@@ 24,6 24,15 @@ type Type interface {
 	fmt.Stringer
 }
 
+// typeContainer is an optional interface that must be implemented by Types that
+// contain other identifiers. It defines the typeof method that must return
+// the type of the selected identifier inside the container type. It returns nil
+// as Type and Invalid as TypeContext if the selector does not exist.
+type typeContainer interface {
+	Type
+	typeof(sel *Ident, ctxHint TypeContext) (Type, TypeContext)
+}
+
 // List of types implementing the Type interface.
 type (
 	// BasicType represents a basic type such as int, bool or string. All basic types


@@ 56,7 65,7 @@ type (
 		lookup map[*GenericType]Type
 	}
 
-	// InterfaceType represents a named interface type.
+	// InterfaceType represents an interface type.
 	InterfaceType struct {
 		// Decl represents the interface declaration of this type.
 		Decl *Interface


@@ 368,10 377,26 @@ func (t *TupleType) ResolveGeneric(resolve map[*GenericType]Type) Type {
 	return t
 }
 
+func (t *TupleType) typeof(sel *Ident, ctxHint TypeContext) (Type, TypeContext) {
+	if sel.Index < 0 || sel.Index >= len(t.Fields) {
+		return nil, Invalid
+	}
+	return t.Fields[sel.Index], ctxHint
+}
+
 // ========> implement Type for StructType
 
-func (s *StructType) AssignableTo(T Type) bool { return s.IdenticalTo(T) }
-func (s *StructType) Valid() bool              { return true }
+func (s *StructType) AssignableTo(T Type) bool {
+	switch T.(type) {
+	case *StructType:
+		return s.IdenticalTo(T)
+	case *InterfaceType:
+		// TODO : struct can be assignable to an interface type if it satisfies it
+	}
+	return false
+}
+
+func (s *StructType) Valid() bool { return true }
 
 func (s *StructType) String() string {
 	var buf strings.Builder


@@ 431,6 456,15 @@ func (s *StructType) ResolveGeneric(resolve map[*GenericType]Type) Type {
 	return s
 }
 
+func (s *StructType) typeof(sel *Ident, _ TypeContext) (Type, TypeContext) {
+	decl := s.Decl.BodyScope.Lookup(sel.Name)
+	if decl == nil {
+		return nil, Invalid
+	}
+	sel.Ref = decl
+	return s.typeOfSel(decl.Type()), decl.TypeContext()
+}
+
 // typeOfSel returns the actual type of a selection inside the struct.
 // If the struct is not generic, then T is returned unchanged, otherwise
 // if T is a GenericType it is replaced by the type of its corresponding


@@ 461,8 495,19 @@ func makeGenericResolveMap(gc *GenericClause, types []Type) map[*GenericType]Typ
 
 // TODO: should be assignable to any interface type that is a subset
 // of this interface (or identical).
-func (i *InterfaceType) AssignableTo(T Type) bool { return i.IdenticalTo(T) }
-func (i *InterfaceType) Valid() bool              { return true }
+func (i *InterfaceType) AssignableTo(T Type) bool {
+	return i.IdenticalTo(T)
+}
+
+// an interface type is valid if all of its methods' types are valid.
+func (i *InterfaceType) Valid() bool {
+	for _, m := range i.Decl.Methods {
+		if !m.Type().Valid() {
+			return false
+		}
+	}
+	return true
+}
 
 func (i *InterfaceType) String() string {
 	var buf strings.Builder


@@ 522,6 567,18 @@ func (i *InterfaceType) ResolveGeneric(resolve map[*GenericType]Type) Type {
 	return i
 }
 
+func (i *InterfaceType) typeof(sel *Ident, ctxHint TypeContext) (Type, TypeContext) {
+	for _, m := range i.Decl.Methods {
+		if m.Ident() == sel.Name {
+			// TODO: first, the type must be generic-resolved much like typeOfSel for SturctType,
+			// then the TypeContext might not be ok, this would always give immutable (the method
+			// is a func decl). Maybe it should be ctxHint? TBD...
+			return m.Type(), m.TypeContext()
+		}
+	}
+	return nil, Invalid
+}
+
 // ========> implement Type for GenericType
 
 func (g *GenericType) String() string { return g.Name }

@@ 144,9 144,6 @@ func (t *typeassignVisitor) Visit(n Node) Visitor {
 		n.typ = it
 
 	case *GenericElem:
-		// TODO: when traits/capabilities are added, this would lookup the matching
-		// generic clause on the parent, and add the specific traits constraints on
-		// the type.
 		n.typ = &GenericType{
 			Name:    n.Ident(),
 			ScopeID: n.Scope().ID,


@@ 335,6 332,7 @@ func (t *typeassignVisitor) instantiateGeneric(gi *GenericInst, gen GenericDecl,
 		t.errh(gi.Pos(), fmt.Sprintf("wrong number of types provided in generic instantiation, want %d, got %d", len(gc.Elems), len(types)))
 	}
 
+	// TODO: need to support Interface too, could be methods on the semantic.Decl instead
 	switch gen := gen.(type) {
 	case *Fn:
 		if gen.GenericInsts == nil {


@@ 378,7 376,7 @@ func (t *typeassignVisitor) typeAssignGenericInst(gi *GenericInst) {
 	ctx := genRef.TypeContext()
 	switch {
 	case ctx == Typ:
-		gi.ctx = Typ // instantiated struct generics are types
+		gi.ctx = Typ // instantiated struct / interface generics are types
 	case ctx.isAnyOf(TypeContextValues...):
 		gi.ctx = Value // instantiated fn generics are values
 	}


@@ 390,27 388,13 @@ func (t *typeassignVisitor) typeAssignSelector(left Type, leftCtx TypeContext, s
 	// of the Selector expr. If Sel is marked unresolved, then the Selector
 	// expression will be unresolved too and will be properly added to deferred.
 
-	switch left := left.(type) {
-	case *StructType:
-		decl := left.Decl.BodyScope.Lookup(sel.Name)
-		if decl != nil {
-			sel.ctx = decl.TypeContext()
-			sel.typ = left.typeOfSel(decl.Type())
-			sel.Ref = decl
-			return
-		}
-		t.errUndefined(sel, left)
-
-	case *TupleType:
-		sel.ctx = leftCtx
-		if sel.Index < 0 || sel.Index >= len(left.Fields) {
+	if tof, ok := left.(typeContainer); ok {
+		sel.typ, sel.ctx = tof.typeof(sel, leftCtx)
+		if sel.typ == nil {
 			t.errUndefined(sel, left)
-			return
 		}
-		sel.typ = left.Fields[sel.Index]
-
-	default:
-		sel.ctx = Invalid
-		sel.typ = unresolvedType{}
+		return
 	}
+	sel.ctx = Invalid
+	sel.typ = unresolvedType{}
 }