diff --git a/src/ast.ts b/src/ast.ts
index cfba0da..fd74c0c 100644
--- a/src/ast.ts
+++ b/src/ast.ts
@@ -1,5 +1,6 @@
 import { Span } from "./error";
 import { LitIntType } from "./lexer";
+import { ComplexMap } from "./utils";
 
 export type Phase = {
   res: unknown;
@@ -52,7 +53,7 @@ export type CrateId = number;
 export type Crate<P extends Phase> = {
   id: CrateId;
   rootItems: Item<P>[];
-  itemsById: Map<ItemId, Item<P>>;
+  itemsById: ComplexMap<ItemId, Item<P>>;
   packageName: string;
 } & P["typeckResults"];
 
@@ -66,7 +67,23 @@ export type IdentWithRes<P extends Phase> = {
   span: Span;
 } & P["res"];
 
-export type ItemId = number;
+export class ItemId {
+  public crateId: number;
+  public itemIdx: number;
+
+  constructor(crateId: number, itemIdx: number) {
+    this.crateId = crateId;
+    this.itemIdx = itemIdx;
+  }
+
+  static dummy(): ItemId {
+    return new ItemId(999999, 999999);
+  }
+
+  toString(): string {
+    return `[${this.crateId}@${this.itemIdx}]`;
+  }
+}
 
 export type ItemKind<P extends Phase> =
   | {
@@ -495,7 +512,7 @@ export type TypeckResults = {
 export type FoldFn<From, To> = (value: From) => To;
 
 export type Folder<From extends Phase, To extends Phase> = {
-  newItemsById: Map<ItemId, Item<To>>;
+  newItemsById: ComplexMap<ItemId, Item<To>>;
   /**
    * This should not be overridden.
    */
@@ -507,7 +524,7 @@ export type Folder<From extends Phase, To extends Phase> = {
 };
 
 type ItemFolder<From extends Phase, To extends Phase> = {
-  newItemsById: Map<ItemId, Item<To>>;
+  newItemsById: ComplexMap<ItemId, Item<To>>;
   item: FoldFn<Item<From>, Item<To>>;
   itemInner: FoldFn<Item<From>, Item<To>>;
 };
@@ -519,7 +536,7 @@ export function mkDefaultFolder<
   To extends Phase
 >(): ItemFolder<From, To> {
   const folder: ItemFolder<From, To> = {
-    newItemsById: new Map(),
+    newItemsById: new ComplexMap(),
     item(item) {
       const newItem = this.itemInner(item);
       this.newItemsById.set(newItem.id, newItem);
diff --git a/src/error.ts b/src/error.ts
index b499361..677d5e1 100644
--- a/src/error.ts
+++ b/src/error.ts
@@ -23,7 +23,11 @@ export class CompilerError extends Error {
   }
 }
 
-export function withErrorPrinter(input: string, filename: string, f: () => void): void {
+export function withErrorPrinter(
+  input: string,
+  filename: string,
+  f: () => void
+): void {
   try {
     f();
   } catch (e) {
@@ -50,7 +54,6 @@ function renderError(input: string, filename: string, e: CompilerError) {
   const lineNo = lineIdx + 1;
   console.error(`error: ${e.message}`);
   console.error(` --> ${filename}:${lineNo}`);
-  
 
   console.error(`${lineNo} | ${spanToSnippet(input, line)}`);
   const startRelLine =
diff --git a/src/index.ts b/src/index.ts
index 4aae1ea..4a4e871 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -72,7 +72,7 @@ function main() {
     console.log(resolvedPrinted);
 
     console.log("-----AST typecked------");
-    const typecked: Crate<Typecked> = typeck(resolved);
+    const typecked: Crate<Typecked> = typeck(resolved, crates);
     const typeckPrinted = printAst(typecked);
     console.log(typeckPrinted);
 
@@ -135,7 +135,8 @@ function loadCrate(
     const tokens = tokenize(input);
     const ast = parse(name, tokens, crateId.next());
     const [resolved, crates] = resolve(ast, loadCrate);
-    const typecked = typeck(resolved);
+
+    const typecked = typeck(resolved, [...existingCrates, ...crates]);
     return [typecked, crates];
   } catch (e) {
     withErrorPrinter(input, filename, () => {
diff --git a/src/parser.ts b/src/parser.ts
index 7f213c0..2783ca0 100644
--- a/src/parser.ts
+++ b/src/parser.ts
@@ -28,10 +28,11 @@ import {
   Built,
   Parsed,
   ExternItem,
+  ItemId,
 } from "./ast";
 import { CompilerError, Span, spanMerge } from "./error";
 import { BaseToken, Token, TokenIdent, TokenLitString } from "./lexer";
-import { Ids } from "./utils";
+import { ComplexMap, ComplexSet, Ids } from "./utils";
 
 type Parser<T> = (t: Token[]) => [Token[], T];
 
@@ -81,7 +82,7 @@ function parseItem(t: Token[]): [Token[], Item<Parsed>] {
         node: def,
         span: tok.span,
         // Assigned later.
-        id: 0,
+        id: ItemId.dummy(),
       },
     ];
   } else if (tok.kind === "type") {
@@ -116,7 +117,10 @@ function parseItem(t: Token[]): [Token[], Item<Parsed>] {
       fields,
     };
 
-    return [t, { kind: "type", node: def, span: name.span, id: 0 }];
+    return [
+      t,
+      { kind: "type", node: def, span: name.span, id: ItemId.dummy() },
+    ];
   } else if (tok.kind === "import") {
     [t] = expectNext(t, "(");
     let module;
@@ -136,7 +140,10 @@ function parseItem(t: Token[]): [Token[], Item<Parsed>] {
       ...sig,
     };
 
-    return [t, { kind: "import", node: def, span: tok.span, id: 0 }];
+    return [
+      t,
+      { kind: "import", node: def, span: tok.span, id: ItemId.dummy() },
+    ];
   } else if (tok.kind === "extern") {
     [t] = expectNext(t, "mod");
     let name;
@@ -148,7 +155,7 @@ function parseItem(t: Token[]): [Token[], Item<Parsed>] {
 
     [t] = expectNext(t, ";");
 
-    return [t, { kind: "extern", node, span: name.span, id: 0 }];
+    return [t, { kind: "extern", node, span: name.span, id: ItemId.dummy() }];
   } else if (tok.kind === "mod") {
     let name;
     [t, name] = expectNext<TokenIdent>(t, "identifier");
@@ -172,7 +179,7 @@ function parseItem(t: Token[]): [Token[], Item<Parsed>] {
       contents,
     };
 
-    return [t, { kind: "mod", node, span: name.span, id: 0 }];
+    return [t, { kind: "mod", node, span: name.span, id: ItemId.dummy() }];
   } else {
     unexpectedToken(tok, "item");
   }
@@ -663,13 +670,15 @@ function unexpectedToken(token: Token, expected: string): never {
 }
 
 function validateAst(ast: Crate<Built>) {
-  const seenItemIds = new Set();
+  const seenItemIds = new ComplexSet();
 
   const validator: Folder<Built, Built> = {
     ...mkDefaultFolder(),
     itemInner(item: Item<Built>): Item<Built> {
       if (seenItemIds.has(item.id)) {
-        throw new Error(`duplicate item id: ${item.id} for ${item.node.name}`);
+        throw new Error(
+          `duplicate item id: ${item.id.toString()} for ${item.node.name}`
+        );
       }
       seenItemIds.add(item.id);
       return superFoldItem(item, this);
@@ -734,14 +743,14 @@ function buildCrate(
   const ast: Crate<Built> = {
     id: crateId,
     rootItems,
-    itemsById: new Map(),
+    itemsById: new ComplexMap(),
     packageName,
   };
 
   const assigner: Folder<Parsed, Built> = {
     ...mkDefaultFolder(),
     itemInner(item: Item<Parsed>): Item<Built> {
-      const id = itemId.next();
+      const id = new ItemId(crateId, itemId.next());
       return { ...superFoldItem(item, this), id };
     },
     expr(expr: Expr<Parsed>): Expr<Built> {
diff --git a/src/resolve.ts b/src/resolve.ts
index b46a9b6..fd47ec8 100644
--- a/src/resolve.ts
+++ b/src/resolve.ts
@@ -20,7 +20,7 @@ import {
   Typecked,
 } from "./ast";
 import { CompilerError, Span, spanMerge } from "./error";
-import { Ids, unwrap } from "./utils";
+import { ComplexMap, Ids, unwrap } from "./utils";
 
 const BUILTIN_SET = new Set<string>(BUILTINS);
 
@@ -34,8 +34,8 @@ export type CrateLoader = (
 type Context = {
   ast: Crate<Built>;
   crates: Crate<Typecked>[];
-  modContentsCache: Map<ItemId, Map<string, ItemId>>;
-  newItemsById: Map<ItemId, Item<Resolved>>;
+  modContentsCache: ComplexMap<ItemId, Map<string, ItemId>>;
+  newItemsById: ComplexMap<ItemId, Item<Resolved>>;
   crateLoader: CrateLoader;
   crateId: Ids;
 };
@@ -56,15 +56,21 @@ function resolveModItem(
   if ("contents" in mod) {
     contents = new Map(mod.contents.map((item) => [item.node.name, item.id]));
   } else {
+    console.log("resolve...");
+
     const [loadedCrate, itsDeps] = cx.crateLoader(
       item.node.name,
       item.span,
       cx.crateId,
       cx.crates
     );
+    console.log("hmm");
+
     cx.crates.push(loadedCrate);
     cx.crates.push(...itsDeps);
 
+    console.log(cx.crates);
+
     contents = new Map(
       loadedCrate.rootItems.map((item) => [item.node.name, item.id])
     );
@@ -83,8 +89,8 @@ export function resolve(
   const cx: Context = {
     ast,
     crates: [],
-    modContentsCache: new Map(),
-    newItemsById: new Map(),
+    modContentsCache: new ComplexMap(),
+    newItemsById: new ComplexMap(),
     crateLoader,
     crateId,
   };
@@ -106,7 +112,7 @@ function resolveModule(
   modName: string[],
   contents: Item<Built>[]
 ): Item<Resolved>[] {
-  const items = new Map<string, number>();
+  const items = new Map<string, ItemId>();
 
   contents.forEach((item) => {
     const existing = items.get(item.node.name);
@@ -266,11 +272,7 @@ function resolveModule(
 
             if (res.kind === "item") {
               const module = unwrap(cx.ast.itemsById.get(res.id));
-              console.log("nested", module.kind, res.id, cx.ast.itemsById);
-              
               if (module.kind === "mod" || module.kind === "extern") {
-                console.log("resolve");
-                
                 if (typeof expr.field.value === "number") {
                   throw new CompilerError(
                     "module contents cannot be indexed with a number",
diff --git a/src/typeck.ts b/src/typeck.ts
index 0fb2ca6..2d2dc75 100644
--- a/src/typeck.ts
+++ b/src/typeck.ts
@@ -32,7 +32,7 @@ import {
 } from "./ast";
 import { CompilerError, Span } from "./error";
 import { printTy } from "./printer";
-import { unwrap } from "./utils";
+import { ComplexMap, unwrap } from "./utils";
 
 function mkTyFn(params: Ty[], returnTy: Ty): Ty {
   return { kind: "fn", params, returnTy };
@@ -89,7 +89,7 @@ function typeOfBuiltinValue(name: BuiltinName, span: Span): Ty {
 function lowerAstTyBase(
   type: Type<Resolved>,
   lowerIdentTy: (ident: IdentWithRes<Resolved>) => Ty,
-  typeOfItem: (index: number, cause: Span) => Ty
+  typeOfItem: (itemId: ItemId, cause: Span) => Ty
 ): Ty {
   switch (type.kind) {
     case "ident": {
@@ -115,19 +115,51 @@ function lowerAstTyBase(
   }
 }
 
-export function typeck(ast: Crate<Resolved>): Crate<Typecked> {
-  const itemTys = new Map<number, Ty | null>();
-  function typeOfItem(index: ItemId, cause: Span): Ty {
-    const item = unwrap(ast.itemsById.get(index));
+export function typeck(
+  ast: Crate<Resolved>,
+  otherCrates: Crate<Typecked>[]
+): Crate<Typecked> {
+  const itemTys = new ComplexMap<ItemId, Ty | null>();
+  function typeOfItem(itemId: ItemId, cause: Span): Ty {
+    if (itemId.crateId !== ast.id) {
+      console.log(otherCrates);
 
-    const ty = itemTys.get(index);
+      const crate = unwrap(
+        otherCrates.find((crate) => crate.id === itemId.crateId)
+      );
+      const item = unwrap(crate.itemsById.get(itemId));
+      switch (item.kind) {
+        case "function":
+        case "import":
+        case "type":
+          return item.node.ty!;
+        case "mod": {
+          throw new CompilerError(
+            `module ${item.node.name} cannot be used as a type or value`,
+            cause
+          );
+        }
+        case "extern": {
+          throw new CompilerError(
+            `extern declaration ${item.node.name} cannot be used as a type or value`,
+            cause
+          );
+        }
+      }
+    }
+
+    const item = unwrap(ast.itemsById.get(itemId));
+    const ty = itemTys.get(itemId);
     if (ty) {
       return ty;
     }
     if (ty === null) {
-      throw new CompilerError(`cycle computing type of #G${index}`, item.span);
+      throw new CompilerError(
+        `cycle computing type of #G${itemId.toString()}`,
+        item.span
+      );
     }
-    itemTys.set(index, null);
+    itemTys.set(itemId, null);
     switch (item.kind) {
       case "function":
       case "import": {
@@ -528,7 +560,7 @@ export class InferContext {
 export function checkBody(
   body: Expr<Resolved>,
   fnTy: TyFn,
-  typeOfItem: (index: number, cause: Span) => Ty
+  typeOfItem: (itemId: ItemId, cause: Span) => Ty
 ): Expr<Typecked> {
   const localTys = [...fnTy.params];
   const loopState: { hasBreak: boolean; loopId: LoopId }[] = [];
diff --git a/src/utils.ts b/src/utils.ts
index 826e62e..d760905 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -22,20 +22,32 @@ export function unwrap<T>(value: T | undefined): T {
  * It uses JSON+string equality instead of refernece equality.
  */
 export class ComplexMap<K, V> {
-  inner = new Map<string | number, V>();
+  private inner = new Map<string | number, V>();
 
   public get(key: K): V | undefined {
-    return this.inner.get(this.mangleKey(key));
+    return this.inner.get(mangleKey(key));
   }
 
   public set(key: K, value: V): void {
-    this.inner.set(this.mangleKey(key), value);
-  }
-
-  private mangleKey(key: K): string | number {
-    if (typeof key === "string" || typeof key === "number") {
-      return key;
-    }
-    return JSON.stringify(key);
+    this.inner.set(mangleKey(key), value);
   }
 }
+
+export class ComplexSet<K> {
+  private inner = new Set();
+
+  public has(key: K): boolean {
+    return this.inner.has(mangleKey(key));
+  }
+
+  public add(key: K): void {
+    this.inner.add(mangleKey(key));
+  }
+}
+
+function mangleKey<K>(key: K): string | number {
+  if (typeof key === "string" || typeof key === "number") {
+    return key;
+  }
+  return JSON.stringify(key);
+}