diff --git a/implementations/javascript/packages/schema/package.json b/implementations/javascript/packages/schema/package.json
index 932a9bb..83a51bb 100644
--- a/implementations/javascript/packages/schema/package.json
+++ b/implementations/javascript/packages/schema/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@preserves/schema",
-  "version": "0.9.0",
+  "version": "0.11.0",
   "description": "Schema support for Preserves data serialization format",
   "homepage": "https://gitlab.com/preserves/preserves",
   "license": "Apache-2.0",
diff --git a/implementations/javascript/packages/schema/src/checker.ts b/implementations/javascript/packages/schema/src/checker.ts
new file mode 100644
index 0000000..77fcf49
--- /dev/null
+++ b/implementations/javascript/packages/schema/src/checker.ts
@@ -0,0 +1,128 @@
+import { typeFor } from './gentype';
+import { ANY_TYPE } from './type';
+import * as M from './meta';
+
+export function checkSchema(schema: M.Schema): M.Schema {
+    const checker = new Checker();
+    schema.definitions.forEach(checker.checkDefinition.bind(checker));
+    if (checker.problems.length > 0) {
+        throw new Error(`Cannot produce unconverter: insufficient information in:\n` +
+            checker.problems.map(c => ' - ' + c).join('\n'));
+    }
+    return schema;
+}
+
+class Checker {
+    problems: Array<string> = [];
+
+    recordProblem(context: string, detail: string): void {
+        this.problems.push(`${detail} in ${context}`);
+    }
+
+    checkBinding(scope: Set<string>, sym: symbol, context: string): void {
+        const name = sym.description!;
+        if (scope.has(name)) {
+            this.recordProblem(context, `duplicate binding named ${JSON.stringify(name)}`);
+        } else {
+            scope.add(name);
+        }
+    }
+
+    checkDefinition(def: M.Definition, name: symbol): void {
+        switch (def._variant) {
+            case 'or':
+                [def.pattern0, def.pattern1, ... def.patternN].forEach(({ variantLabel, pattern }) =>
+                    this.checkPattern(new Set(), pattern, `variant ${variantLabel} of ${name.description!}`));
+                break;
+            case 'and': {
+                const scope = new Set<string>();
+                [def.pattern0, def.pattern1, ... def.patternN].forEach((p) =>
+                    this.checkNamedPattern(scope, p, name.description!));
+                break;
+            }
+            case 'Pattern':
+                this.checkPattern(new Set(), def.value, name.description!);
+                break;
+        }
+    }
+
+    checkNamedPattern(scope: Set<string>, p: M.NamedPattern, context: string): void {
+        switch (p._variant) {
+            case 'named':
+                this.checkPattern(scope,
+                                  M.Pattern.SimplePattern(p.value.pattern),
+                                  `${p.value.name.description!} of ${context}`);
+                break;
+            case 'anonymous':
+                this.checkPattern(scope, p.value, context);
+                break;
+        }
+    }
+
+    checkPattern(scope: Set<string>, p: M.Pattern, context: string): void {
+        const t = typeFor((_ref) => ANY_TYPE, p);
+        switch (p._variant) {
+            case 'SimplePattern':
+                if (p.value._variant !== 'lit' && (t.kind === 'record' || t.kind === 'unit')) {
+                    this.recordProblem(context, 'cannot recover serialization of non-literal pattern');
+                }
+                break;
+            case 'CompoundPattern':
+                ((p: M.CompoundPattern): void => {
+                    switch (p._variant) {
+                        case 'rec':
+                            this.checkNamedPattern(scope, p.label, `label of ${context}`);
+                            this.checkNamedPattern(scope, p.fields, `fields of ${context}`);
+                            break;
+                        case 'tuple':
+                            p.patterns.forEach((pp, i) =>
+                                this.checkNamedPattern(scope, pp, `item ${i} of ${context}`));
+                            break;
+                        case 'tuple*':
+                            if (p.variable._variant === 'named') {
+                                this.checkBinding(scope, p.variable.value.name, context);
+                                this.checkPattern(scope,
+                                                  M.Pattern.SimplePattern(p.variable.value.pattern),
+                                                  `${JSON.stringify(p.variable.value.name.description!)} of ${context}`);
+                            } else {
+                                if (t.kind !== 'array') {
+                                    this.recordProblem(context, 'unable to reconstruct tail of tuple* pattern');
+                                }
+                                this.checkPattern(scope,
+                                                  M.Pattern.SimplePattern(p.variable.value),
+                                                  `variable-length portion of ${context}`);
+                            }
+                            p.fixed.forEach((pp, i) =>
+                                this.checkNamedPattern(scope, pp, `item ${i} of ${context}`));
+                            break;
+                        case 'setof':
+                            if (t.kind !== 'set') {
+                                this.recordProblem(context, 'unable to reconstruct set');
+                            }
+                            this.checkPattern(scope,
+                                              M.Pattern.SimplePattern(p.pattern),
+                                              `set in ${context}`);
+                            break;
+                        case 'dictof':
+                            if (t.kind !== 'dictionary') {
+                                this.recordProblem(context, 'unable to reconstruct dictionary');
+                            }
+                            this.checkPattern(scope,
+                                              M.Pattern.SimplePattern(p.key),
+                                              `key in dictionary in ${context}`);
+                            this.checkPattern(scope,
+                                              M.Pattern.SimplePattern(p.value),
+                                              `value in dictionary in ${context}`);
+                            break;
+                        case 'dict':
+                            p.entries.forEach((np, key) =>
+                                this.checkNamedPattern(
+                                    scope,
+                                    M.promoteNamedSimplePattern(M.addNameIfAbsent(np, key)),
+                                    `entry ${key.asPreservesText()} in dictionary in ${context}`));
+                            break;
+                    }
+                })(p.value);
+        }
+    }
+}
diff --git a/implementations/javascript/packages/schema/src/compiler.ts b/implementations/javascript/packages/schema/src/compiler.ts
index adc7fbb..ee8f4bc 100644
--- a/implementations/javascript/packages/schema/src/compiler.ts
+++ b/implementations/javascript/packages/schema/src/compiler.ts
@@ -2,9 +2,10 @@ import { stringify } from "@preserves/core";
 import * as M from "./meta";
 import { CompilerOptions, ModuleContext } from "./compiler/context";
 import { Formatter, block, seq } from "./compiler/block";
-import { typeForDefinition } from "./compiler/gentype";
+import { typeForDefinition } from "./gentype";
 import { converterForDefinition } from "./compiler/genconverter";
-import { renderType, Type } from "./compiler/type";
+import { Type } from "./type";
+import { renderType } from "./compiler/rendertype";
 import { genConstructor } from "./compiler/genctor";
 import { unconverterForDefinition } from "./compiler/genunconverter";
 import { sourceCodeFor } from "./compiler/value";
@@ -16,12 +17,12 @@ export function compile(env: M.Environment, schema: M.Schema, options: CompilerO
     mod.defineType(seq(`export type _embedded = `,
                        renderType(embeddedName._variant === 'false'
                            ? Type.ref('any')
-                           : typeForDefinition(mod, M.Definition.Pattern(M.Pattern.SimplePattern(M.SimplePattern.Ref(embeddedName.value))))),
+                           : typeForDefinition(mod.resolver(), M.Definition.Pattern(M.Pattern.SimplePattern(M.SimplePattern.Ref(embeddedName.value))))),
                        `;`));
     mod.defineType(`export type _val = _.Value<_embedded>;`);
 
     for (const [name, def] of schema.definitions) {
-        const t = typeForDefinition(mod, def);
+        const t = typeForDefinition(mod.resolver(), def);
         const nameStr = stringify(name);
 
         mod.defineType(seq(`export type ${nameStr} = `, renderType(t), `;`));
@@ -60,7 +61,7 @@ export function compile(env: M.Environment, schema: M.Schema, options: CompilerO
         mod.defineFunction(ctx =>
             seq(`export function from${name.description!}`,
                 '(_v: ', name.description!, '): _val ',
-                ctx.block(() => unconverterForDefinition(ctx, name.description!, def, '_v'))));
+                ctx.block(() => unconverterForDefinition(ctx, def, '_v'))));
     }
 
     const f = new Formatter();
diff --git a/implementations/javascript/packages/schema/src/compiler/context.ts b/implementations/javascript/packages/schema/src/compiler/context.ts
index 692b07b..13b36a8 100644
--- a/implementations/javascript/packages/schema/src/compiler/context.ts
+++ b/implementations/javascript/packages/schema/src/compiler/context.ts
@@ -2,7 +2,8 @@ import { Dictionary, KeyedSet, Position } from "@preserves/core";
 import { refPosition } from "../reader";
 import * as M from "../meta";
 import { block, braces, commas, formatItems, Item, keyvalue, seq } from "./block";
-import { ANY_TYPE, renderType, Type, variantInitFor } from "./type";
+import { ANY_TYPE, AtomicType, Type } from "../type";
+import { renderType, variantInitFor } from "./rendertype";
 
 export interface CompilerOptions {
     preservesModule?: string;
@@ -67,6 +68,15 @@ export class ModuleContext {
     defineFunction(f: (ctx: FunctionContext) => Item): void {
         this.functiondefs.push(f(new FunctionContext(this)));
     }
+
+    resolver(): (ref: M.Ref) => AtomicType {
+        return (ref) => M.lookup(refPosition(ref), ref, this.env,
+                                 (_p) => Type.ref(ref.name.description!),
+                                 (modId, modPath,_p) => {
+                                     this.imports.add([modId, modPath]);
+                                     return Type.ref(`${modId}.${ref.name.description!}`);
+                                 });
+    }
 }
 
 export class FunctionContext {
diff --git a/implementations/javascript/packages/schema/src/compiler/genconverter.ts b/implementations/javascript/packages/schema/src/compiler/genconverter.ts
index 320ccb5..0e6a014 100644
--- a/implementations/javascript/packages/schema/src/compiler/genconverter.ts
+++ b/implementations/javascript/packages/schema/src/compiler/genconverter.ts
@@ -1,9 +1,9 @@
 import { FunctionContext } from "./context";
 import * as M from '../meta';
 import { block, Item, seq } from "./block";
-import { simpleType, dictionaryType, setType, typeFor } from "./gentype";
+import { simpleType, dictionaryType, setType, typeFor } from "../gentype";
 import { refPosition } from "../reader";
-import { ANY_TYPE, Type } from "./type";
+import { ANY_TYPE, Type } from "../type";
 
 export function converterForDefinition(
     ctx: FunctionContext,
@@ -48,7 +48,7 @@ function converterForPattern(
         if (simpleValue === void 0) {
             return [ctx.buildCapturedCompound(dest)];
         } else if (ctx.variantName !== void 0) {
-            if (typeFor(ctx.mod, p).kind === 'unit') {
+            if (typeFor(ctx.mod.resolver(), p).kind === 'unit') {
                 return [ctx.buildCapturedCompound(dest)];
             } else {
                 return [ctx.withCapture('value',
@@ -98,7 +98,7 @@ function converterForArray(ctx: FunctionContext,
                            k: (dest: string) => Item[]): Item
 {
     const postCheck = () => {
-        const r = ctx.gentemp(Type.array(simpleType(ctx.mod, M.unnameSimplePattern(arrayType))));
+        const r = ctx.gentemp(Type.array(simpleType(ctx.mod.resolver(), M.unnameSimplePattern(arrayType))));
         const v = ctx.gentempname();
         return [
             seq(`${r} = []`),
@@ -125,14 +125,14 @@ function converterFor(
     let maybeName = M.nameFor(np);
 
     if (p._variant === 'SimplePattern') {
-        const dest = ctx.gentemp(simpleType(ctx.mod, p.value));
+        const dest = ctx.gentemp(simpleType(ctx.mod.resolver(), p.value));
         return [... converterForSimple(ctx, p.value, src, dest),
                 ctx.convertCapture(maybeName, dest, ks)];
     } else {
         switch (p.value._variant) {
             case 'setof': {
                 const setPattern = p.value.pattern;
-                const r = ctx.gentemp(setType(ctx.mod, setPattern));
+                const r = ctx.gentemp(setType(ctx.mod.resolver(), setPattern));
                 const v = ctx.gentempname();
                 return [
                     seq(`if (_.Set.isSet<_embedded>(${src})) `, ctx.block(() => [
@@ -147,7 +147,7 @@ function converterFor(
             case 'dictof': {
                 const keyPattern = p.value.key;
                 const valPattern = p.value.value;
-                const r = ctx.gentemp(dictionaryType(ctx.mod, keyPattern, valPattern));
+                const r = ctx.gentemp(dictionaryType(ctx.mod.resolver(), keyPattern, valPattern));
                 const v = ctx.gentempname();
                 const k = ctx.gentempname();
                 return [
diff --git a/implementations/javascript/packages/schema/src/compiler/genctor.ts b/implementations/javascript/packages/schema/src/compiler/genctor.ts
index 29d8c05..ebdb859 100644
--- a/implementations/javascript/packages/schema/src/compiler/genctor.ts
+++ b/implementations/javascript/packages/schema/src/compiler/genctor.ts
@@ -1,6 +1,7 @@
 import * as M from '../meta';
 import { block, braces, Item, keyvalue, parens, seq } from "./block";
-import { FieldType, renderType, SimpleType } from "./type";
+import { FieldType, SimpleType } from "../type";
+import { renderType } from "./rendertype";
 
 export function genConstructor(
     name: string,
diff --git a/implementations/javascript/packages/schema/src/compiler/genunconverter.ts b/implementations/javascript/packages/schema/src/compiler/genunconverter.ts
index f6106f7..a8259e0 100644
--- a/implementations/javascript/packages/schema/src/compiler/genunconverter.ts
+++ b/implementations/javascript/packages/schema/src/compiler/genunconverter.ts
@@ -2,12 +2,12 @@ import { refPosition } from '../reader';
 import * as M from '../meta';
 import { block, brackets, formatItems, Item, parens, seq } from './block';
 import { FunctionContext } from "./context";
-import { FieldType, renderType, SimpleType } from './type';
-import { typeFor, typeForIntersection } from './gentype';
+import { FieldType, SimpleType } from '../type';
+import { typeFor, typeForIntersection } from '../gentype';
+import { renderType } from "./rendertype";
 
 export function unconverterForDefinition(
     ctx: FunctionContext,
-    name: string,
     def: M.Definition,
     src: string): Item[]
 {
@@ -18,53 +18,30 @@ export function unconverterForDefinition(
                     seq(`case `, JSON.stringify(p.variantLabel), `: `, ctx.block(() => {
                         const hasValueField = p.pattern._variant === 'SimplePattern';
                         return [seq(`return `, unconverterForPattern(
-                            ctx, name, p.pattern, hasValueField ? `${src}.value` : src))];
+                            ctx, p.pattern, hasValueField ? `${src}.value` : src))];
                     })))))];
         case 'and': {
             const ps = [def.pattern0, def.pattern1, ... def.patternN];
-            const t = typeForIntersection(ctx.mod, ps);
-            const errs: [string, InsufficientInformationError][] = [];
+            const t = typeForIntersection(ctx.mod.resolver(), ps);
             const cs = ps.flatMap(p => {
                 if (p._variant === 'anonymous' && p.value._variant === 'SimplePattern') {
                     return [];
                 } else {
-                    try {
-                        return [unconverterForNamed(ctx, p, src, t)];
-                    } catch (e) {
-                        if (e instanceof InsufficientInformationError) {
-                            errs.push([name + '/' + (M.nameFor(p) ?? '<anonymous>'), e]);
-                            return [];
-                        }
-                        throw e;
-                    }
+                    return [unconverterForNamed(ctx, p, src, t)];
                 }
             });
-            if (cs.length === 0 || errs.length > 0) {
-                throw new Error(`Cannot produce unconverter for ${name}: ` +
-                    errs.map(e => `${e[0]}: ${e[1].message}`).join(', '));
-            }
             return [seq(`return `, (cs.length === 1)
                 ? cs[0]
                 : seq(`_.merge`, parens(`(a, b) => (a === b) ? a : void 0`, ... cs)))];
         }
         case 'Pattern':
-            return [seq(`return `, unconverterForPattern(ctx, name, def.value, `${src}`))];
+            return [seq(`return `, unconverterForPattern(ctx, def.value, `${src}`))];
     }
 }
 
-export class InsufficientInformationError extends Error {}
-
-function unconverterForPattern(ctx: FunctionContext, name: string, a: M.Pattern, src: string): Item
+function unconverterForPattern(ctx: FunctionContext, a: M.Pattern, src: string): Item
 {
-    const t = typeFor(ctx.mod, a);
-    try {
-        return unconverterFor(ctx, a, src, t);
-    } catch (e) {
-        if (e instanceof InsufficientInformationError) {
-            throw new Error(`Cannot produce unconverter for ${name}: ${e.message}`);
-        }
-        throw e;
-    }
+    return unconverterFor(ctx, a, src, typeFor(ctx.mod.resolver(), a));
 }
 
 function stepSource(
@@ -74,8 +51,7 @@ function stepSource(
 {
     if (t.kind !== 'record' || !t.fields.has(key)) {
         throw new Error(
-            `Internal error: attempt to step type ` +
-                `${formatItems([renderType(t)])} with key ${key}`);
+            `Internal error: attempt to step type ${JSON.stringify(t)} with key ${key}`);
     }
     return {
         steppedSrc: `${src}[${JSON.stringify(key)}]`,
@@ -87,9 +63,6 @@ function unconverterFor(ctx: FunctionContext, p: M.Pattern, src: string, t: Simp
     switch (p._variant) {
         case 'SimplePattern':
             return ((p: M.SimplePattern) => {
-                if (p._variant !== 'lit' && (t.kind === 'record' || t.kind === 'unit')) {
-                    throw new InsufficientInformationError(`A Cannot produce unconverter TODO ${src} ${formatItems([renderType(t)])}`);
-                }
                 switch (p._variant) {
                     case 'any':
                         return `${src}`;
@@ -141,10 +114,7 @@ function unconverterFor(ctx: FunctionContext, p: M.Pattern, src: string, t: Simp
                                     `v`,
                                     steppedType.type))));
                         } else {
-                            if (t.kind !== 'array') {
-                                throw new InsufficientInformationError(
-                                    `B Cannot produce unconverter TODO ${src} ${formatItems([renderType(t)])}`);
-                            }
+                            if (t.kind !== 'array') throw new Error("Internal error");
                             varexp = seq(src, `.map`, parens(
                                 seq(`v => `, unconverterFor(
                                     ctx,
@@ -161,19 +131,11 @@ function unconverterFor(ctx: FunctionContext, p: M.Pattern, src: string, t: Simp
                         }
                     }
                     case 'setof':
-                        if (t.kind !== 'set') {
-                            throw new InsufficientInformationError(
-                                `C Cannot produce unconverter TODO ${src} ${formatItems([renderType(t)])}`);
-                        }
                         return seq(`new _.Set<_embedded>`, parens(
                             `_.Array.from(${src}.values()).map(v => `,
                             unconverterFor(ctx, M.Pattern.SimplePattern(p.pattern), 'v', t),
                             `)`));
                     case 'dictof':
-                        if (t.kind !== 'dictionary') {
-                            throw new InsufficientInformationError(
-                                `D Cannot produce unconverter TODO ${src} ${formatItems([renderType(t)])}`);
-                        }
                         return seq(`new _.Dictionary<_embedded>`, parens(seq(
                             `_.Array.from(${src}.entries()).map(([k, v]) => `,
                             brackets(
diff --git a/implementations/javascript/packages/schema/src/compiler/type.ts b/implementations/javascript/packages/schema/src/compiler/rendertype.ts
similarity index 56%
rename from implementations/javascript/packages/schema/src/compiler/type.ts
rename to implementations/javascript/packages/schema/src/compiler/rendertype.ts
index 1a38ef8..b2fc868 100644
--- a/implementations/javascript/packages/schema/src/compiler/type.ts
+++ b/implementations/javascript/packages/schema/src/compiler/rendertype.ts
@@ -1,41 +1,6 @@
+import { SimpleType, Type } from "../type";
 import { anglebrackets, braces, Item, keyvalue, opseq, seq } from "./block";
 
-export type Type =
-    | { kind: 'union', variants: VariantMap } // zero: never
-    | SimpleType
-
-export type SimpleType = AtomicType | CompoundType
-export type FieldType = AtomicType | CollectionType;
-
-export type AtomicType =
-    | { kind: 'unit' }
-    | { kind: 'ref', typeName: string } // also for base types
-
-export type CompoundType =
-    | CollectionType
-    | { kind: 'record', fields: FieldMap }
-
-export type CollectionType =
-    | { kind: 'array', type: AtomicType }
-    | { kind: 'set', type: AtomicType }
-    | { kind: 'dictionary', key: AtomicType, value: AtomicType }
-
-export type VariantMap = Map<string, SimpleType>;
-export type FieldMap = Map<string, FieldType>;
-
-export namespace Type {
-    export const union = (variants: VariantMap): Type => ({ kind: 'union', variants });
-    export const unit = (): AtomicType => ({ kind: 'unit' });
-    export const ref = (typeName: string): AtomicType => ({ kind: 'ref', typeName });
-    export const record = (fields: FieldMap): CompoundType => ({ kind: 'record', fields });
-    export const array = (type: AtomicType): CollectionType => ({ kind: 'array', type });
-    export const set = (type: AtomicType): CollectionType => ({ kind: 'set', type });
-    export const dictionary = (key: AtomicType, value: AtomicType): CollectionType => (
-        { kind: 'dictionary', key, value });
-}
-
-export const ANY_TYPE: AtomicType = Type.ref('_val');
-
 export function variantInitFor(variantName: string | undefined) : Item[] {
     return variantName === void 0 ? [] : [variantFor(variantName)];
 }
diff --git a/implementations/javascript/packages/schema/src/compiler/gentype.ts b/implementations/javascript/packages/schema/src/gentype.ts
similarity index 52%
rename from implementations/javascript/packages/schema/src/compiler/gentype.ts
rename to implementations/javascript/packages/schema/src/gentype.ts
index 69ad8db..22ccea5 100644
--- a/implementations/javascript/packages/schema/src/compiler/gentype.ts
+++ b/implementations/javascript/packages/schema/src/gentype.ts
@@ -1,62 +1,62 @@
-import { refPosition } from "../reader";
-import * as M from "../meta";
-import { ModuleContext } from "./context";
+import * as M from "./meta";
 import { ANY_TYPE, AtomicType, CollectionType, FieldMap, SimpleType, Type } from "./type";
 
-export function typeForDefinition(mod: ModuleContext, d: M.Definition): Type {
+export type RefResolver = (ref: M.Ref) => AtomicType;
+
+export function typeForDefinition(resolver: RefResolver, d: M.Definition): Type {
     switch (d._variant) {
         case 'or':
             return Type.union(
                 new Map([d.pattern0, d.pattern1, ... d.patternN].map(a =>
-                    [a.variantLabel, typeFor(mod, a.pattern)])));
+                    [a.variantLabel, typeFor(resolver, a.pattern)])));
         case 'and':
-            return typeForIntersection(mod, [d.pattern0, d.pattern1, ... d.patternN]);
+            return typeForIntersection(resolver, [d.pattern0, d.pattern1, ... d.patternN]);
         case 'Pattern':
-            return typeFor(mod, d.value);
+            return typeFor(resolver, d.value);
     }
 }
 
-export function typeForIntersection(mod: ModuleContext, ps: M.NamedPattern[]): SimpleType {
+export function typeForIntersection(resolver: RefResolver, ps: M.NamedPattern[]): SimpleType {
     const fs = new Map();
-    ps.forEach(p => gatherFields(fs, mod, p));
+    ps.forEach(p => gatherFields(fs, resolver, p));
     return fs.size > 0 ? Type.record(fs) : Type.unit();
 }
 
-export function setType(mod: ModuleContext, p: M.SimplePattern): CollectionType {
-    return Type.set(simpleType(mod, p));
+export function setType(resolver: RefResolver, p: M.SimplePattern): CollectionType {
+    return Type.set(simpleType(resolver, p));
 }
 
-export function dictionaryType(mod: ModuleContext,
+export function dictionaryType(resolver: RefResolver,
                                kp: M.SimplePattern,
                                vp: M.SimplePattern): CollectionType
 {
-    return Type.dictionary(simpleType(mod, kp), simpleType(mod, vp));
+    return Type.dictionary(simpleType(resolver, kp), simpleType(resolver, vp));
 }
 
-export function typeFor(mod: ModuleContext, p: M.Pattern): SimpleType {
+export function typeFor(resolver: RefResolver, p: M.Pattern): SimpleType {
     if (p._variant === 'SimplePattern') {
-        return simpleType(mod, p.value);
+        return simpleType(resolver, p.value);
     } else {
         switch (p.value._variant) {
             case 'setof':
-                return setType(mod, p.value.pattern);
+                return setType(resolver, p.value.pattern);
             case 'dictof':
-                return dictionaryType(mod, p.value.key, p.value.value);
+                return dictionaryType(resolver, p.value.key, p.value.value);
             default: {
                 const arrayType = M.simpleArray(p.value);
                 if (arrayType === void 0) {
                     const fs = new Map();
-                    compoundFields(fs, mod, p.value);
+                    compoundFields(fs, resolver, p.value);
                     return fs.size > 0 ? Type.record(fs) : Type.unit();
                 } else {
-                    return Type.array(simpleType(mod, arrayType));
+                    return Type.array(simpleType(resolver, arrayType));
                 }
             }
         }
     }
 }
 
-export function simpleType(mod: ModuleContext, p: M.SimplePattern): AtomicType {
+export function simpleType(resolver: RefResolver, p: M.SimplePattern): AtomicType {
     switch (p._variant) {
         case 'any':
             return ANY_TYPE;
@@ -75,32 +75,27 @@ export function simpleType(mod: ModuleContext, p: M.SimplePattern): AtomicType {
         case 'lit':
             return Type.unit();
         case 'Ref':
-            return M.lookup(refPosition(p.value), p.value, mod.env,
-                            (_p) => Type.ref(p.value.name.description!),
-                            (modId, modPath,_p) => {
-                                mod.imports.add([modId, modPath]);
-                                return Type.ref(`${modId}.${p.value.name.description!}`);
-                            });
+            return resolver(p.value);
         default:
             ((_p: never) => {})(p);
             throw new Error("Unreachable");
     }
 }
 
-function compoundFields(fs: FieldMap, mod: ModuleContext, p: M.CompoundPattern): void {
+function compoundFields(fs: FieldMap, resolver: RefResolver, p: M.CompoundPattern): void {
     switch (p._variant) {
         case 'rec':
-            gatherFields(fs, mod, p.label);
-            gatherFields(fs, mod, p.fields);
+            gatherFields(fs, resolver, p.label);
+            gatherFields(fs, resolver, p.fields);
             break;
         case 'tuple':
-            p.patterns.forEach(pp => gatherFields(fs, mod, pp));
+            p.patterns.forEach(pp => gatherFields(fs, resolver, pp));
             break;
         case 'tuple*': {
-            p.fixed.forEach(pp => gatherFields(fs, mod, pp));
+            p.fixed.forEach(pp => gatherFields(fs, resolver, pp));
             const n = p.variable;
             if (n._variant === 'named') {
-                fs.set(n.value.name.description!, Type.array(simpleType(mod, n.value.pattern)));
+                fs.set(n.value.name.description!, Type.array(simpleType(resolver, n.value.pattern)));
             }
             break;
         }
@@ -109,7 +104,7 @@ function compoundFields(fs: FieldMap, mod: ModuleContext, p: M.CompoundPattern):
             break;
         case 'dict':
             p.entries.forEach((n, k) =>
-                gatherFields(fs, mod, M.promoteNamedSimplePattern(M.addNameIfAbsent(n, k))));
+                gatherFields(fs, resolver, M.promoteNamedSimplePattern(M.addNameIfAbsent(n, k))));
             break;
         default:
             ((_p: never) => {})(p);
@@ -117,10 +112,10 @@ function compoundFields(fs: FieldMap, mod: ModuleContext, p: M.CompoundPattern):
     }
 }
 
-function gatherFields(fs: FieldMap, mod: ModuleContext, n: M.NamedPattern): void {
+function gatherFields(fs: FieldMap, resolver: RefResolver, n: M.NamedPattern): void {
     if (n._variant === 'named') {
-        fs.set(n.value.name.description!, simpleType(mod, n.value.pattern));
+        fs.set(n.value.name.description!, simpleType(resolver, n.value.pattern));
     } else if (n.value._variant === 'CompoundPattern') {
-        compoundFields(fs, mod, n.value.value);
+        compoundFields(fs, resolver, n.value.value);
     }
 }
diff --git a/implementations/javascript/packages/schema/src/reader.ts b/implementations/javascript/packages/schema/src/reader.ts
index a040769..ce47e54 100644
--- a/implementations/javascript/packages/schema/src/reader.ts
+++ b/implementations/javascript/packages/schema/src/reader.ts
@@ -2,6 +2,7 @@ import { Reader, Annotated, Dictionary, is, peel, preserves, Record, strip, Tupl
 import { Input, Pattern, Schema, Definition, CompoundPattern, SimplePattern } from './meta';
 import * as M from './meta';
 import { SchemaSyntaxError } from './error';
+import { checkSchema } from './checker';
 
 const positionTable = new WeakMap<object, Position>();
 
@@ -58,7 +59,7 @@ function _readSchema(source: string, options?: ReaderOptions<never>): Array<Inpu
 export function readSchema(source: string,
                            options?: ReaderOptions<never> & SchemaReaderOptions): Schema
 {
-    return parseSchema(_readSchema(source, options), options ?? {});
+    return checkSchema(parseSchema(_readSchema(source, options), options ?? {}));
 }
 
 export function parseSchema(toplevelTokens: Array<Input>,
diff --git a/implementations/javascript/packages/schema/src/type.ts b/implementations/javascript/packages/schema/src/type.ts
new file mode 100644
index 0000000..e03d1de
--- /dev/null
+++ b/implementations/javascript/packages/schema/src/type.ts
@@ -0,0 +1,35 @@
+export type Type =
+    | { kind: 'union', variants: VariantMap } // zero: never
+    | SimpleType
+
+export type SimpleType = AtomicType | CompoundType
+export type FieldType = AtomicType | CollectionType;
+
+export type AtomicType =
+    | { kind: 'unit' }
+    | { kind: 'ref', typeName: string } // also for base types
+
+export type CompoundType =
+    | CollectionType
+    | { kind: 'record', fields: FieldMap }
+
+export type CollectionType =
+    | { kind: 'array', type: AtomicType }
+    | { kind: 'set', type: AtomicType }
+    | { kind: 'dictionary', key: AtomicType, value: AtomicType }
+
+export type VariantMap = Map<string, SimpleType>;
+export type FieldMap = Map<string, FieldType>;
+
+export namespace Type {
+    export const union = (variants: VariantMap): Type => ({ kind: 'union', variants });
+    export const unit = (): AtomicType => ({ kind: 'unit' });
+    export const ref = (typeName: string): AtomicType => ({ kind: 'ref', typeName });
+    export const record = (fields: FieldMap): CompoundType => ({ kind: 'record', fields });
+    export const array = (type: AtomicType): CollectionType => ({ kind: 'array', type });
+    export const set = (type: AtomicType): CollectionType => ({ kind: 'set', type });
+    export const dictionary = (key: AtomicType, value: AtomicType): CollectionType => (
+        { kind: 'dictionary', key, value });
+}
+
+export const ANY_TYPE: AtomicType = Type.ref('_val');