diff --git a/src/go/ast/ast.go b/src/go/ast/ast.go
index b595573851..a931d1b48b 100644
--- a/src/go/ast/ast.go
+++ b/src/go/ast/ast.go
@@ -440,9 +440,11 @@ type (
 	}
 
 	// An InterfaceType node represents an interface type.
+	// The Types list is an experimental extension for interfaces that serve as type bounds (like contracts).
 	InterfaceType struct {
 		Interface  token.Pos  // position of "interface" keyword
 		Methods    *FieldList // list of methods
+		Types      []Expr     // list of types (TODO(gri) for now they are all lumped together, loosing syntax info)
 		Incomplete bool       // true if (source) methods are missing in the Methods list
 	}
 
diff --git a/src/go/parser/parser.go b/src/go/parser/parser.go
index e87757f164..3578a2f67a 100644
--- a/src/go/parser/parser.go
+++ b/src/go/parser/parser.go
@@ -1231,9 +1231,20 @@ func (p *parser) parseInterfaceType() *ast.InterfaceType {
 	pos := p.expect(token.INTERFACE)
 	lbrace := p.expect(token.LBRACE)
 	scope := ast.NewScope(nil) // interface scope
-	var list []*ast.Field
-	for p.tok == token.IDENT {
-		list = append(list, p.parseMethodSpec(scope))
+	var mlist []*ast.Field
+	var tlist []ast.Expr
+L:
+	for {
+		switch p.tok {
+		case token.IDENT:
+			mlist = append(mlist, p.parseMethodSpec(scope))
+		case token.TYPE:
+			p.next()
+			tlist = append(tlist, p.parseTypeList()...)
+			p.expectSemi()
+		default:
+			break L
+		}
 	}
 	rbrace := p.expect(token.RBRACE)
 
@@ -1241,9 +1252,10 @@ func (p *parser) parseInterfaceType() *ast.InterfaceType {
 		Interface: pos,
 		Methods: &ast.FieldList{
 			Opening: lbrace,
-			List:    list,
+			List:    mlist,
 			Closing: rbrace,
 		},
+		Types: tlist,
 	}
 }
 
diff --git a/src/go/parser/short_test.go b/src/go/parser/short_test.go
index 461434eaca..b39fae7d76 100644
--- a/src/go/parser/short_test.go
+++ b/src/go/parser/short_test.go
@@ -97,6 +97,10 @@ var valids = []string{
 	`package p; type C contract(T){ T int, imported.T, chan<-int; T m(x int) float64; C0(); imported.C1(int, T,) }`,
 	`package p; func _(type T1, T2 interface{})(x T1) T2`,
 	`package p; func _(type T1 interface{ m() }, T2, T3 interface{})(x T1, y T3) T2`,
+
+	// interfaces as contracts (experimental)
+	`package p; type _ interface{type int}`,
+	`package p; type _ interface{type int, float32; type bool; m(); type string;}`,
 }
 
 func TestValid(t *testing.T) {
diff --git a/src/go/types/contracts.go b/src/go/types/contracts.go
index 2723fd8195..c5bc9d381c 100644
--- a/src/go/types/contracts.go
+++ b/src/go/types/contracts.go
@@ -79,24 +79,7 @@ func (check *Checker) contractType(contr *Contract, e *ast.ContractType) {
 			switch nmethods {
 			case 0:
 				// type constraints
-				for _, texpr := range c.Types {
-					if texpr == nil {
-						check.invalidAST(pos, "missing type constraint")
-						continue
-					}
-					typ := check.typ(texpr)
-					// A type constraint may be a predeclared type or a
-					// composite type composed only of predeclared types.
-					// TODO(gri) should we keep this restriction?
-					var why string
-					if !check.typeConstraint(typ, &why) {
-						check.errorf(texpr.Pos(), "invalid type constraint %s (%s)", typ, why)
-						continue
-					}
-					// add type
-					iface.types = append(iface.types, typ)
-
-				}
+				iface.types = check.collectTypeConstraints(pos, iface.types, c.Types)
 
 			case 1:
 				// method constraint
@@ -161,6 +144,27 @@ func (check *Checker) contractType(contr *Contract, e *ast.ContractType) {
 	contr.IFaces = ifaces
 }
 
+func (check *Checker) collectTypeConstraints(pos token.Pos, list []Type, types []ast.Expr) []Type {
+	for _, texpr := range types {
+		if texpr == nil {
+			check.invalidAST(pos, "missing type constraint")
+			continue
+		}
+		typ := check.typ(texpr)
+		// A type constraint may be a predeclared type or a
+		// composite type composed only of predeclared types.
+		// TODO(gri) should we keep this restriction?
+		var why string
+		if !check.typeConstraint(typ, &why) {
+			check.errorf(texpr.Pos(), "invalid type constraint %s (%s)", typ, why)
+			continue
+		}
+		// add type
+		list = append(list, typ)
+	}
+	return list
+}
+
 // TODO(gri) does this simply check for the absence of defined types?
 //           (if so, should choose a better name)
 func (check *Checker) typeConstraint(typ Type, why *string) bool {
@@ -213,7 +217,8 @@ func (check *Checker) typeConstraint(typ Type, why *string) bool {
 		*why = check.sprintf("%s is not a type", t)
 		return false
 	case *TypeParam:
-		// ok
+		// TODO(gri) should this be ok? need a good use case
+		// ok for now
 	default:
 		unreachable()
 	}
diff --git a/src/go/types/exprstring.go b/src/go/types/exprstring.go
index e89950e19b..c6c44b6da6 100644
--- a/src/go/types/exprstring.go
+++ b/src/go/types/exprstring.go
@@ -98,12 +98,7 @@ func WriteExpr(buf *bytes.Buffer, x ast.Expr) {
 	case *ast.CallExpr:
 		WriteExpr(buf, x.Fun)
 		buf.WriteByte('(')
-		for i, arg := range x.Args {
-			if i > 0 {
-				buf.WriteString(", ")
-			}
-			WriteExpr(buf, arg)
-		}
+		writeExprList(buf, x.Args)
 		if x.Ellipsis.IsValid() {
 			buf.WriteString("...")
 		}
@@ -144,6 +139,13 @@ func WriteExpr(buf *bytes.Buffer, x ast.Expr) {
 	case *ast.InterfaceType:
 		buf.WriteString("interface{")
 		writeFieldList(buf, x.Methods, "; ", true)
+		if len(x.Types) > 0 {
+			if len(x.Methods.List) > 0 {
+				buf.WriteString("; ")
+			}
+			buf.WriteString("type ")
+			writeExprList(buf, x.Types)
+		}
 		buf.WriteByte('}')
 
 	case *ast.MapType:
@@ -232,3 +234,12 @@ func writeIdentList(buf *bytes.Buffer, list []*ast.Ident) {
 		buf.WriteString(x.Name)
 	}
 }
+
+func writeExprList(buf *bytes.Buffer, list []ast.Expr) {
+	for i, x := range list {
+		if i > 0 {
+			buf.WriteString(", ")
+		}
+		WriteExpr(buf, x)
+	}
+}
diff --git a/src/go/types/testdata/contracts.go2 b/src/go/types/testdata/contracts.go2
index 28af96227e..4973a9ee85 100644
--- a/src/go/types/testdata/contracts.go2
+++ b/src/go/types/testdata/contracts.go2
@@ -161,3 +161,25 @@ func _(type F, C FloatComplex)(c C) {
         var fim F = F(im)
         c = C(complex(fre, fim))
 }
+
+// --------------------------------------------------------------------------------------
+// Parameterized interfaces as contracts
+
+type Int interface {
+        type int8, int16, int32, int64, int
+        type uint8, uint16, uint32, uint64, uint
+}
+
+type Adder(type T) interface {
+        Add(T) T
+}
+
+func adderSum(type T Adder)(data []T) T {
+        var s T
+        for _, x := range data {
+                // TODO(gri) make this work
+                // s = s.Add(x)
+                _ = x
+        }
+        return s
+}
diff --git a/src/go/types/typestring.go b/src/go/types/typestring.go
index 4ad4bf8ffe..4b27045282 100644
--- a/src/go/types/typestring.go
+++ b/src/go/types/typestring.go
@@ -201,11 +201,14 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 			if !empty && len(t.types) > 0 {
 				buf.WriteString("; ")
 			}
-			for i, typ := range t.types {
-				if i > 0 {
-					buf.WriteString(", ")
+			if len(t.types) > 0 {
+				buf.WriteString("type ")
+				for i, typ := range t.types {
+					if i > 0 {
+						buf.WriteString(", ")
+					}
+					writeType(buf, typ, qf, visited)
 				}
-				writeType(buf, typ, qf, visited)
 				empty = false
 			}
 			if !empty && len(t.embeddeds) > 0 {
diff --git a/src/go/types/typexpr.go b/src/go/types/typexpr.go
index 7262bdc364..e22966fc47 100644
--- a/src/go/types/typexpr.go
+++ b/src/go/types/typexpr.go
@@ -623,6 +623,9 @@ func (check *Checker) interfaceType(ityp *Interface, iface *ast.InterfaceType, d
 		}
 	}
 
+	// type constraints
+	ityp.types = check.collectTypeConstraints(iface.Pos(), ityp.types, iface.Types)
+
 	if len(ityp.methods) == 0 && len(ityp.embeddeds) == 0 {
 		// empty interface
 		ityp.allMethods = markComplete