create();
 
@@ -139,18 +203,15 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
     private final Map<String, URI> javaFiles = new HashMap<>();
 
     /**
-     * This will contain the "dummy" module we use for the compilation. All
-     * queries present in an VSM file will have a corresponding [query] block in
-     * this dummy.
+     * This will contain the "dummy" module we use for the compilation. All queries present in an VSM file will have a
+     * corresponding [query] block in this dummy.
      */
     private final DynamicAcceleoModule module = new DynamicAcceleoModule();
 
     /**
-     * This map will hold the values associated to given variable names. Note
-     * that even if this is a multimap, the variables are considered as a stack
-     * in order to be coherent with other interpreters : evaluation will
-     * consider the value to be a Collection, but setting/unsetting will only
-     * work one object by one object.
+     * This map will hold the values associated to given variable names. Note that even if this is a multimap, the
+     * variables are considered as a stack in order to be coherent with other interpreters : evaluation will consider
+     * the value to be a Collection, but setting/unsetting will only work one object by one object.
      */
     private final ListMultimap<String, Object> variables = AcceleoCollections.newCircularArrayDequeMultimap();
 
@@ -159,14 +220,12 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
     private final Set<String> variableNsURIs = new LinkedHashSet<>();
 
     /**
-     * This will be updated with the list of accessible viewpoint plugins, if
-     * any.
+     * This will be updated with the list of accessible viewpoint plugins, if any.
      */
     private final Set<String> viewpointPlugins = new LinkedHashSet<>();
 
     /**
-     * This will be updated with the list of accessible viewpoint projects
-     * present in the workspace, if any.
+     * This will be updated with the list of accessible viewpoint projects present in the workspace, if any.
      */
     private final Set<String> viewpointProjects = new LinkedHashSet<>();
 
@@ -176,6 +235,11 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
     private CrossReferencerProviderAdapterFactory adapterFactory;
 
     /**
+     * The converter used to coerce raw results.
+     */
+    private IConverter converter = new MTLConverter();
+
+    /**
      * Default constructor, will be called from the extension point's registry.
      */
     public AcceleoMTLInterpreter() {
@@ -183,17 +247,15 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
     }
 
     /**
-     * Tries and coerce the given <em>object</em> to an instance of the given
-     * class.
-     * 
+     * Tries and coerce the given <em>object</em> to an instance of the given class.
+     *
      * @param <T>
      *            Type to which we need to coerce <em>object</em>.
      * @param object
      *            The object we need to coerce to a given {@link Class}.
      * @param clazz
      *            Class to which we are to cast <em>object</em>.
-     * @return <em>object</em> cast to type <em>T</em> if possible,
-     *         <code>null</code> if not.
+     * @return <em>object</em> cast to type <em>T</em> if possible, <code>null</code> if not.
      */
     @SuppressWarnings("unchecked")
     private static <T> T coerceValue(Object object, Class<T> clazz) {
@@ -217,11 +279,10 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
 
     /**
      * Checks whether the given path exists in the plugins.
-     * 
+     *
      * @param path
      *            The path we need to check.
-     * @return <code>true</code> if <em>path</em> denotes an existing plugin
-     *         resource, <code>false</code> otherwise.
+     * @return <code>true</code> if <em>path</em> denotes an existing plugin resource, <code>false</code> otherwise.
      */
     private static boolean existsInPlugins(String path) {
         try {
@@ -234,11 +295,10 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
 
     /**
      * Checks whether the given path exists in the workspace.
-     * 
+     *
      * @param path
      *            The path we need to check.
-     * @return <code>true</code> if <em>path</em> denotes an existing workspace
-     *         resource, <code>false</code> otherwise.
+     * @return <code>true</code> if <em>path</em> denotes an existing workspace resource, <code>false</code> otherwise.
      */
     private static boolean existsInWorkspace(String path) {
         if (path == null || path.length() == 0) {
@@ -250,11 +310,10 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
 
     /**
      * Extracts the NsURI of the given EObject's metamodel.
-     * 
+     *
      * @param object
      *            The EObject for which we need the metamodel URI.
-     * @return The NsURI of the given EObject's metamodel, <code>null</code> if
-     *         we could not retrieve it.
+     * @return The NsURI of the given EObject's metamodel, <code>null</code> if we could not retrieve it.
      */
     private static String extractNsURI(EObject object) {
         final EPackage pack = object.eClass().getEPackage();
@@ -268,9 +327,8 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
     }
 
     /**
-     * Tries and extract the NsURIs of the given object if it is an EObject, or
-     * of its children if it is a Collection.
-     * 
+     * Tries and extract the NsURIs of the given object if it is an EObject, or of its children if it is a Collection.
+     *
      * @param object
      *            The object from which to try and detect metamodel URIs.
      * @return The extracted URIs.
@@ -279,13 +337,13 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
         Set<String> uris = new LinkedHashSet<>();
 
         if (object instanceof EObject) {
-            final String uri = extractNsURI((EObject) object);
+            final String uri = AcceleoMTLInterpreter.extractNsURI((EObject) object);
             if (uri != null) {
                 uris = Collections.singleton(uri);
             }
         } else if (object instanceof Collection<?>) {
             for (Object child : (Collection<?>) object) {
-                final Set<String> childURIs = extractNsURIs(child);
+                final Set<String> childURIs = AcceleoMTLInterpreter.extractNsURIs(child);
                 if (!childURIs.isEmpty()) {
                     uris = Sets.union(uris, childURIs);
                 }
@@ -300,7 +358,7 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
      * <p>
      * Makes no effort to try and check whether the argument is valid.
      * </p>
-     * 
+     *
      * @param values
      *            List from which we need the last value.
      * @param <V>
@@ -315,21 +373,20 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
     /**
      * Returns the qualified name of the given eObject's EClass, OCL style.
      * <p>
-     * For example, passing an instance of "eClass" to this would yield
-     * <code>ecore::EClass</code>.
+     * For example, passing an instance of "eClass" to this would yield <code>ecore::EClass</code>.
      * </p>
-     * 
+     *
      * @param eObject
      *            The EObject for which we need the qualified type name.
      * @return The qualified name of the given eObject's EClass.
      */
     private static String getQualifiedType(EObject eObject) {
-        return getQualifiedName(eObject.eClass());
+        return AcceleoMTLInterpreter.getQualifiedName(eObject.eClass());
     }
 
     /**
      * Returns the qualified name of the given EClassifier, OCL style.
-     * 
+     *
      * @param classifier
      *            The classifier for which we need the qualified type name.
      * @return The qualified name of the given EClassifier.
@@ -355,11 +412,10 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
 
     /**
      * Tries and infer the OCL type of the given Object.
-     * 
+     *
      * @param obj
      *            Object for which we need an OCL type.
-     * @return The inferred OCL type. OCLAny if we could not infer anything more
-     *         sensible.
+     * @return The inferred OCL type. OCLAny if we could not infer anything more sensible.
      */
     private static String inferOCLType(Object obj) {
         String oclType = "OCLAny"; //$NON-NLS-1$
@@ -374,27 +430,25 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
             } else if (obj instanceof Bag<?>) {
                 oclType = "Bag("; //$NON-NLS-1$
             }
-            oclType += getQualifiedName(elementType) + ')';
+            oclType += AcceleoMTLInterpreter.getQualifiedName(elementType) + ')';
         } else {
-            oclType = getQualifiedName(DynamicAcceleoModule.getOCLType(env, obj));
+            oclType = AcceleoMTLInterpreter.getQualifiedName(DynamicAcceleoModule.getOCLType(env, obj));
         }
         return oclType;
     }
 
     /**
-     * Checks whether the given <em>candidatePath</em> ends with the given
-     * <em>suffix</em>.
-     * 
+     * Checks whether the given <em>candidatePath</em> ends with the given <em>suffix</em>.
+     *
      * @param candidatePath
      *            Path for which we need to check the suffix.
      * @param suffix
      *            Suffix we need <em>candidatePath</em> to have.
-     * @return <code>true</code> if <em>candidatePath</em> ends with
-     *         <em>suffix</em>, <code>false</code> otherwise.
+     * @return <code>true</code> if <em>candidatePath</em> ends with <em>suffix</em>, <code>false</code> otherwise.
      */
     private static boolean pathEndsWith(String candidatePath, String suffix) {
-        final String[] candidateSegments = candidatePath.split(FILE_SEPARATOR_REGEX);
-        final String[] suffixSegments = suffix.split(FILE_SEPARATOR_REGEX);
+        final String[] candidateSegments = candidatePath.split(AcceleoMTLInterpreter.FILE_SEPARATOR_REGEX);
+        final String[] suffixSegments = suffix.split(AcceleoMTLInterpreter.FILE_SEPARATOR_REGEX);
 
         int candidateIndex = candidateSegments.length - 1;
         int suffixIndex = suffixSegments.length - 1;
@@ -410,11 +464,6 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
         return match;
     }
 
-    /**
-     * {@inheritDoc}
-     * 
-     * @see org.eclipse.sirius.common.tools.api.interpreter.IInterpreter#activateMetamodels(java.util.Collection)
-     */
     @Override
     public void activateMetamodels(Collection<MetamodelDescriptor> metamodels) {
         Set<EPackage> additionalEPackages = new LinkedHashSet<>();
@@ -431,11 +480,10 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
 
     /**
      * {@inheritDoc}
-     * 
+     *
      * @param dependency
-     *            Acceleo 3 dependencies should be of the form
-     *            <em>my::package::myModule</em>. Java dependencies should be in
-     *            the form <em>my.package.MyClass</em>.
+     *            Acceleo 3 dependencies should be of the form <em>my::package::myModule</em>. Java dependencies should
+     *            be in the form <em>my.package.MyClass</em>.
      * @see org.eclipse.sirius.common.tools.api.interpreter.IInterpreter#addImport(java.lang.String)
      */
     @Override
@@ -451,11 +499,6 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
         }
     }
 
-    /**
-     * {@inheritDoc}
-     * 
-     * @see org.eclipse.sirius.common.tools.api.interpreter.IInterpreter#clearImports()
-     */
     @Override
     public void clearImports() {
         mtlDependencies.clear();
@@ -464,11 +507,6 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
         invalidateModule();
     }
 
-    /**
-     * {@inheritDoc}
-     * 
-     * @see org.eclipse.sirius.common.tools.api.interpreter.IInterpreter#clearVariables()
-     */
     @Override
     public void clearVariables() {
         variables.clear();
@@ -477,9 +515,8 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
     }
 
     /**
-     * Creates an Acceleo compilation context for the current variables and the
-     * given contextual information.
-     * 
+     * Creates an Acceleo compilation context for the current variables and the given contextual information.
+     *
      * @param context
      *            The target EObject of this compilation.
      * @param expression
@@ -487,17 +524,17 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
      * @return The newly created context.
      */
     public CompilationContext createCompilationContext(EObject context, String expression) {
-        final String targetType = getQualifiedType(context);
+        final String targetType = AcceleoMTLInterpreter.getQualifiedType(context);
 
         String targetNsURI = null;
         final EObject root = EcoreUtil.getRootContainer(context);
         if (root != null) {
-            targetNsURI = extractNsURI(root);
+            targetNsURI = AcceleoMTLInterpreter.extractNsURI(root);
         }
 
         /*
-         * We will only iterate once on all variables, registering their nsURI
-         * and determining their OCL type along the way.
+         * We will only iterate once on all variables, registering their nsURI and determining their OCL type along the
+         * way.
          */
         if (compilationVariables.isEmpty() && !variables.isEmpty()) {
             for (Map.Entry<String, Collection<Object>> entry : variables.asMap().entrySet()) {
@@ -511,9 +548,9 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
                 }
                 List<Object> values = (List<Object>) entry.getValue();
                 if (!values.isEmpty()) {
-                    final Object actualValue = getLast(values);
-                    compilationVariables.put(entry.getKey(), inferOCLType(actualValue));
-                    variableNsURIs.addAll(extractNsURIs(actualValue));
+                    final Object actualValue = AcceleoMTLInterpreter.getLast(values);
+                    compilationVariables.put(entry.getKey(), AcceleoMTLInterpreter.inferOCLType(actualValue));
+                    variableNsURIs.addAll(AcceleoMTLInterpreter.extractNsURIs(actualValue));
                 }
             }
         }
@@ -529,9 +566,8 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
     }
 
     /**
-     * Creates an Acceleo compilation context for the current variables and the
-     * given contextual information.
-     * 
+     * Creates an Acceleo compilation context for the current variables and the given contextual information.
+     *
      * @param context
      *            Context of the current compilation.
      * @param expression
@@ -553,11 +589,6 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
         return new CompilationContext(expression, targetType, variableTypes, nsURIs, mtlDependencies, Sets.newLinkedHashSet(extendedDependencies));
     }
 
-    /**
-     * {@inheritDoc}
-     * 
-     * @see org.eclipse.sirius.common.tools.api.interpreter.IInterpreter#dispose()
-     */
     @Override
     public void dispose() {
         mtlDependencies.clear();
@@ -575,12 +606,6 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
         }
     }
 
-    /**
-     * {@inheritDoc}
-     * 
-     * @see org.eclipse.sirius.common.tools.api.interpreter.IInterpreter#evaluate(org.eclipse.emf.ecore.EObject,
-     *      java.lang.String)
-     */
     @Override
     public Object evaluate(EObject target, String expression) throws EvaluationException {
         EvaluationResult evaluationResult = internalEvaluate(target, expression);
@@ -588,114 +613,50 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
         return evaluationResult.getEvaluationResult();
     }
 
-    /**
-     * {@inheritDoc}
-     * 
-     * @see org.eclipse.sirius.common.tools.api.interpreter.IInterpreter#evaluateBoolean(org.eclipse.emf.ecore.EObject,
-     *      java.lang.String)
-     */
     @Override
     public boolean evaluateBoolean(EObject context, String expression) throws EvaluationException {
         EvaluationResult evaluationResult = internalEvaluate(context, expression);
         // Ignore potential problems for now
         Object result = evaluationResult.getEvaluationResult();
-
-        final Boolean coerced;
-        if (result instanceof Boolean) {
-            coerced = (Boolean) result;
-        } else {
-            coerced = coerceValue(result, Boolean.class);
-        }
-
-        if (coerced != null) {
-            return coerced.booleanValue();
-        }
-        return false;
+        return converter.toBoolean(result).orElse(Boolean.FALSE);
     }
 
-    /**
-     * {@inheritDoc}
-     * 
-     * @see org.eclipse.sirius.common.tools.api.interpreter.IInterpreter#evaluateCollection(org.eclipse.emf.ecore.EObject,
-     *      java.lang.String)
-     */
     @Override
     public Collection<EObject> evaluateCollection(EObject context, String expression) throws EvaluationException {
         EvaluationResult evaluationResult = internalEvaluate(context, expression);
         Object result = evaluationResult.getEvaluationResult();
-
-        Collection<EObject> coercedResult = new ArrayList<>();
-        if (result instanceof Collection<?>) {
-            Iterables.addAll(coercedResult, Iterables.filter((Collection<?>) result, EObject.class));
-        } else if (result != null && result.getClass().isArray()) {
-            Iterables.addAll(coercedResult, Iterables.filter(new ArrayList<Object>(Arrays.asList((Object[]) result)), EObject.class));
-        } else {
-            EObject coerced = coerceValue(result, EObject.class);
-            if (coerced != null) {
-                coercedResult.add(coerced);
-            }
-        }
-
-        return coercedResult;
+        return converter.toEObjectCollection(result).orElse(Collections.emptyList());
     }
 
-    /**
-     * {@inheritDoc}
-     * 
-     * @see org.eclipse.sirius.common.tools.api.interpreter.IInterpreter#evaluateEObject(org.eclipse.emf.ecore.EObject,
-     *      java.lang.String)
-     */
     @Override
     public EObject evaluateEObject(EObject context, String expression) throws EvaluationException {
         EvaluationResult evaluationResult = internalEvaluate(context, expression);
         // Ignore potential problems for now
         Object result = evaluationResult.getEvaluationResult();
-
-        EObject coerced = coerceValue(result, EObject.class);
-
-        return coerced;
+        return converter.toEObject(result).orElse(null);
     }
 
-    /**
-     * {@inheritDoc}
-     * 
-     * @see org.eclipse.sirius.common.tools.api.interpreter.IInterpreter#evaluateInteger(org.eclipse.emf.ecore.EObject,
-     *      java.lang.String)
-     */
     @Override
     public Integer evaluateInteger(EObject context, String expression) throws EvaluationException {
         EvaluationResult evaluationResult = internalEvaluate(context, expression);
         // Ignore potential problems for now
         Object result = evaluationResult.getEvaluationResult();
-
-        Integer coerced = coerceValue(result, Integer.class);
-
-        return coerced;
+        OptionalInt coerced = converter.toInt(result);
+        if (coerced.isPresent()) {
+            return Integer.valueOf(coerced.getAsInt());
+        } else {
+            return null;
+        }
     }
 
-    /**
-     * {@inheritDoc}
-     * 
-     * @see org.eclipse.sirius.common.tools.api.interpreter.IInterpreter#evaluateString(org.eclipse.emf.ecore.EObject,
-     *      java.lang.String)
-     */
     @Override
     public String evaluateString(EObject context, String expression) throws EvaluationException {
         EvaluationResult evaluationResult = internalEvaluate(context, expression);
         // Ignore potential problems for now
         Object result = evaluationResult.getEvaluationResult();
-
-        if (result != null) {
-            return result.toString();
-        }
-        return null;
+        return converter.toString(result).orElse(null);
     }
 
-    /**
-     * {@inheritDoc}
-     * 
-     * @see org.eclipse.sirius.common.tools.api.interpreter.IInterpreter#getImports()
-     */
     @Override
     public Collection<String> getImports() {
         Set<String> extendedImports = new LinkedHashSet<>();
@@ -707,79 +668,49 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
     }
 
     /**
-     * Gives access to the current module maintained by this interpreter. Mainly
-     * used to provide completion without re-compiling.
-     * 
+     * Gives access to the current module maintained by this interpreter. Mainly used to provide completion without
+     * re-compiling.
+     *
      * @return The current in-memory module maintained by this interpreter.
      */
     public DynamicAcceleoModule getModule() {
         return module;
     }
 
-    /**
-     * {@inheritDoc}
-     * 
-     * @see org.eclipse.sirius.common.tools.api.interpreter.IInterpreter#getPrefix()
-     */
     @Override
     public String getPrefix() {
-        return ACCELEO_EXPRESSION_PREFIX;
+        return AcceleoMTLInterpreter.ACCELEO_EXPRESSION_PREFIX;
     }
 
-    /**
-     * {@inheritDoc}
-     * 
-     * @see org.eclipse.sirius.common.tools.api.interpreter.IInterpreter#getVariable(java.lang.String)
-     */
     @Override
     public Object getVariable(String name) {
         if (variables.containsKey(name)) {
             final List<Object> values = variables.get(name);
             if (!values.isEmpty()) {
-                return getLast(values);
+                return AcceleoMTLInterpreter.getLast(values);
             }
         }
         return null;
     }
 
-    /**
-     * {@inheritDoc}
-     * 
-     * @see org.eclipse.sirius.common.tools.api.interpreter.IInterpreter#getVariablePrefix()
-     */
     @Override
     public String getVariablePrefix() {
         return null;
     }
 
-    /**
-     * {@inheritDoc}
-     * 
-     * @see org.eclipse.sirius.common.tools.api.interpreter.IInterpreter#getVariables()
-     */
     @Override
     public Map<String, ?> getVariables() {
         return variables.asMap();
     }
 
-    /**
-     * {@inheritDoc}
-     * 
-     * @see org.eclipse.sirius.common.tools.api.interpreter.IInterpreter#provides(java.lang.String)
-     */
     @Override
     public boolean provides(String expression) {
         if (expression != null) {
-            return expression.startsWith(ACCELEO_EXPRESSION_PREFIX) && expression.endsWith(ACCELEO_EXPRESSION_SUFFIX);
+            return expression.startsWith(AcceleoMTLInterpreter.ACCELEO_EXPRESSION_PREFIX) && expression.endsWith(AcceleoMTLInterpreter.ACCELEO_EXPRESSION_SUFFIX);
         }
         return false;
     }
 
-    /**
-     * {@inheritDoc}
-     * 
-     * @see org.eclipse.sirius.common.tools.api.interpreter.IInterpreter#removeImport(java.lang.String)
-     */
     @Override
     public void removeImport(String dependency) {
         boolean removed = false;
@@ -795,11 +726,6 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
         }
     }
 
-    /**
-     * {@inheritDoc}
-     * 
-     * @see org.eclipse.sirius.common.tools.api.interpreter.IInterpreter#setCrossReferencer(org.eclipse.emf.ecore.util.ECrossReferenceAdapter)
-     */
     @Override
     public void setCrossReferencer(ECrossReferenceAdapter crossReferencer) {
         if (adapterFactory == null) {
@@ -808,29 +734,17 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
         }
     }
 
-    /**
-     * {@inheritDoc}
-     * 
-     * @see org.eclipse.sirius.common.tools.api.interpreter.IInterpreter#setModelAccessor(org.eclipse.sirius.ecore.extender.business.api.accessor.ModelAccessor)
-     */
     @Override
     public void setModelAccessor(ModelAccessor modelAccessor) {
         // Nothing to do
     }
 
-    /**
-     * {@inheritDoc}
-     * 
-     * @see org.eclipse.sirius.common.tools.api.interpreter.IInterpreter#setProperty(java.lang.Object,
-     *      java.lang.Object)
-     */
     @Override
     public void setProperty(Object key, Object value) {
         /*
-         * This is called by the framework with the FILES key in order to pass
-         * us all the VSM files as a Collection.
+         * This is called by the framework with the FILES key in order to pass us all the VSM files as a Collection.
          */
-        if (FILES.equals(key)) {
+        if (IInterpreter.FILES.equals(key)) {
             if (value == null) {
                 viewpointProjects.clear();
                 viewpointPlugins.clear();
@@ -838,9 +752,9 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
                 for (final String odesignPath : Iterables.filter((Collection<?>) value, String.class)) {
                     final URI workspaceCandidate = URI.createPlatformResourceURI(odesignPath, true);
                     final URI pluginCandidate = URI.createPlatformPluginURI(odesignPath, true);
-                    if (existsInWorkspace(workspaceCandidate.toPlatformString(true))) {
+                    if (AcceleoMTLInterpreter.existsInWorkspace(workspaceCandidate.toPlatformString(true))) {
                         viewpointProjects.add(workspaceCandidate.segment(1));
-                    } else if (existsInPlugins(URI.decode(pluginCandidate.toString()))) {
+                    } else if (AcceleoMTLInterpreter.existsInPlugins(URI.decode(pluginCandidate.toString()))) {
                         viewpointPlugins.add(pluginCandidate.segment(1));
                     }
                 }
@@ -848,12 +762,6 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
         }
     }
 
-    /**
-     * {@inheritDoc}
-     * 
-     * @see org.eclipse.sirius.common.tools.api.interpreter.IInterpreter#setVariable(java.lang.String,
-     *      java.lang.Object)
-     */
     @Override
     public void setVariable(String name, Object value) {
         variables.put(name, value);
@@ -861,21 +769,11 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
         compilationVariables.clear();
     }
 
-    /**
-     * {@inheritDoc}
-     * 
-     * @see org.eclipse.sirius.common.tools.api.interpreter.IInterpreter#supportsValidation()
-     */
     @Override
     public boolean supportsValidation() {
         return true;
     }
 
-    /**
-     * {@inheritDoc}
-     * 
-     * @see org.eclipse.sirius.common.tools.api.interpreter.IInterpreter#unSetVariable(java.lang.String)
-     */
     @Override
     public void unSetVariable(String name) {
         if (variables.containsKey(name)) {
@@ -890,12 +788,6 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
         }
     }
 
-    /**
-     * {@inheritDoc}
-     * 
-     * @see org.eclipse.sirius.common.tools.api.interpreter.IInterpreter#validateExpression(org.eclipse.sirius.common.tools.api.interpreter.IInterpreterContext,
-     *      java.lang.String)
-     */
     @Override
     public Collection<IInterpreterStatus> validateExpression(IInterpreterContext context, String expression) {
         return analyzeExpression(context, expression).getStatuses();
@@ -910,7 +802,7 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
 
     /**
      * Retrieves the list of imports from the given interpreter context.
-     * 
+     *
      * @param context
      *            The context from which to retrieve a list of dependencies.
      */
@@ -920,9 +812,9 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
             Resource resource = element.eResource();
             if (resource != null) {
                 final URI uri = resource.getURI();
-                if (existsInWorkspace(uri.toPlatformString(true))) {
+                if (AcceleoMTLInterpreter.existsInWorkspace(uri.toPlatformString(true))) {
                     viewpointProjects.add(uri.segment(1));
-                } else if (existsInPlugins(uri.toPlatformString(true))) {
+                } else if (AcceleoMTLInterpreter.existsInPlugins(uri.toPlatformString(true))) {
                     viewpointPlugins.add(uri.segment(1));
                 }
             }
@@ -934,13 +826,12 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
     }
 
     /**
-     * Asks the registered import handlers whether they know about the given
-     * dependency.
-     * 
+     * Asks the registered import handlers whether they know about the given dependency.
+     *
      * @param dependency
      *            The dependency to try and add to this context.
-     * @return <code>true</code> if we found a corresponding import handler and
-     *         managed to create the dependency, <code>false</code> otherwise.
+     * @return <code>true</code> if we found a corresponding import handler and managed to create the dependency,
+     *         <code>false</code> otherwise.
      */
     private boolean addExtendedImport(String dependency) {
         // Consider that this will always change the dependency if it previously
@@ -971,13 +862,12 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
     }
 
     /**
-     * Tries and add the given dependency to the context, considering that it is
-     * an MTL file.
-     * 
+     * Tries and add the given dependency to the context, considering that it is an MTL file.
+     *
      * @param dependency
      *            The dependency to try and add.
-     * @return <code>true</code> if we found a corresponding Acceleo module and
-     *         added the dependency, <code>false</code> otherwise.
+     * @return <code>true</code> if we found a corresponding Acceleo module and added the dependency, <code>false</code>
+     *         otherwise.
      */
     private boolean addMTLImport(String dependency) {
         String actualDependency = dependency;
@@ -993,10 +883,8 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
         Set<URI> candidates = Sets.union(findProjectCandidates(emtlPath), findPluginCandidates(emtlPath));
 
         /*
-         * We may have multiple candidates for a single emtl path. Candidates
-         * located in the workspace will be considered before candidates located
-         * in the plugins but, other than that, we let Acceleo do its
-         * resolution.
+         * We may have multiple candidates for a single emtl path. Candidates located in the workspace will be
+         * considered before candidates located in the plugins but, other than that, we let Acceleo do its resolution.
          */
         if (!candidates.isEmpty()) {
             return mtlDependencies.putAll(actualDependency, candidates);
@@ -1006,7 +894,7 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
 
     /**
      * Validates that the given expression compiles in the given context.
-     * 
+     *
      * @param context
      *            Context of the current compilation.
      * @param compilationContext
@@ -1028,8 +916,7 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
                         severity = IInterpreterStatus.ERROR;
                     } else {
                         /*
-                         * IInterpreterStatus only understand "ERROR" and
-                         * "WARNING". Log infos as warnings.
+                         * IInterpreterStatus only understand "ERROR" and "WARNING". Log infos as warnings.
                          */
                         severity = IInterpreterStatus.WARNING;
                     }
@@ -1043,9 +930,8 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
     }
 
     /**
-     * Checks whether a viewpoint specification plugin contains an emtl
-     * corresponding to the given path.
-     * 
+     * Checks whether a viewpoint specification plugin contains an emtl corresponding to the given path.
+     *
      * @param emtlPath
      *            Path of the emtl for which we need potential URIs.
      * @return
@@ -1064,7 +950,7 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
                     while (emtlEntries.hasMoreElements()) {
                         final String candidatePath = emtlEntries.nextElement().getPath();
 
-                        if (pathEndsWith(candidatePath, emtlPath)) {
+                        if (AcceleoMTLInterpreter.pathEndsWith(candidatePath, emtlPath)) {
                             final StringBuilder fullPath = new StringBuilder(viewpointPluginSymbolicName);
                             fullPath.append('/').append(candidatePath);
 
@@ -1079,13 +965,11 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
     }
 
     /**
-     * Checks if the workspace contains an IProject that could hold an emtl file
-     * located at the given path.
-     * 
+     * Checks if the workspace contains an IProject that could hold an emtl file located at the given path.
+     *
      * @param emtlPath
      *            Path of the emtl for which we need potential URIs.
-     * @return The list of all projects that could hold a file at the given
-     *         path.
+     * @return The list of all projects that could hold a file at the given path.
      */
     private Set<URI> findProjectCandidates(String emtlPath) {
         final IWorkspace workspace = ResourcesPlugin.getWorkspace();
@@ -1112,15 +996,13 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
     }
 
     /**
-     * Evaluates the given expression and return the evaluation result and
-     * potential issues.
-     * 
+     * Evaluates the given expression and return the evaluation result and potential issues.
+     *
      * @param context
      *            Target EObject of this evaluation.
      * @param expression
      *            Expression we are to evaluate.
-     * @return The evaluation result along with all problems that may have
-     *         occurred during evaluation.
+     * @return The evaluation result along with all problems that may have occurred during evaluation.
      */
     private EvaluationResult internalEvaluate(EObject context, String expression) {
         final CompilationContext compilationContext = createCompilationContext(context, expression);
@@ -1136,16 +1018,15 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
     }
 
     /**
-     * Create a new IIntepreter preparing the environment by registering the
-     * required EPackages and resource factories in EMF.
-     * 
-     * @return an {@link AcceleoMTLInterpreter} suitable for querying in a
-     *         standalone environment.
+     * Create a new IIntepreter preparing the environment by registering the required EPackages and resource factories
+     * in EMF.
+     *
+     * @return an {@link AcceleoMTLInterpreter} suitable for querying in a standalone environment.
      */
     public static AcceleoMTLInterpreter createStandaloneInterpreter() {
-        registerPackages(EPackage.Registry.INSTANCE);
-        registerResourceFactories(Resource.Factory.Registry.INSTANCE);
-        registerLibraries();
+        AcceleoMTLInterpreter.registerPackages(EPackage.Registry.INSTANCE);
+        AcceleoMTLInterpreter.registerResourceFactories(Resource.Factory.Registry.INSTANCE);
+        AcceleoMTLInterpreter.registerLibraries();
         return new AcceleoMTLInterpreter();
 
     }
@@ -1159,7 +1040,7 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
         registry.put(MtlPackage.eINSTANCE.getNsURI(), MtlPackage.eINSTANCE);
 
         registry.put("http://www.eclipse.org/ocl/1.1.0/oclstdlib.ecore", //$NON-NLS-1$
-                getOCLStdLibPackage());
+                AcceleoMTLInterpreter.getOCLStdLibPackage());
     }
 
     private static void registerResourceFactories(Resource.Factory.Registry registry) {
@@ -1196,12 +1077,10 @@ public class AcceleoMTLInterpreter implements IInterpreter, TypedValidation {
     @Override
     public ValidationResult analyzeExpression(IInterpreterContext context, String expression) {
         /*
-         * The interpreter is created as a global singleton : one single
-         * interpreter for all operations, whatever the number of VSM or
-         * representation files. Validation is called on expression granularity,
-         * and we are never warned of the validation "start" or "end" events. We
-         * thus cannot keep any state for the in-memory module as it could be
-         * reflecting operations located on other VSM files.
+         * The interpreter is created as a global singleton : one single interpreter for all operations, whatever the
+         * number of VSM or representation files. Validation is called on expression granularity, and we are never
+         * warned of the validation "start" or "end" events. We thus cannot keep any state for the in-memory module as
+         * it could be reflecting operations located on other VSM files.
          */
         invalidateModule();
 
diff --git a/plugins/org.eclipse.sirius.common.ocl/src/org/eclipse/sirius/common/ocl/business/internal/interpreter/OclInterpreter.java b/plugins/org.eclipse.sirius.common.ocl/src/org/eclipse/sirius/common/ocl/business/internal/interpreter/OclInterpreter.java
index 320c4b1234..cf8a6a0ac5 100644
--- a/plugins/org.eclipse.sirius.common.ocl/src/org/eclipse/sirius/common/ocl/business/internal/interpreter/OclInterpreter.java
+++ b/plugins/org.eclipse.sirius.common.ocl/src/org/eclipse/sirius/common/ocl/business/internal/interpreter/OclInterpreter.java
@@ -18,6 +18,8 @@ import java.util.Collections;
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
+import java.util.Optional;
+import java.util.OptionalInt;
 
 import org.eclipse.emf.ecore.EClass;
 import org.eclipse.emf.ecore.EClassifier;
@@ -48,6 +50,7 @@ import org.eclipse.sirius.common.tools.api.interpreter.IInterpreterContext;
 import org.eclipse.sirius.common.tools.api.interpreter.IInterpreterProvider;
 import org.eclipse.sirius.common.tools.api.interpreter.IInterpreterStatus;
 import org.eclipse.sirius.common.tools.api.interpreter.VariableManager;
+import org.eclipse.sirius.common.tools.internal.interpreter.IConverter;
 import org.eclipse.sirius.ecore.extender.business.api.accessor.MetamodelDescriptor;
 import org.eclipse.sirius.ecore.extender.business.api.accessor.ModelAccessor;
 
@@ -58,6 +61,86 @@ import org.eclipse.sirius.ecore.extender.business.api.accessor.ModelAccessor;
  */
 public class OclInterpreter implements IInterpreter, IInterpreterProvider, IProposalProvider {
 
+    /**
+     * Coercion rules used to convert raw values obtained from OCL into the values returned to Sirius.
+     * 
+     * @author pcdavid
+     */
+    private static class OclConverter implements IConverter {
+
+        @Override
+        public OptionalInt toInt(Object rawValue) {
+            Integer result = null;
+            if (rawValue instanceof Integer) {
+                result = (Integer) rawValue;
+            } else if (rawValue instanceof String) {
+                try {
+                    result = new Integer((String) rawValue);
+                } catch (final NumberFormatException nfe) {
+                    DslOclPlugin.getPlugin().error(Messages.OclInterpreter_OclNotANumber, nfe);
+                }
+            }
+            if (result != null) {
+                return OptionalInt.of(result);
+            } else {
+                return OptionalInt.empty();
+            }
+        }
+
+        @Override
+        public Optional<Boolean> toBoolean(Object rawValue) {
+            boolean result = false;
+            if (rawValue instanceof Boolean) {
+                result = ((Boolean) rawValue).booleanValue();
+            } else if (rawValue instanceof String) {
+                result = Boolean.parseBoolean((String) rawValue);
+            }
+            return Optional.of(result);
+        }
+
+        @Override
+        public Optional<String> toString(Object rawValue) {
+            if (rawValue != null) {
+                return Optional.of(rawValue.toString());
+            }
+            return Optional.empty();
+        }
+
+        @Override
+        public Optional<EObject> toEObject(Object rawValue) {
+            if (rawValue instanceof EObject) {
+                return Optional.of((EObject) rawValue);
+            } else {
+                return Optional.empty();
+            }
+        }
+
+        @Override
+        public Optional<Collection<EObject>> toEObjectCollection(Object rawValue) {
+            Collection<EObject> result = Collections.emptyList();
+            if (rawValue instanceof EObject) {
+                result = new ArrayList<>(1);
+                result.add((EObject) rawValue);
+            } else if (rawValue instanceof Collection) {
+                result = new ArrayList<>(((Collection<?>) rawValue).size());
+                for (final Object object : (Collection<?>) rawValue) {
+                    if (object instanceof EObject) {
+                        result.add((EObject) object);
+                    }
+                }
+            } else if (rawValue != null && rawValue.getClass().isArray()) {
+                result = new ArrayList<>(((Object[]) rawValue).length);
+                for (final Object object : (Object[]) rawValue) {
+                    if (object instanceof EObject) {
+                        result.add((EObject) object);
+                    }
+                }
+            }
+            return Optional.of(result);
+        }
+
+    }
+
     /** The OCL expression discrimant. */
     public static final String OCL_DISCRIMINANT = "ocl:"; //$NON-NLS-1$
 
@@ -70,6 +153,8 @@ public class OclInterpreter implements IInterpreter, IInterpreterProvider, IProp
     /** The variables. */
     private final VariableManager variables = new VariableManager();
 
+    private final IConverter converter = new OclConverter();
+
     @Override
     public void activateMetamodels(Collection<MetamodelDescriptor> metamodels) {
         // Nothing to do
@@ -127,73 +212,32 @@ public class OclInterpreter implements IInterpreter, IInterpreterProvider, IProp
 
     @Override
     public boolean evaluateBoolean(final EObject context, final String expression) throws EvaluationException {
-        final Object value = evaluate(context, expression);
-        boolean result = false;
-        if (value instanceof Boolean) {
-            result = ((Boolean) value).booleanValue();
-        } else if (value instanceof String) {
-            result = Boolean.parseBoolean((String) value);
-        }
-        return result;
+        Object value = evaluate(context, expression);
+        return converter.toBoolean(value).orElse(false);
     }
 
     @Override
     public Collection<EObject> evaluateCollection(final EObject context, final String expression) throws EvaluationException {
-        final Object value = evaluate(context, expression);
-        Collection<EObject> result = Collections.emptyList();
-        if (value instanceof EObject) {
-            result = new ArrayList<EObject>(1);
-            result.add((EObject) value);
-        } else if (value instanceof Collection) {
-            result = new ArrayList<EObject>(((Collection<?>) value).size());
-            for (final Object object : (Collection<?>) value) {
-                if (object instanceof EObject) {
-                    result.add((EObject) object);
-                }
-            }
-        } else if (value != null && value.getClass().isArray()) {
-            result = new ArrayList<EObject>(((Object[]) value).length);
-            for (final Object object : (Object[]) value) {
-                if (object instanceof EObject) {
-                    result.add((EObject) object);
-                }
-            }
-        }
-        return result;
+        Object value = evaluate(context, expression);
+        return converter.toEObjectCollection(value).orElse(Collections.emptySet());
     }
 
     @Override
     public EObject evaluateEObject(final EObject context, final String expression) throws EvaluationException {
-        final Object value = evaluate(context, expression);
-        if (value instanceof EObject) {
-            return (EObject) value;
-        }
-        return null;
+        Object value = evaluate(context, expression);
+        return converter.toEObject(value).orElse(null);
     }
 
     @Override
     public Integer evaluateInteger(final EObject context, final String expression) throws EvaluationException {
         final Object value = evaluate(context, expression);
-        Integer result = null;
-        if (value instanceof Integer) {
-            result = (Integer) value;
-        } else if (value instanceof String) {
-            try {
-                result = new Integer((String) value);
-            } catch (final NumberFormatException nfe) {
-                DslOclPlugin.getPlugin().error(Messages.OclInterpreter_OclNotANumber, nfe);
-            }
-        }
-        return result;
+        return converter.toInt(value).orElse(0);
     }
 
     @Override
     public String evaluateString(final EObject context, final String expression) throws EvaluationException {
-        final Object value = evaluate(context, expression);
-        if (value != null) {
-            return value.toString();
-        }
-        return null;
+        Object value = evaluate(context, expression);
+        return converter.toString(value).orElse(null);
     }
 
     /**
@@ -274,9 +318,6 @@ public class OclInterpreter implements IInterpreter, IInterpreterProvider, IProp
         this.variables.clearVariables();
     }
 
-    /**
-     * {@inheritDoc}
-     */
     @Override
     public void setModelAccessor(final ModelAccessor modelAccessor) {
         // ignore
@@ -284,8 +325,7 @@ public class OclInterpreter implements IInterpreter, IInterpreterProvider, IProp
 
     @Override
     public List<ContentProposal> getProposals(IInterpreter interpreter, ContentContext context) {
-        List<ContentProposal> computeCompletionEntry = OclCompletionEntry.computeCompletionEntry(context);
-        return computeCompletionEntry;
+        return OclCompletionEntry.computeCompletionEntry(context);
     }
 
     @Override
@@ -322,8 +362,8 @@ public class OclInterpreter implements IInterpreter, IInterpreterProvider, IProp
 
                 // Ask OCLHelper for proposals and process them
                 final List<Choice> choices = getOCLHelper().getSyntaxHelp(ConstraintKind.INVARIANT, evaluationString);
-                if (choices != null && choices.size() > 0) {
-                    final List<ContentProposal> contentProposals = new ArrayList<ContentProposal>();
+                if (choices != null && !choices.isEmpty()) {
+                    final List<ContentProposal> contentProposals = new ArrayList<>();
                     for (Choice choice : choices) {
                         contentProposals.add(new ContentProposal(choice.getName(), choice.getName() + " (" + choice.getKind().name() + ")", choice.getDescription())); //$NON-NLS-1$//$NON-NLS-2$
                     }
@@ -337,7 +377,7 @@ public class OclInterpreter implements IInterpreter, IInterpreterProvider, IProp
 
     @Override
     public Collection<String> getImports() {
-        return Collections.<String> emptyList();
+        return Collections.emptyList();
     }
 
     @Override
@@ -347,7 +387,7 @@ public class OclInterpreter implements IInterpreter, IInterpreterProvider, IProp
 
     @Override
     public Collection<IInterpreterStatus> validateExpression(IInterpreterContext context, String expression) {
-        return new LinkedHashSet<IInterpreterStatus>();
+        return new LinkedHashSet<>();
     }
 
     @Override
diff --git a/plugins/org.eclipse.sirius.common/src/org/eclipse/sirius/common/tools/internal/interpreter/AbstractInterpreter.java b/plugins/org.eclipse.sirius.common/src/org/eclipse/sirius/common/tools/internal/interpreter/AbstractInterpreter.java
index 34071661e9..c04259010c 100644
--- a/plugins/org.eclipse.sirius.common/src/org/eclipse/sirius/common/tools/internal/interpreter/AbstractInterpreter.java
+++ b/plugins/org.eclipse.sirius.common/src/org/eclipse/sirius/common/tools/internal/interpreter/AbstractInterpreter.java
@@ -28,9 +28,6 @@ import org.eclipse.sirius.common.tools.api.interpreter.TypedValidation;
 import org.eclipse.sirius.ecore.extender.business.api.accessor.MetamodelDescriptor;
 import org.eclipse.sirius.ecore.extender.business.api.accessor.ModelAccessor;
 
-import com.google.common.collect.Iterables;
-import com.google.common.collect.Lists;
-
 /**
  * Abstract base class for implementations of {@link IInterpreter}.
  *
@@ -41,6 +38,12 @@ public abstract class AbstractInterpreter implements IInterpreter, TypedValidati
     /** The separator between EPackage name and EClass name for domain class. */
     protected static final String SEPARATOR = "."; //$NON-NLS-1$
 
+    /**
+     * The converter to use to coerce raw values returned by the actual implementation into the type requested expected
+     * for a given method.
+     */
+    private final IConverter converter = new DefaultConverter();
+
     @Override
     public boolean provides(String expression) {
         return expression != null && expression.startsWith(getPrefix());
@@ -49,73 +52,31 @@ public abstract class AbstractInterpreter implements IInterpreter, TypedValidati
     @Override
     public Collection<EObject> evaluateCollection(EObject context, String expression) throws EvaluationException {
         Object raw = evaluate(context, expression);
-        final Collection<EObject> result;
-        if (raw instanceof Collection<?>) {
-            result = Lists.newArrayList(Iterables.filter((Collection<?>) raw, EObject.class));
-        } else if (raw instanceof EObject) {
-            result = Collections.singleton((EObject) raw);
-        } else if (raw != null && raw.getClass().isArray()) {
-            result = Lists.newArrayList(Iterables.filter(Lists.newArrayList((Object[]) raw), EObject.class));
-        } else {
-            result = Collections.emptySet();
-        }
-        return result;
+        return converter.toEObjectCollection(raw).orElse(Collections.emptySet());
     }
 
     @Override
     public boolean evaluateBoolean(EObject context, String expression) throws EvaluationException {
         Object raw = evaluate(context, expression);
-        final boolean result;
-        if (raw == null) {
-            result = false;
-        } else if (raw instanceof Boolean) {
-            result = ((Boolean) raw).booleanValue();
-        } else {
-            String toString = raw.toString();
-            if ("true".equalsIgnoreCase(toString)) { //$NON-NLS-1$
-                result = true;
-            } else if ("false".equalsIgnoreCase(toString)) { //$NON-NLS-1$
-                result = false;
-            } else {
-                /*
-                 * raw is != null and its toString is neither true or false, this happens when the user expect the
-                 * condition to check that a value is existing, then we consider any non null value returns true and
-                 * null returns false.
-                 */
-                result = true;
-            }
-        }
-        return result;
+        return converter.toBoolean(raw).orElse(false);
     }
 
     @Override
     public EObject evaluateEObject(EObject context, String expression) throws EvaluationException {
         Object raw = evaluate(context, expression);
-        if (raw instanceof EObject) {
-            return (EObject) raw;
-        } else {
-            return null;
-        }
+        return converter.toEObject(raw).orElse(null);
     }
 
     @Override
     public String evaluateString(EObject context, String expression) throws EvaluationException {
         Object raw = evaluate(context, expression);
-        if (raw != null) {
-            return String.valueOf(raw);
-        } else {
-            return ""; //$NON-NLS-1$
-        }
+        return converter.toString(raw).orElse(""); //$NON-NLS-1$
     }
 
     @Override
     public Integer evaluateInteger(EObject context, String expression) throws EvaluationException {
         Object raw = evaluate(context, expression);
-        try {
-            return Integer.parseInt(String.valueOf(raw));
-        } catch (NumberFormatException e) {
-            return Integer.valueOf(0);
-        }
+        return converter.toInt(raw).orElse(0);
     }
 
     @Override
diff --git a/plugins/org.eclipse.sirius.common/src/org/eclipse/sirius/common/tools/internal/interpreter/DefaultConverter.java b/plugins/org.eclipse.sirius.common/src/org/eclipse/sirius/common/tools/internal/interpreter/DefaultConverter.java
new file mode 100644
index 0000000000..754b24e7f0
--- /dev/null
+++ b/plugins/org.eclipse.sirius.common/src/org/eclipse/sirius/common/tools/internal/interpreter/DefaultConverter.java
@@ -0,0 +1,100 @@
+/*******************************************************************************
+ * Copyright (c) 2019 Obeo.
+ * This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * which accompanies this distribution, and is available at
+ * https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ * Contributors:
+ *    Obeo - initial API and implementation
+ *******************************************************************************/
+
+package org.eclipse.sirius.common.tools.internal.interpreter;
+
+import java.util.Collection;
+import java.util.Collections;
+import java.util.Optional;
+import java.util.OptionalInt;
+
+import org.eclipse.emf.ecore.EObject;
+
+import com.google.common.collect.Iterables;
+import com.google.common.collect.Lists;
+
+/**
+ * The default implementation of type conversion for interperters. It should be the one used for all implementations
+ * unless there are good reasons to do otherwise to ensure consistency between the various languages.
+ * 
+ * @author pcdavid
+ *
+ */
+public class DefaultConverter implements IConverter {
+    @Override
+    public Optional<Boolean> toBoolean(Object rawValue) {
+        final boolean result;
+        if (rawValue == null) {
+            result = false;
+        } else if (rawValue instanceof Boolean) {
+            result = ((Boolean) rawValue).booleanValue();
+        } else {
+            String toString = rawValue.toString();
+            if ("true".equalsIgnoreCase(toString)) { //$NON-NLS-1$
+                result = true;
+            } else if ("false".equalsIgnoreCase(toString)) { //$NON-NLS-1$
+                result = false;
+            } else {
+                /*
+                 * raw is != null and its toString is neither true or false, this happens when the user expect the
+                 * condition to check that a value is existing, then we consider any non null value returns true and
+                 * null returns false.
+                 */
+                result = true;
+            }
+        }
+        return Optional.of(result);
+    }
+    
+    @Override
+    public Optional<EObject> toEObject(Object rawValue) {
+        if (rawValue instanceof EObject) {
+            return Optional.of((EObject) rawValue);
+        } else {
+            return Optional.empty();
+        }
+    }
+    
+    @Override
+    public Optional<String> toString(Object rawValue) {
+        if (rawValue != null) {
+            return Optional.of(String.valueOf(rawValue));
+        } else {
+            return Optional.empty();
+        }
+    }
+    
+    @Override
+    public OptionalInt toInt(Object rawValue) {
+        try {
+            return OptionalInt.of(Integer.parseInt(String.valueOf(rawValue)));
+        } catch (NumberFormatException e) {
+            return OptionalInt.empty();
+        }
+    }
+    
+    @Override
+    public Optional<Collection<EObject>> toEObjectCollection(Object rawValue) {
+        final Collection<EObject> result;
+        if (rawValue instanceof Collection<?>) {
+            result = Lists.newArrayList(Iterables.filter((Collection<?>) rawValue, EObject.class));
+        } else if (rawValue instanceof EObject) {
+            result = Collections.singleton((EObject) rawValue);
+        } else if (rawValue != null && rawValue.getClass().isArray()) {
+            result = Lists.newArrayList(Iterables.filter(Lists.newArrayList((Object[]) rawValue), EObject.class));
+        } else {
+            result = Collections.emptySet();
+        }
+        return Optional.of(result);
+    }
+}
diff --git a/plugins/org.eclipse.sirius.common/src/org/eclipse/sirius/common/tools/internal/interpreter/IConverter.java b/plugins/org.eclipse.sirius.common/src/org/eclipse/sirius/common/tools/internal/interpreter/IConverter.java
new file mode 100644
index 0000000000..95c137f864
--- /dev/null
+++ b/plugins/org.eclipse.sirius.common/src/org/eclipse/sirius/common/tools/internal/interpreter/IConverter.java
@@ -0,0 +1,77 @@
+/*******************************************************************************
+ * Copyright (c) 2019 Obeo.
+ * This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * which accompanies this distribution, and is available at
+ * https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ * Contributors:
+ *    Obeo - initial API and implementation
+ *******************************************************************************/
+package org.eclipse.sirius.common.tools.internal.interpreter;
+
+import java.util.Collection;
+import java.util.Optional;
+import java.util.OptionalInt;
+
+import org.eclipse.emf.ecore.EObject;
+
+/**
+ * Provides coercion from raw values obtained from evaluating an expression. The rules followed for the conversions are
+ * implementation-defined.
+ * 
+ * @author pcdavid
+ */
+public interface IConverter {
+    /**
+     * Try to convert a raw value into an {@code int}.
+     * 
+     * @param rawValue
+     *            the raw value.
+     * @return an int "equivalent" to the raw value, or an empty optional of the value could not be converted
+     *         meaningfully.
+     */
+    OptionalInt toInt(Object rawValue);
+
+    /**
+     * Try to convert a raw value into a {@code Boolean}.
+     * 
+     * @param rawValue
+     *            the raw value.
+     * @return a boolean "equivalent" to the raw value, or an empty optional of the value could not be converted
+     *         meaningfully.
+     */
+    Optional<Boolean> toBoolean(Object rawValue);
+
+    /**
+     * Try to convert a raw value into an {@code String}.
+     * 
+     * @param rawValue
+     *            the raw value.
+     * @return an string "equivalent" to the raw value, or an empty optional of the value could not be converted
+     *         meaningfully.
+     */
+    Optional<String> toString(Object rawValue);
+
+    /**
+     * Try to convert a raw value into an {@code EObject}.
+     * 
+     * @param rawValue
+     *            the raw value.
+     * @return an EObject "equivalent" to the raw value, or an empty optional of the value could not be converted
+     *         meaningfully.
+     */
+    Optional<EObject> toEObject(Object rawValue);
+
+    /**
+     * Try to convert a raw value into a collection of {@link EObject}s.
+     * 
+     * @param rawValue
+     *            the raw value.
+     * @return a collection of {@link EObject}s "equivalent" to the raw value, or an empty optional of the value could
+     *         not be converted meaningfully.
+     */
+    Optional<Collection<EObject>> toEObjectCollection(Object rawValue);
+}