Minor improvements, add some experimental type class logic and refactor diagnostics

src/checker.ts

@@ -1,3 +1,7 @@
+
+// TODO support rigid vs free variables
+//      https://www.reddit.com/r/haskell/comments/d4v83/comment/c0xmc3r/
+
 import {
   ClassDeclaration,
   EnumDeclaration,
@@ -23,14 +27,14 @@ import {
   describeType,
   BindingNotFoundDiagnostic,
   Diagnostics,
-  FieldMissingDiagnostic,
+  FieldNotFoundDiagnostic,
-  UnificationFailedDiagnostic,
+  TypeMismatchDiagnostic,
   KindMismatchDiagnostic,
   ModuleNotFoundDiagnostic,
   TypeclassNotFoundDiagnostic,
-  FieldTypeMismatchDiagnostic,
+  TypeclassDeclaredTwiceDiagnostic,
 } from "./diagnostics";
-import { assert, assertNever, first, isEmpty, last, MultiMap } from "./util";
+import { assert, isDebug, assertNever, first, isEmpty, last, MultiMap } from "./util";
 import { Analyser } from "./analysis";
 
 const MAX_TYPE_ERROR_COUNT = 5;
@@ -440,6 +444,49 @@ export type Type
   | TPresent
   | TAbsent
 
+export class Qual {
+
+  public constructor(
+    public preds: Pred[],
+    public type: Type,
+  ) {
+
+  }
+
+  public substitute(sub: TVSub): Qual {
+    return new Qual(
+      this.preds.map(pred => pred.substitute(sub)),
+      this.type.substitute(sub),
+    );
+  }
+
+  public *getTypeVars() {
+    for (const pred of this.preds) {
+      yield* pred.type.getTypeVars();
+    }
+    yield* this.type.getTypeVars();
+  }
+
+}
+
+class IsInPred {
+
+  public constructor(
+    public id: string,
+    public type: Type,
+  ) {
+
+  }
+
+  public substitute(sub: TVSub): Pred {
+    return new IsInPred(this.id, this.type.substitute(sub));
+
+  }
+
+}
+
+type Pred = IsInPred;
+
 export const enum KindType {
   Star,
   Arrow,
@@ -552,6 +599,14 @@ class TVSet {
 
   private mapping = new Map<number, TVar>();
 
+  public constructor(iterable?: Iterable<TVar>) {
+    if (iterable !== undefined) {
+      for (const tv of iterable) {
+        this.add(tv);
+      }
+    }
+  }
+
   public add(tv: TVar): void {
     this.mapping.set(tv.id, tv);
   }
@@ -573,6 +628,10 @@ class TVSet {
     this.mapping.delete(tv.id);
   }
 
+  public get size(): number {
+    return this.mapping.size;
+  }
+
   public [Symbol.iterator](): Iterator<TVar> {
     return this.mapping.values();
   }
@@ -690,10 +749,6 @@ class CMany extends ConstraintBase {
 
 }
 
-interface InstanceRef {
-  node: InstanceDeclaration | null;
-}
-
 type Constraint
   = CEqual
   | CMany
@@ -706,12 +761,19 @@ abstract class SchemeBase {
 
 class Forall extends SchemeBase {
 
+  public typeVars: TVSet;
+
   public constructor(
-    public typeVars: Iterable<TVar>,
+    typeVars: Iterable<TVar>,
     public constraints: Iterable<Constraint>,
     public type: Type,
   ) {
     super();
+    if (typeVars instanceof TVSet) {
+      this.typeVars = typeVars;
+    } else { 
+      this.typeVars = new TVSet(typeVars);
+    }
   }
 
 }
@@ -726,6 +788,23 @@ type NodeWithReference
   | ReferenceExpression
   | ReferenceTypeExpression
 
+function validateScheme(scheme: Scheme): void {
+  const isMonoVar = scheme.type.kind === TypeKind.Var && scheme.typeVars.size === 0;
+  if (!isMonoVar) {
+    const tvs = new TVSet(scheme.type.getTypeVars())
+    for (const tv of tvs) {
+      if (!scheme.typeVars.has(tv)) {
+        throw new Error(`Type variable ${describeType(tv)} is free because does not appear in the scheme's type variable list`);
+      }
+    }
+    for (const tv of scheme.typeVars) {
+      if (!tvs.has(tv)) {
+        throw new Error(`Polymorphic type variable ${describeType(tv)} does not occur anywhere in scheme's type ${describeType(scheme.type)}`);
+      }
+    }
+  }
+}
+
 class TypeEnv {
 
   private mapping = new MultiMap<string, [Symkind, Scheme]>();
@@ -735,6 +814,9 @@ class TypeEnv {
   }
 
   public add(name: string, scheme: Scheme, kind: Symkind): void {
+    if (isDebug) {
+      validateScheme(scheme);
+    }
     this.mapping.add(name, [kind, scheme]);
   }
 
@@ -819,6 +901,10 @@ export class Checker {
 
   private contexts: InferContext[] = [];
 
+  private classDecls = new Map<string, ClassDeclaration>();
+  private globalKindEnv = new KindEnv();
+  private globalTypeEnv = new TypeEnv();
+
   private solution = new TVSub();
   private kindSolution = new KVSub();
 
@@ -827,6 +913,26 @@ export class Checker {
     private diagnostics: Diagnostics
   ) {
 
+    this.globalKindEnv.set('Int', new KType());
+    this.globalKindEnv.set('String', new KType());
+    this.globalKindEnv.set('Bool', new KType());
+
+    const a = new TVar(this.nextTypeVarId++);
+    const b = new TVar(this.nextTypeVarId++);
+
+    this.globalTypeEnv.add('$', new Forall([ a, b ], [], new TArrow(new TArrow(new TArrow(a, b), a), b)), Symkind.Var);
+    this.globalTypeEnv.add('String', new Forall([], [], this.stringType), Symkind.Type);
+    this.globalTypeEnv.add('Int', new Forall([], [], this.intType), Symkind.Type);
+    this.globalTypeEnv.add('Bool', new Forall([], [], this.boolType), Symkind.Type);
+    this.globalTypeEnv.add('True', new Forall([], [], this.boolType), Symkind.Var);
+    this.globalTypeEnv.add('False', new Forall([], [], this.boolType), Symkind.Var);
+    this.globalTypeEnv.add('+', new Forall([], [], TArrow.build([ this.intType, this.intType ], this.intType)), Symkind.Var);
+    this.globalTypeEnv.add('-', new Forall([], [], TArrow.build([ this.intType, this.intType ], this.intType)), Symkind.Var);
+    this.globalTypeEnv.add('*', new Forall([], [], TArrow.build([ this.intType, this.intType ], this.intType)), Symkind.Var);
+    this.globalTypeEnv.add('/', new Forall([], [], TArrow.build([ this.intType, this.intType ], this.intType)), Symkind.Var);
+    this.globalTypeEnv.add('==', new Forall([ a ], [], TArrow.build([ a, a ], this.boolType)), Symkind.Var);
+    this.globalTypeEnv.add('not', new Forall([], [], new TArrow(this.boolType, this.boolType)), Symkind.Var);
+
   }
 
   public getIntType(): Type {
@@ -874,7 +980,7 @@ export class Checker {
       let maxIndex = 0;
       let currUp = node.getEnclosingModule();
       outer: for (;;) {
-        let currDown: SourceFile | ModuleDeclaration = currUp;
+        let currDown = currUp;
         for (let i = 0; i < modulePath.length; i++) {
           const moduleName = modulePath[i];
           const nextDown = currDown.resolveModule(moduleName.text);
@@ -939,7 +1045,7 @@ export class Checker {
       let maxIndex = 0;
       let currUp = node.getEnclosingModule();
       outer: for (;;) {
-        let currDown: SourceFile | ModuleDeclaration = currUp;
+        let currDown = currUp;
         for (let i = 0; i < modulePath.length; i++) {
           const moduleName = modulePath[i];
           const nextDown = currDown.resolveModule(moduleName.text);
@@ -1000,7 +1106,8 @@ export class Checker {
 
   private createSubstitution(scheme: Scheme): TVSub {
     const sub = new TVSub();
-    for (const tv of scheme.typeVars) {
+  const tvs = [...scheme.typeVars]
+    for (const tv of tvs) {
       sub.set(tv, this.createTypeVar());
     }
     return sub;
@@ -1211,14 +1318,14 @@ export class Checker {
       case SyntaxKind.ClassDeclaration:
       case SyntaxKind.InstanceDeclaration:
       {
-        if (node.constraints !== null) {
+        if (node.constraintClause !== null) {
-          for (const constraint of node.constraints.constraints) {
+          for (const constraint of node.constraintClause.constraints) {
             for (const typeExpr of constraint.types) {
               this.unifyKind(this.inferKindFromTypeExpression(typeExpr, env), new KType(), typeExpr);
             }
           }
         }
-        for (const typeExpr of node.constraint.types) {
+        for (const typeExpr of node.types) {
           this.unifyKind(this.inferKindFromTypeExpression(typeExpr, env), new KType(), typeExpr);
         }
         for (const element of node.elements) {
@@ -1374,9 +1481,9 @@ export class Checker {
 
       case SyntaxKind.InstanceDeclaration:
       {
-        const cls = node.getScope().lookup(node.constraint.name.text, Symkind.Typeclass) as ClassDeclaration | null;
+        const cls = node.getScope().lookup(node.name.text, Symkind.Typeclass) as ClassDeclaration | null;
         if (cls === null) {
-          this.diagnostics.add(new TypeclassNotFoundDiagnostic(node.constraint.name.text, node.constraint.name));
+          this.diagnostics.add(new TypeclassNotFoundDiagnostic(node.name));
         }
         for (const element of node.elements) {
           this.infer(element);
@@ -1695,7 +1802,7 @@ export class Checker {
 
         case SyntaxKind.TupleTypeExpression:
         {
-          type = new TTuple(node.elements.map(el => this.inferTypeExpression(el)), node);
+          type = new TTuple(node.elements.map(el => this.inferTypeExpression(el, introduceTypeVars)), node);
           break;
         }
 
@@ -1708,7 +1815,7 @@ export class Checker {
           const scheme = this.lookup(node.name, Symkind.Type);
           if (scheme === null) {
             if (!introduceTypeVars) {
-              // this.diagnostics.add(new BindingNotFoudDiagnostic(node.name.text, node.name));
+              this.diagnostics.add(new BindingNotFoundDiagnostic([], node.name.text, node.name));
             }
             type = this.createTypeVar();
             this.addBinding(node.name.text, new Forall([], [], type), Symkind.Type);
@@ -1873,11 +1980,37 @@ export class Checker {
         break;
       }
 
-      case SyntaxKind.InstanceDeclaration:
       case SyntaxKind.ClassDeclaration:
       {
+        const other = this.classDecls.get(node.name.text);
+        if (other !== undefined) {
+          this.diagnostics.add(new TypeclassDeclaredTwiceDiagnostic(node.name, other));
+        } else {
+          if (node.constraintClause !== null) {
+            for (const constraint of node.constraintClause.constraints) {
+              if (!this.classDecls.has(constraint.name.text)) {
+                this.diagnostics.add(new TypeclassNotFoundDiagnostic(constraint.name));
+              }
+            }
+          }
+          this.classDecls.set(node.name.text, node);
+        }
+        const env = node.typeEnv = new TypeEnv(parentEnv);
+        for (const tv of node.types) {
+          assert(tv.kind === SyntaxKind.VarTypeExpression);
+          env.add(tv.name.text, new Forall([], [], this.createTypeVar(tv)), Symkind.Type);
+        }
         for (const element of node.elements) {
-          this.initialize(element, parentEnv);
+          this.initialize(element, env);
+        }
+        break;
+      }
+
+      case SyntaxKind.InstanceDeclaration:
+      {
+        const env = node.typeEnv = new TypeEnv(parentEnv);
+        for (const element of node.elements) {
+          this.initialize(element, env);
         }
         break;
       }
@@ -1911,41 +2044,44 @@ export class Checker {
         }
         this.pushContext(context);
         const kindArgs = [];
-        for (const varExpr of node.varExps) {
+        for (const name of node.varExps) {
           const kindArg = this.createTypeVar();
-          env.add(varExpr.text, new Forall([], [], kindArg), Symkind.Type);
+          env.add(name.text, new Forall([], [], kindArg), Symkind.Type);
           kindArgs.push(kindArg);
         }
+        const type = TApp.build(new TNominal(node, node), kindArgs);
+        parentEnv.add(node.name.text, new Forall(typeVars, constraints, type), Symkind.Type);
         let elementTypes: Type[] = [];
-        const type = new TNominal(node, node);
         if (node.members !== null) {
           for (const member of node.members) {
-            let ctorType;
+            let ctorType, elementType;
             switch (member.kind) {
               case SyntaxKind.EnumDeclarationTupleElement:
               {
-                const argTypes = member.elements.map(el => this.inferTypeExpression(el));
+                const argTypes = member.elements.map(el => this.inferTypeExpression(el, false));
-                ctorType = TArrow.build(argTypes, TApp.build(type, kindArgs), member);
+                elementType = new TTuple(argTypes, member);
+                ctorType = TArrow.build(argTypes, type, member);
                 break;
               }
               case SyntaxKind.EnumDeclarationStructElement:
               {
-                ctorType = new TNil(member);
+                elementType = new TNil(member);
                 for (const field of member.fields) {
-                  ctorType = new TField(field.name.text, new TPresent(this.inferTypeExpression(field.typeExpr)), ctorType, member);
+                  elementType = new TField(field.name.text, new TPresent(this.inferTypeExpression(field.typeExpr, false)), elementType, member);
                 }
-                ctorType = new TArrow(TField.sort(ctorType), TApp.build(type, kindArgs));
+                elementType = TField.sort(elementType);
+                ctorType = new TArrow(elementType, type);
                 break;
               }
               default:
                 throw new Error(`Unexpected ${member}`);
             }
+            // FIXME `typeVars` may contain too much irrelevant type variables
             parentEnv.add(member.name.text, new Forall(typeVars, constraints, ctorType), Symkind.Var);
-            elementTypes.push(ctorType);
+            elementTypes.push(elementType);
           }
         }
         this.popContext(context);
-        parentEnv.add(node.name.text, new Forall(typeVars, constraints, type), Symkind.Type);
         break;
       }
 
@@ -2013,37 +2149,17 @@ export class Checker {
 
   public check(node: SourceFile): void {
 
-    const kenv = new KindEnv();
+    const kenv = new KindEnv(this.globalKindEnv);
-    kenv.set('Int', new KType());
+    this.forwardDeclareKind(node, kenv);
-    kenv.set('String', new KType());
+    this.inferKind(node, kenv);
-    kenv.set('Bool', new KType());
-    const skenv = new KindEnv(kenv);
-    this.forwardDeclareKind(node, skenv);
-    this.inferKind(node, skenv);
 
     const typeVars = new TVSet();
     const constraints = new ConstraintSet();
-    const env = new TypeEnv();
+    const env = new TypeEnv(this.globalTypeEnv);
     const context: InferContext = { typeVars, constraints, env, returnType: null };
 
     this.pushContext(context);
 
-    const a = this.createTypeVar();
-    const b = this.createTypeVar();
-
-    env.add('$', new Forall([ a, b ], [], new TArrow(new TArrow(new TArrow(a, b), a), b)), Symkind.Var);
-    env.add('String', new Forall([], [], this.stringType), Symkind.Type);
-    env.add('Int', new Forall([], [], this.intType), Symkind.Type);
-    env.add('Bool', new Forall([], [], this.boolType), Symkind.Type);
-    env.add('True', new Forall([], [], this.boolType), Symkind.Var);
-    env.add('False', new Forall([], [], this.boolType), Symkind.Var);
-    env.add('+', new Forall([], [], TArrow.build([ this.intType, this.intType ], this.intType)), Symkind.Var);
-    env.add('-', new Forall([], [], TArrow.build([ this.intType, this.intType ], this.intType)), Symkind.Var);
-    env.add('*', new Forall([], [], TArrow.build([ this.intType, this.intType ], this.intType)), Symkind.Var);
-    env.add('/', new Forall([], [], TArrow.build([ this.intType, this.intType ], this.intType)), Symkind.Var);
-    env.add('==', new Forall([ a ], [], TArrow.build([ a, a ], this.boolType)), Symkind.Var);
-    env.add('not', new Forall([], [], new TArrow(this.boolType, this.boolType)), Symkind.Var);
-
     this.initialize(node, env);
 
     this.pushContext({
@@ -2141,7 +2257,6 @@ export class Checker {
     }
 
     const visitElements = (elements: Syntax[]) => {
-
       for (const element of elements) {
         if (element.kind === SyntaxKind.LetDeclaration
             && isFunctionDeclarationLike(element)) {
@@ -2152,14 +2267,21 @@ export class Checker {
             this.instantiate(scheme, null);
           }
         } else {
+          const elementHasTypeEnv = hasTypeEnv(element);
+          if (elementHasTypeEnv) {
+            this.pushContext({ ...this.getContext(), env: element.typeEnv! });
+          }
           this.infer(element);
+          if(elementHasTypeEnv) {
+            this.contexts.pop();
+          }
         }
       }
     }
 
     for (const nodes of sccs) {
 
-      if (nodes.some(n => n.kind === SyntaxKind.SourceFile)) {
+      if (nodes[0].kind === SyntaxKind.SourceFile) {
         assert(nodes.length === 1);
         continue;
       }
@@ -2222,23 +2344,29 @@ export class Checker {
 
   }
   
+  private lookupClass(name: string): ClassDeclaration | null {
+    return this.classDecls.get(name) ?? null;
+  }
+
   private *findInstanceContext(type: TCon, clazz: ClassDeclaration): Iterable<ClassDeclaration[]> {
     for (const instance of clazz.getInstances()) {
-      assert(instance.constraint.types.length === 1);
+      assert(instance.types.length === 1);
-      const instTy0 = instance.constraint.types[0];
+      const instTy0 = instance.types[0];
-      if (instTy0.kind === SyntaxKind.AppTypeExpression
+      if ((instTy0.kind === SyntaxKind.AppTypeExpression
           && instTy0.operator.kind === SyntaxKind.ReferenceTypeExpression
-          && instTy0.operator.name.text === type.displayName) {
+          && instTy0.operator.name.text === type.displayName)
-        if (instance.constraints === null) {
+         || (instTy0.kind === SyntaxKind.ReferenceTypeExpression
+          && instTy0.name.text === type.displayName)) {
+        if (instance.constraintClause === null) {
           return;
         }
         for (const argType of type.argTypes) {
           const classes = [];
-          for (const constraint of instance.constraints.constraints) {
+          for (const constraint of instance.constraintClause.constraints) {
             assert(constraint.types.length === 1);
-            const classDecl = this.lookupClass(constraint.name);
+            const classDecl = this.lookupClass(constraint.name.text);
             if (classDecl === null) {
-              this.diagnostics.add(new TypeclassNotFoundDiagnostic(constraint.name.text, constraint.name));
+              this.diagnostics.add(new TypeclassNotFoundDiagnostic(constraint.name));
             } else {
               classes.push(classDecl);
             }
@@ -2296,7 +2424,7 @@ export class Checker {
               assert(right.kind === TypeKind.Present);
               const fieldName = path[path.length-1];
               this.diagnostics.add(
-                new FieldMissingDiagnostic(fieldName, left.node, right.type.node, constraint.firstNode)
+                new FieldNotFoundDiagnostic(fieldName, left.node, right.type.node, constraint.firstNode)
               );
               return false;
             }
@@ -2305,6 +2433,13 @@ export class Checker {
             return unify(left.type, right.type);
           }
 
+          const unifyPred = (left: Pred, right: Pred) => {
+            if (left.id === right.id) {
+              return unify(left.type, right.type);
+            }
+            throw new Error(`Classes do not match and no diagnostic defined`);
+          }
+
           const unify = (left: Type, right: Type): boolean => {
 
             left = find(left);
@@ -2342,6 +2477,7 @@ export class Checker {
                     propagateClassTCon(constraint, type);
                   }
                 } else {
+                  //assert(false);
                   //this.diagnostics.add(new );
                 }
               }
@@ -2475,7 +2611,7 @@ export class Checker {
             }
 
             this.diagnostics.add(
-              new UnificationFailedDiagnostic(
+              new TypeMismatchDiagnostic(
                 left.substitute(solution),
                 right.substitute(solution),
                 [...constraint.getNodes()],
@@ -2512,3 +2648,18 @@ function getVariadicMember(node: StructPattern) {
   return null;
 }
 
+type HasTypeEnv
+  = ClassDeclaration
+  | InstanceDeclaration
+  | LetDeclaration
+  | ModuleDeclaration
+  | SourceFile
+
+function hasTypeEnv(node: Syntax): node is HasTypeEnv {
+  return node.kind === SyntaxKind.ClassDeclaration
+      || node.kind === SyntaxKind.InstanceDeclaration
+      || node.kind === SyntaxKind.LetDeclaration
+      || node.kind === SyntaxKind.ModuleDeclaration
+      || node.kind === SyntaxKind.SourceFile
+}
+

src/cst.ts

@@ -2,7 +2,7 @@
 import type stream from "stream";
 import path from "path"
 
-import { assert, IndentWriter, JSONObject, JSONValue } from "./util";
+import { assert, implementationLimitation, IndentWriter, JSONObject, JSONValue } from "./util";
 import { isNodeWithScope, Scope } from "./scope"
 import { InferContext, Kind, KindEnv, Scheme, Type, TypeEnv } from "./checker"
 import { Emitter } from "./emitter";
@@ -2828,22 +2828,33 @@ export class ClassDeclaration extends SyntaxBase {
 
   public readonly kind = SyntaxKind.ClassDeclaration;
 
+  public typeEnv?: TypeEnv;
+
   public constructor(
     public pubKeyword: PubKeyword | null,
     public classKeyword: ClassKeyword,
-    public constraints: ClassConstraintClause | null,
+    public constraintClause: ClassConstraintClause | null,
-    public constraint: ClassConstraint,
+    public name: IdentifierAlt,
+    public types: VarTypeExpression[],
     public elements: ClassDeclarationElement[],
   ) {
     super();
   }
 
+  public *getSupers(): Iterable<IdentifierAlt> {
+    if (this.constraintClause !== null) {
+      for (const constraint of this.constraintClause.constraints) {
+        yield constraint.name;
+      }
+    }
+  }
+
   public lookup(element: InstanceDeclarationElement): ClassDeclarationElement | null {
 
     switch (element.kind) {
 
       case SyntaxKind.LetDeclaration:
-        assert(element.pattern.kind === SyntaxKind.NamedPattern);
+        implementationLimitation(element.pattern.kind === SyntaxKind.NamedPattern);
         for (const other of this.elements) {
           if (other.kind === SyntaxKind.LetDeclaration
               && other.pattern.kind === SyntaxKind.NamedPattern
@@ -2875,7 +2886,7 @@ export class ClassDeclaration extends SyntaxBase {
         curr = curr.parent!;
       }
       for (const element of getElements(curr)) {
-        if (element.kind === SyntaxKind.InstanceDeclaration && element.constraint.name === this.constraint.name) {
+        if (element.kind === SyntaxKind.InstanceDeclaration && element.name.text === this.name.text) {
           yield element;
         }
       }
@@ -2886,8 +2897,9 @@ export class ClassDeclaration extends SyntaxBase {
     return new ClassDeclaration(
       this.pubKeyword !== null ? this.pubKeyword.clone() : null,
       this.classKeyword.clone(),
-      this.constraints !== null ? this.constraints.clone() : null,
+      this.constraintClause !== null ? this.constraintClause.clone() : null,
-      this.constraint.clone(),
+      this.name.clone(),
+      this.types.map(t => t.clone()),
       this.elements.map(element => element.clone()),
     );
   }
@@ -2903,7 +2915,10 @@ export class ClassDeclaration extends SyntaxBase {
     if (this.elements.length > 0) {
       return this.elements[this.elements.length-1].getLastToken();
     }
-    return this.constraint.getLastToken();
+    if (this.types.length > 0) {
+      return this.types[this.types.length-1].getLastToken();
+    }
+    return this.name;
   }
 
 }
@@ -2916,11 +2931,14 @@ export class InstanceDeclaration extends SyntaxBase {
 
   public readonly kind = SyntaxKind.InstanceDeclaration;
 
+  public typeEnv?: TypeEnv;
+
   public constructor(
     public pubKeyword: PubKeyword | null,
     public classKeyword: InstanceKeyword,
-    public constraints: ClassConstraintClause | null,
+    public constraintClause: ClassConstraintClause | null,
-    public constraint: ClassConstraint,
+    public name: IdentifierAlt,
+    public types: TypeExpression[],
     public elements: InstanceDeclarationElement[],
   ) {
     super();
@@ -2930,8 +2948,9 @@ export class InstanceDeclaration extends SyntaxBase {
     return new InstanceDeclaration(
       this.pubKeyword !== null ? this.pubKeyword.clone() : null,
       this.classKeyword.clone(),
-      this.constraints !== null ? this.constraints.clone() : null,
+      this.constraintClause !== null ? this.constraintClause.clone() : null,
-      this.constraint.clone(),
+      this.name.clone(),
+      this.types.map(t => t.clone()),
       this.elements.map(element => element.clone()),
     );
   }
@@ -2947,7 +2966,10 @@ export class InstanceDeclaration extends SyntaxBase {
     if (this.elements.length > 0) {
       return this.elements[this.elements.length-1].getLastToken();
     }
-    return this.constraint.getLastToken();
+    if (this.types.length > 0) {
+      return this.types[this.types.length-1].getLastToken();
+    }
+    return this.name;
   }
 
 }
@@ -3095,7 +3117,7 @@ export function vistEachChild<T extends Syntax>(node: T, proc: (node: Syntax) =>
   if (!changed) {
     return node;
   }
-  return new node.constructor(...newArgs);
+  return new (node as any).constructor(...newArgs);
 }
 
 export function canHaveInstanceDeclaration(node: Syntax): boolean {

src/diagnostics.ts

@@ -1,7 +1,7 @@
 
-import { TypeKind, type Type, type TArrow, TRecord, Kind, KindType } from "./checker";
+import { TypeKind, type Type, Kind, KindType } from "./checker";
-import { Syntax, SyntaxKind, TextFile, TextPosition, TextRange, Token } from "./cst";
+import { ClassConstraint, ClassDeclaration, IdentifierAlt, InstanceDeclaration, Syntax, SyntaxKind, TextFile, TextPosition, TextRange, Token } from "./cst";
-import { countDigits, IndentWriter } from "./util";
+import { assertNever, countDigits, IndentWriter } from "./util";
 
 const ANSI_RESET = "\u001b[0m"
 const ANSI_BOLD = "\u001b[1m"
@@ -35,9 +35,28 @@ const enum Level {
   Fatal,
 }
 
-export class UnexpectedCharDiagnostic {
+const enum DiagnosticKind {
+  UnexpectedChar,
+  UnexpectedToken,
+  KindMismatch,
+  TypeMismatch,
+  TypeclassNotFound,
+  TypeclassDecaredTwice,
+  BindingNotFound,
+  ModuleNotFound,
+  FieldNotFound,
+}
 
-  public readonly level = Level.Error;
+interface DiagnosticBase {
+  level: Level;
+  readonly kind: DiagnosticKind;
+}
+
+export class UnexpectedCharDiagnostic implements DiagnosticBase {
+
+  public readonly kind = DiagnosticKind.UnexpectedChar;
+
+  public level = Level.Error;
 
   public constructor(
     public file: TextFile,
@@ -47,12 +66,322 @@ export class UnexpectedCharDiagnostic {
 
   }
 
-  public format(out: IndentWriter): void {
+}
-    const endPos = this.position.clone();
+
-    endPos.advance(this.actual);
+
-    out.write(ANSI_FG_RED + ANSI_BOLD + 'error: ' + ANSI_RESET);
+export class UnexpectedTokenDiagnostic implements DiagnosticBase {
-    out.write(`unexpected character sequence '${this.actual}'.\n\n`);
+
-    out.write(printExcerpt(this.file, new TextRange(this.position, endPos)) + '\n');
+  public readonly kind = DiagnosticKind.UnexpectedToken;
+
+  public level = Level.Error;
+
+  public constructor(
+    public file: TextFile,
+    public actual: Token,
+    public expected: SyntaxKind[],
+  ) {
+
+  }
+
+}
+
+export class TypeclassDeclaredTwiceDiagnostic implements DiagnosticBase {
+
+  public readonly kind = DiagnosticKind.TypeclassDecaredTwice;
+  
+  public level = Level.Error;
+
+  public constructor(
+    public name: IdentifierAlt,
+    public origDecl: ClassDeclaration,
+  ) {
+
+  } 
+
+}
+
+export class TypeclassNotFoundDiagnostic implements DiagnosticBase {
+
+  public readonly kind = DiagnosticKind.TypeclassNotFound;
+
+  public level = Level.Error;
+
+  public constructor(
+    public name: IdentifierAlt,
+    public origin: InstanceDeclaration | ClassConstraint | null = null,
+  ) {
+
+  }
+
+}
+
+export class BindingNotFoundDiagnostic implements DiagnosticBase {
+
+  public readonly kind = DiagnosticKind.BindingNotFound;
+
+  public level = Level.Error;
+
+  public constructor(
+    public modulePath: string[],
+    public name: string,
+    public node: Syntax,
+  ) {
+
+  }
+
+}
+
+export class TypeMismatchDiagnostic implements DiagnosticBase {
+
+  public readonly kind = DiagnosticKind.TypeMismatch;
+
+  public level = Level.Error;
+
+  public constructor(
+    public left: Type,
+    public right: Type,
+    public trace: Syntax[],
+    public fieldPath: string[],
+  ) {
+
+  }
+
+}
+
+export class FieldNotFoundDiagnostic implements DiagnosticBase {
+
+  public readonly kind = DiagnosticKind.FieldNotFound;
+
+  public level = Level.Error;
+
+  public constructor(
+    public fieldName: string,
+    public missing: Syntax | null,
+    public present: Syntax | null,
+    public cause: Syntax | null = null,
+  ) {
+
+  }
+
+}
+
+export class KindMismatchDiagnostic implements DiagnosticBase {
+
+  public readonly kind = DiagnosticKind.KindMismatch;
+
+  public level = Level.Error;
+
+  public constructor(
+    public left: Kind,
+    public right: Kind,
+    public origin: Syntax | null,
+  ) {
+  
+  }
+
+}
+
+export class ModuleNotFoundDiagnostic implements DiagnosticBase {
+
+  public readonly kind = DiagnosticKind.ModuleNotFound;
+
+  public level = Level.Error;
+
+  public constructor(
+    public modulePath: string[],
+    public node: Syntax,
+  ) {
+
+  }
+
+}
+
+export type Diagnostic
+  = UnexpectedCharDiagnostic
+  | TypeclassNotFoundDiagnostic
+  | TypeclassDeclaredTwiceDiagnostic
+  | BindingNotFoundDiagnostic
+  | TypeMismatchDiagnostic
+  | UnexpectedTokenDiagnostic
+  | FieldNotFoundDiagnostic
+  | KindMismatchDiagnostic
+  | ModuleNotFoundDiagnostic
+
+export interface Diagnostics {
+  readonly hasError: boolean;
+  readonly hasFatal: boolean;
+  add(diagnostic: Diagnostic): void;
+}
+
+export class DiagnosticStore implements Diagnostics {
+
+  private storage: Diagnostic[] = [];
+  
+  public hasError = false;
+  public hasFatal = false;
+
+  public add(diagnostic: Diagnostic): void {
+    this.storage.push(diagnostic);
+    if (diagnostic.level >= Level.Error) {
+      this.hasError = true;
+    }
+    if (diagnostic.level >= Level.Fatal) {
+      this.hasFatal = true;
+    }
+  }
+
+  public getDiagnostics(): Iterable<Diagnostic> {
+    return this.storage;
+  }
+
+}
+
+export class ConsoleDiagnostics implements Diagnostics {
+  
+  private writer = new IndentWriter(process.stderr);
+
+  public hasError = false;
+  public hasFatal = false;
+
+  public add(diagnostic: Diagnostic): void {
+
+    if (diagnostic.level >= Level.Error) {
+      this.hasError = true;
+    }
+    if (diagnostic.level >= Level.Fatal) {
+      this.hasFatal = true;
+    }
+
+    switch (diagnostic.level) {
+      case Level.Fatal: 
+        this.writer.write(ANSI_FG_RED + ANSI_BOLD + 'fatal: ' + ANSI_RESET);
+        break;
+      case Level.Error:
+        this.writer.write(ANSI_FG_RED + ANSI_BOLD + 'error: ' + ANSI_RESET);
+        break;
+      case Level.Warning:
+        this.writer.write(ANSI_FG_RED + ANSI_BOLD + 'warning: ' + ANSI_RESET);
+        break;
+      case Level.Info:
+        this.writer.write(ANSI_FG_YELLOW + ANSI_BOLD + 'info: ' + ANSI_RESET);
+        break;
+      case Level.Verbose:
+        this.writer.write(ANSI_FG_CYAN + ANSI_BOLD + 'verbose: ' + ANSI_RESET);
+        break;
+    }
+
+    switch (diagnostic.kind) {
+
+      case DiagnosticKind.UnexpectedChar:
+        const endPos = diagnostic.position.clone();
+        endPos.advance(diagnostic.actual);
+        this.writer.write(`unexpected character sequence '${diagnostic.actual}'.\n\n`);
+        this.writer.write(printExcerpt(diagnostic.file, new TextRange(diagnostic.position, endPos)) + '\n');
+        break;
+
+      case DiagnosticKind.UnexpectedToken:
+        this.writer.write(`expected ${describeExpected(diagnostic.expected)} but got ${describeActual(diagnostic.actual)}\n\n`);
+        this.writer.write(printExcerpt(diagnostic.file, diagnostic.actual.getRange()) + '\n');
+        break;
+
+      case DiagnosticKind.TypeclassDecaredTwice:
+        this.writer.write(`type class '${diagnostic.name.text}' was already declared somewhere else.\n\n`);
+        this.writer.write(ANSI_FG_YELLOW + ANSI_BOLD + 'info: ' + ANSI_RESET);
+        this.writer.write(`type class '${diagnostic.name.text}' is already declared here\n\n`);
+        this.writer.write(printNode(diagnostic.origDecl) + '\n');
+        break;
+
+      case DiagnosticKind.TypeclassNotFound:
+        this.writer.write(`the type class ${ANSI_FG_MAGENTA + diagnostic.name.text + ANSI_RESET} was not found.\n\n`);
+        this.writer.write(printNode(diagnostic.name) + '\n');
+        if (diagnostic.origin !== null) {
+          this.writer.indent();
+          this.writer.write(ANSI_FG_YELLOW + ANSI_BOLD + 'info: ' + ANSI_RESET);
+          this.writer.write(`${ANSI_FG_MAGENTA + diagnostic.name.text + ANSI_RESET} is required by ${ANSI_FG_MAGENTA + diagnostic.origin.name.text + ANSI_RESET}\n\n`);
+          this.writer.write(printNode(diagnostic.origin.name) + '\n');
+          this.writer.dedent();
+        }
+        break;
+
+      case DiagnosticKind.BindingNotFound:
+        this.writer.write(`binding '${diagnostic.name}' was not found`);
+        if (diagnostic.modulePath.length > 0) {
+          this.writer.write(` in module ${ANSI_FG_BLUE + diagnostic.modulePath.join('.') + ANSI_RESET}`);
+        }
+        this.writer.write(`.\n\n`);
+        this.writer.write(printNode(diagnostic.node) + '\n');
+        break;
+
+      case DiagnosticKind.TypeMismatch:
+        const leftNode = getFirstNodeInTypeChain(diagnostic.left);
+        const rightNode = getFirstNodeInTypeChain(diagnostic.right);
+        const node = diagnostic.trace[0];
+        this.writer.write(`unification of ` + ANSI_FG_GREEN + describeType(diagnostic.left) + ANSI_RESET);
+        this.writer.write(' and ' + ANSI_FG_GREEN + describeType(diagnostic.right) + ANSI_RESET + ' failed');
+        if (diagnostic.fieldPath.length > 0) {
+          this.writer.write(` in field '${diagnostic.fieldPath.join('.')}'`);
+        }
+        this.writer.write('.\n\n');
+        this.writer.write(printNode(node) + '\n');
+        for (let i = 1; i < diagnostic.trace.length; i++) {
+          const node = diagnostic.trace[i];
+          this.writer.write('  ... in an instantiation of the following expression\n\n');
+          this.writer.write(printNode(node, { indentation: i === 0 ? '  ' : '    ' }) + '\n');
+        }
+        if (leftNode !== null) {
+          this.writer.indent();
+          this.writer.write(ANSI_FG_YELLOW + ANSI_BOLD + `info: ` + ANSI_RESET);
+          this.writer.write(`type ` + ANSI_FG_GREEN + describeType(diagnostic.left) + ANSI_RESET + ` was inferred from diagnostic expression:\n\n`);
+          this.writer.write(printNode(leftNode) + '\n');
+          this.writer.dedent();
+        }
+        if (rightNode !== null) {
+          this.writer.indent();
+          this.writer.write(ANSI_FG_YELLOW + ANSI_BOLD + `info: ` + ANSI_RESET);
+          this.writer.write(`type ` + ANSI_FG_GREEN + describeType(diagnostic.right) + ANSI_RESET + ` was inferred from diagnostic expression:\n\n`);
+          this.writer.write(printNode(rightNode) + '\n');
+          this.writer.dedent();
+        }
+        break;
+
+      case DiagnosticKind.KindMismatch:
+        this.writer.write(`kind ${describeKind(diagnostic.left)} does not match with ${describeKind(diagnostic.right)}\n\n`);
+        if (diagnostic.origin !== null) {
+          this.writer.write(printNode(diagnostic.origin) + '\n');
+        }
+        break;
+
+      case DiagnosticKind.ModuleNotFound:
+        this.writer.write(`a module named ${ANSI_FG_BLUE + diagnostic.modulePath.join('.') + ANSI_RESET} was not found.\n\n`);
+        this.writer.write(printNode(diagnostic.node) + '\n');
+        break;
+
+      case DiagnosticKind.FieldNotFound:
+        this.writer.write(`field '${diagnostic.fieldName}' is required in one type but missing in another\n\n`);
+        this.writer.indent();
+        if (diagnostic.missing !== null) {
+          this.writer.write(ANSI_FG_YELLOW + ANSI_BOLD + 'info: ' + ANSI_RESET);
+          this.writer.write(`field '${diagnostic.fieldName}' is missing in diagnostic construct\n\n`);
+          this.writer.write(printNode(diagnostic.missing) + '\n');
+        }
+        if (diagnostic.present !== null) {
+          this.writer.write(ANSI_FG_YELLOW + ANSI_BOLD + 'info: ' + ANSI_RESET);
+          this.writer.write(`field '${diagnostic.fieldName}' is required in diagnostic construct\n\n`);
+          this.writer.write(printNode(diagnostic.present) + '\n');
+        }
+        if (diagnostic.cause !== null) {
+          this.writer.write(ANSI_FG_YELLOW + ANSI_BOLD + 'info: ' + ANSI_RESET);
+          this.writer.write(`because of a constraint on diagnostic node:\n\n`);
+          this.writer.write(printNode(diagnostic.cause) + '\n');
+        }
+        this.writer.dedent();
+        break;
+
+      default:
+        assertNever(diagnostic);
+
+    }
+
   }
 
 }
@@ -60,6 +389,9 @@ export class UnexpectedCharDiagnostic {
 const DESCRIPTIONS: Partial<Record<SyntaxKind, string>> = {
   [SyntaxKind.StringLiteral]: 'a string literal',
   [SyntaxKind.Identifier]: "an identifier",
+  [SyntaxKind.RArrow]: "'->'",
+  [SyntaxKind.RArrowAlt]: '"=>"',
+  [SyntaxKind.VBar]: "'|'",
   [SyntaxKind.Comma]: "','",
   [SyntaxKind.Colon]: "':'",
   [SyntaxKind.Integer]: "an integer",
@@ -86,8 +418,6 @@ const DESCRIPTIONS: Partial<Record<SyntaxKind, string>> = {
   [SyntaxKind.BlockEnd]: 'the end of an indented block',
   [SyntaxKind.LineFoldEnd]: 'the end of the current line-fold',
   [SyntaxKind.EndOfFile]: 'end-of-file',
-  [SyntaxKind.RArrowAlt]: '"=>"',
-  [SyntaxKind.VBar]: "'|'",
 }
 
 function describeSyntaxKind(kind: SyntaxKind): string {
@@ -126,70 +456,6 @@ function describeActual(token: Token): string {
   }
 }
 
-export class UnexpectedTokenDiagnostic {
-
-  public readonly level = Level.Error;
-
-  public constructor(
-    public file: TextFile,
-    public actual: Token,
-    public expected: SyntaxKind[],
-  ) {
-
-  }
-
-  public format(out: IndentWriter): void {
-    out.write(ANSI_FG_RED + ANSI_BOLD + 'fatal: ' + ANSI_RESET);
-    out.write(`expected ${describeExpected(this.expected)} but got ${describeActual(this.actual)}\n\n`);
-    out.write(printExcerpt(this.file, this.actual.getRange()) + '\n');
-  }
-
-}
-
-export class TypeclassNotFoundDiagnostic {
-
-  public readonly level = Level.Error;
-
-  public constructor(
-    public name: string,
-    public node: Syntax,
-  ) {
-
-
-  }
-
-  public format(out: IndentWriter): void {
-    out.write(ANSI_FG_RED + ANSI_BOLD + 'error: ' + ANSI_RESET); 
-    out.write(`could not implement type class because class '${this.name}' was not found.\n\n`);
-    out.write(printNode(this.node) + '\n');
-  }
-
-}
-
-export class BindingNotFoundDiagnostic {
-
-  public readonly level = Level.Error;
-
-  public constructor(
-    public modulePath: string[],
-    public name: string,
-    public node: Syntax,
-  ) {
-
-  }
-
-  public format(out: IndentWriter): void {
-    out.write(ANSI_FG_RED + ANSI_BOLD + 'error: ' + ANSI_RESET); 
-    out.write(`binding '${this.name}' was not found`);
-    if (this.modulePath.length > 0) {
-      out.write(` in module ${ANSI_FG_BLUE + this.modulePath.join('.') + ANSI_RESET}`);
-    }
-    out.write(`.\n\n`);
-    out.write(printNode(this.node) + '\n');
-  }
-
-}
-
 export function describeType(type: Type): string {
   switch (type.kind) {
     case TypeKind.Con:
@@ -267,196 +533,14 @@ function getFirstNodeInTypeChain(type: Type): Syntax | null {
   return curr.node;
 }
 
-export class UnificationFailedDiagnostic {
-
-  public readonly level = Level.Error;
-
-  public constructor(
-    public left: Type,
-    public right: Type,
-    public nodes: Syntax[],
-    public path: string[],
-  ) {
-
-  }
-
-  public format(out: IndentWriter): void {
-    const leftNode = getFirstNodeInTypeChain(this.left);
-    const rightNode = getFirstNodeInTypeChain(this.right);
-    const node = this.nodes[0];
-    out.write(ANSI_FG_RED + ANSI_BOLD + `error: ` + ANSI_RESET);
-    out.write(`unification of ` + ANSI_FG_GREEN + describeType(this.left) + ANSI_RESET);
-    out.write(' and ' + ANSI_FG_GREEN + describeType(this.right) + ANSI_RESET + ' failed');
-    if (this.path.length > 0) {
-      out.write(` in field '${this.path.join('.')}'`);
-    }
-    out.write('.\n\n');
-    out.write(printNode(node) + '\n');
-    for (let i = 1; i < this.nodes.length; i++) {
-      const node = this.nodes[i];
-      out.write('  ... in an instantiation of the following expression\n\n');
-      out.write(printNode(node, { indentation: i === 0 ? '  ' : '    ' }) + '\n');
-    }
-    if (leftNode !== null) {
-      out.indent();
-      out.write(ANSI_FG_YELLOW + ANSI_BOLD + `info: ` + ANSI_RESET);
-      out.write(`type ` + ANSI_FG_GREEN + describeType(this.left) + ANSI_RESET + ` was inferred from this expression:\n\n`);
-      out.write(printNode(leftNode) + '\n');
-      out.dedent();
-    }
-    if (rightNode !== null) {
-      out.indent();
-      out.write(ANSI_FG_YELLOW + ANSI_BOLD + `info: ` + ANSI_RESET);
-      out.write(`type ` + ANSI_FG_GREEN + describeType(this.right) + ANSI_RESET + ` was inferred from this expression:\n\n`);
-      out.write(printNode(rightNode) + '\n');
-      out.dedent();
-    }
-  }
-
-}
-
-export class FieldMissingDiagnostic {
-
-  public readonly level = Level.Error;
-
-  public constructor(
-    public fieldName: string,
-    public missing: Syntax | null,
-    public present: Syntax | null,
-    public cause: Syntax | null = null,
-  ) {
-
-  }
-
-  public format(out: IndentWriter): void {
-    out.write(ANSI_FG_RED + ANSI_BOLD + 'error: ' + ANSI_RESET);
-    out.write(`field '${this.fieldName}' is required in one type but missing in another\n\n`);
-    out.indent();
-    if (this.missing !== null) {
-      out.write(ANSI_FG_YELLOW + ANSI_BOLD + 'info: ' + ANSI_RESET);
-      out.write(`field '${this.fieldName}' is missing in this construct\n\n`);
-      out.write(printNode(this.missing) + '\n');
-    }
-    if (this.present !== null) {
-      out.write(ANSI_FG_YELLOW + ANSI_BOLD + 'info: ' + ANSI_RESET);
-      out.write(`field '${this.fieldName}' is required in this construct\n\n`);
-      out.write(printNode(this.present) + '\n');
-    }
-    if (this.cause !== null) {
-      out.write(ANSI_FG_YELLOW + ANSI_BOLD + 'info: ' + ANSI_RESET);
-      out.write(`because of a constraint on this node:\n\n`);
-      out.write(printNode(this.cause) + '\n');
-    }
-    out.dedent();
-  }
-
-}
-
-export class KindMismatchDiagnostic {
-
-  public readonly level = Level.Error;
-
-  public constructor(
-    public left: Kind,
-    public right: Kind,
-    public node: Syntax | null,
-  ) {
-  
-  }
-
-  public format(out: IndentWriter): void {
-    out.write(ANSI_FG_RED + ANSI_BOLD + 'error: ' + ANSI_RESET);
-    out.write(`kind ${describeKind(this.left)} does not match with ${describeKind(this.right)}\n\n`);
-    if (this.node !== null) {
-      out.write(printNode(this.node) + '\n');
-    }
-  }
-
-}
-
-export class ModuleNotFoundDiagnostic {
-
-  public readonly level = Level.Error;
-
-  public constructor(
-    public modulePath: string[],
-    public node: Syntax,
-  ) {
-
-  }
-
-  public format(out: IndentWriter): void {
-    out.write(ANSI_FG_RED + ANSI_BOLD + 'error: ' + ANSI_RESET);
-    out.write(`a module named ${ANSI_FG_BLUE + this.modulePath.join('.') + ANSI_RESET} was not found.\n\n`);
-    out.write(printNode(this.node) + '\n');
-  }
-
-}
-
-export type Diagnostic
-  = UnexpectedCharDiagnostic
-  | TypeclassNotFoundDiagnostic
-  | BindingNotFoundDiagnostic
-  | UnificationFailedDiagnostic
-  | UnexpectedTokenDiagnostic
-  | FieldMissingDiagnostic
-  | KindMismatchDiagnostic
-  | ModuleNotFoundDiagnostic
-
-export interface Diagnostics {
-  readonly hasError: boolean;
-  readonly hasFatal: boolean;
-  add(diagnostic: Diagnostic): void;
-}
-
-export class DiagnosticStore {
-
-  private storage: Diagnostic[] = [];
-  
-  public hasError = false;
-  public hasFatal = false;
-
-  public add(diagnostic: Diagnostic): void {
-    this.storage.push(diagnostic);
-    if (diagnostic.level >= Level.Error) {
-      this.hasError = true;
-    }
-    if (diagnostic.level >= Level.Fatal) {
-      this.hasFatal = true;
-    }
-  }
-
-  public getDiagnostics(): Iterable<Diagnostic> {
-    return this.storage;
-  }
-
-}
-
-export class ConsoleDiagnostics {
-  
-  private writer = new IndentWriter(process.stderr);
-
-  public hasError = false;
-  public hasFatal = false;
-
-  public add(diagnostic: Diagnostic): void {
-    diagnostic.format(this.writer);
-    if (diagnostic.level >= Level.Error) {
-      this.hasError = true;
-    }
-    if (diagnostic.level >= Level.Fatal) {
-      this.hasFatal = true;
-    }
-  }
-
-}
-
 interface PrintExcerptOptions {
   indentation?: string;
   extraLineCount?: number;
 }
 
-function printNode(node: Syntax, options?: PrintExcerptOptions): string {
+interface PrintNodeOptions extends PrintExcerptOptions { }
+
+function printNode(node: Syntax, options?: PrintNodeOptions): string {
   const file = node.getSourceFile().getFile();
   return printExcerpt(file, node.getRange(), options);
 }

src/emitter.ts

@@ -167,14 +167,18 @@ export class Emitter {
           this.writer.write('pub ');
         }
         this.writer.write(`class `);
-        if (node.constraints) {
+        if (node.constraintClause) {
-          for (const constraint of node.constraints.constraints) {
+          for (const constraint of node.constraintClause.constraints) {
             this.emit(constraint);
             this.writer.write(`, `);
           }
           this.writer.write(' => ');
         }
-        this.emit(node.constraint);
+        this.emit(node.name);
+        for (const type of node.types) {
+          this.writer.write(' ');
+          this.emit(type);
+        }
         if (node.elements !== null) {
           this.writer.write('.\n');
           this.writer.indent();

src/parser.ts

@@ -183,14 +183,18 @@ export class Parser {
     return new ReferenceTypeExpression(modulePath, name);
   }
 
+  public parseVarTypeExpression(): VarTypeExpression {
+    const name = this.expectToken(SyntaxKind.Identifier);
+    return new VarTypeExpression(name);
+  }
+
   public parsePrimitiveTypeExpression(): TypeExpression {
     const t0 = this.peekToken();
     switch (t0.kind) {
       case SyntaxKind.Identifier:
-      {
+        return this.parseVarTypeExpression();
-        this.getToken();
+      case SyntaxKind.IdentifierAlt:
-        return new VarTypeExpression(t0);
+        return this.parseReferenceTypeExpression();
-      }
       case SyntaxKind.LParen:
       {
         this.getToken();
@@ -220,14 +224,12 @@ export class Parser {
         }
         return new TupleTypeExpression(t0, elements, rparen);
       }
-      case SyntaxKind.IdentifierAlt:
-        return this.parseReferenceTypeExpression();
       default:
         this.raiseParseError(t0, [ SyntaxKind.IdentifierAlt ]);
     }
   }
 
-  private tryParseAppTypeExpression(): TypeExpression {
+  private parseAppTypeExpressionOrBelow(): TypeExpression {
     const operator = this.parsePrimitiveTypeExpression();
     const args = [];
     for (;;) {
@@ -251,7 +253,7 @@ export class Parser {
   }
 
   public parseTypeExpression(): TypeExpression {
-    let returnType = this.tryParseAppTypeExpression();
+    let returnType = this.parseAppTypeExpressionOrBelow();
     const paramTypes = [];
     for (;;) {
       const t1 = this.peekToken();
@@ -260,7 +262,7 @@ export class Parser {
       }
       this.getToken();
       paramTypes.push(returnType);
-      returnType = this.tryParseAppTypeExpression();
+      returnType = this.parseAppTypeExpressionOrBelow();
     }
     if (paramTypes.length === 0) {
       return returnType;
@@ -1048,13 +1050,22 @@ export class Parser {
       }
       clause = new ClassConstraintClause(constraints, rarrowAlt);
     }
-    const constraint = this.parseClassConstraint();
+    const name = this.expectToken(SyntaxKind.IdentifierAlt);
+    const types = [];
+    for (;;) {
+      const t3 = this.peekToken();
+      if (t3.kind === SyntaxKind.BlockStart || t3.kind === SyntaxKind.LineFoldEnd) {
+        break;
+      }
+      const type = this.parseTypeExpression();
+      types.push(type);
+    }
     this.expectToken(SyntaxKind.BlockStart);
     const elements = [];
     loop: for (;;) {
-      const t3 = this.peekToken();
+      const t4 = this.peekToken();
       let element;
-      switch (t3.kind) {
+      switch (t4.kind) {
         case SyntaxKind.BlockEnd:
           this.getToken();
           break loop;
@@ -1065,12 +1076,12 @@ export class Parser {
           element = this.parseTypeDeclaration();
           break;
         default:
-          this.raiseParseError(t3, [ SyntaxKind.LetKeyword, SyntaxKind.TypeKeyword, SyntaxKind.BlockEnd ]);
+          this.raiseParseError(t4, [ SyntaxKind.LetKeyword, SyntaxKind.TypeKeyword, SyntaxKind.BlockEnd ]);
       }
       elements.push(element);
     }
     this.expectToken(SyntaxKind.LineFoldEnd);
-    return new InstanceDeclaration(pubKeyword, t0, clause, constraint, elements);
+    return new InstanceDeclaration(pubKeyword, t0, clause, name, types, elements);
   }
 
   public parseClassDeclaration(): ClassDeclaration {
@@ -1097,7 +1108,17 @@ export class Parser {
       }
       clause = new ClassConstraintClause(constraints, rarrowAlt);
     }
-    const constraint = this.parseClassConstraint();
+    const name = this.expectToken(SyntaxKind.IdentifierAlt);
+    const types = [];
+    for (;;) {
+      const t1 = this.peekToken();
+      if (t1.kind === SyntaxKind.Identifier) {
+        const type = this.parseVarTypeExpression();
+        types.push(type);
+      } else {
+        break;
+      }
+    }
     this.expectToken(SyntaxKind.BlockStart);
     const elements = [];
     loop: for (;;) {
@@ -1119,7 +1140,7 @@ export class Parser {
       elements.push(element);
     }
     this.expectToken(SyntaxKind.LineFoldEnd);
-    return new ClassDeclaration(pubKeyword, t0 as ClassKeyword, clause, constraint, elements);
+    return new ClassDeclaration(pubKeyword, t0 as ClassKeyword, clause, name, types, elements);
   }
 
   public parseSourceFileElement(): SourceFileElement {

src/passes/TypeclassDictPass.ts

@@ -45,7 +45,7 @@ function lcfirst(text: string): string {
 export class TypeclassDictPassing implements Pass<SourceFile, SourceFile> {
 
   private mangleInstance(node: InstanceDeclaration): string {
-    return lcfirst(node.constraint.name.text) + '_' + node.constraint.types.map(encode).join('');
+    return lcfirst(node.name.text) + '_' + node.types.map(encode).join('');
   }
 
   private visit(node: Syntax): Syntax {

src/scope.ts

@@ -57,7 +57,7 @@ export class Scope {
     switch (node.kind) {
       case SyntaxKind.ClassDeclaration:
       {
-        this.add(node.constraint.name.text, node, Symkind.Typeclass);
+        this.add(node.name.text, node, Symkind.Typeclass);
       }
       case SyntaxKind.InstanceDeclaration:
       case SyntaxKind.SourceFile:

src/util.ts

@@ -2,6 +2,8 @@
 import path from "path"
 import stream from "stream"
 
+export const isDebug = true;
+
 export function first<T>(iter: Iterator<T>): T | undefined {
   return iter.next().value;
 }
@@ -60,9 +62,17 @@ export class IndentWriter {
 
 }
 
+const GITHUB_ISSUE_URL = 'https://github.com/boltlang/bolt/issues/'
+
 export function assert(test: boolean): asserts test {
   if (!test) {
-    throw new Error(`Assertion failed. See the stack trace for more information.`);
+    throw new Error(`Assertion failed. See the stack trace for more information. You are invited to search this issue on GitHub or to create a new one at ${GITHUB_ISSUE_URL} .`);
+  }
+}
+
+export function implementationLimitation(test: boolean): asserts test {
+  if (!test) {
+    throw new Error(`We encountered a limitation to the implementation of this compiler. You are invited to search this issue on GitHub or to create a new one at ${GITHUB_ISSUE_URL} .`);
   }
 }