diff --git a/src/ast.ts b/src/ast.ts
index 5db6527..805c323 100644
--- a/src/ast.ts
+++ b/src/ast.ts
@@ -1,5 +1,13 @@
 import { Span } from "./error";
 
+export type Ast = Item[];
+
+export type Identifier = {
+  name: string;
+  span: Span;
+  res?: Resolution;
+};
+
 export type ItemKind = {
   kind: "function";
   node: FunctionDef;
@@ -22,40 +30,67 @@ export type FunctionArg = {
   span: Span;
 };
 
+export type ExprEmpty = { kind: "empty" };
+
+export type ExprLet = {
+  kind: "let";
+  name: string;
+  type?: Type;
+  rhs: Expr;
+  after: Expr;
+};
+
+export type ExprBlock = {
+  kind: "block";
+  exprs: Expr[];
+};
+
+export type ExprLiteral = {
+  kind: "literal";
+  value: Literal;
+};
+
+export type ExprIdent = {
+  kind: "ident";
+  value: Identifier;
+};
+
+export type ExprBinary = {
+  kind: "binary";
+  binaryKind: BinaryKind;
+  lhs: Expr;
+  rhs: Expr;
+};
+
+export type ExprUnary = {
+  kind: "unary";
+  unaryKind: UnaryKind;
+  rhs: Expr;
+};
+
+export type ExprCall = {
+  kind: "call";
+  lhs: Expr;
+  args: Expr[];
+};
+
+export type ExprIf = {
+  kind: "if";
+  cond: Expr;
+  then: Expr;
+  else?: Expr;
+};
+
 export type ExprKind =
-  | { kind: "empty" }
-  | { kind: "let"; name: string; type?: Type; rhs: Expr; after: Expr }
-  | { kind: "block"; exprs: Expr[] }
-  | {
-      kind: "literal";
-      value: Literal;
-    }
-  | {
-      kind: "ident";
-      value: string;
-    }
-  | {
-      kind: "binary";
-      binaryKind: BinaryKind;
-      lhs: Expr;
-      rhs: Expr;
-    }
-  | {
-      kind: "unary";
-      unaryKind: UnaryKind;
-      rhs: Expr;
-    }
-  | {
-      kind: "call";
-      lhs: Expr;
-      args: Expr[];
-    }
-  | {
-      kind: "if";
-      cond: Expr;
-      then: Expr;
-      else?: Expr;
-    };
+  | ExprEmpty
+  | ExprLet
+  | ExprBlock
+  | ExprLiteral
+  | ExprIdent
+  | ExprBinary
+  | ExprUnary
+  | ExprCall
+  | ExprIf;
 
 export type Expr = ExprKind & {
   span: Span;
@@ -126,7 +161,7 @@ export const UNARY_KINDS: UnaryKind[] = ["!", "-"];
 export type TypeKind =
   | {
       kind: "ident";
-      value: string;
+      value: Identifier;
     }
   | {
       kind: "list";
@@ -140,3 +175,177 @@ export type TypeKind =
 export type Type = TypeKind & {
   span: Span;
 };
+
+export type Resolution =
+  | {
+      kind: "local";
+      /**
+       * The index of the local variable, from inside out.
+       * ```
+       * let a in let b in (a, b);
+       *     ^        ^
+       *     1        0
+       * ```
+       * When traversing resolutions, a stack of locals has to be kept.
+       * It's similar to a De Bruijn index.
+       */
+      index: number;
+    }
+  | {
+      kind: "item";
+      /**
+       * Items are numbered in the order they appear in.
+       * Right now we only have one scope of items (global)
+       * so this is enough.
+       */
+      index: number;
+    }
+  | {
+      kind: "builtin";
+    };
+
+// folders
+
+export type FoldFn<T> = (value: T) => T;
+
+export type Folder = {
+  item: FoldFn<Item>;
+  expr: FoldFn<Expr>;
+  ident: FoldFn<Identifier>;
+  type: FoldFn<Type>;
+};
+
+export const DEFAULT_FOLDER: Folder = {
+  item(item) {
+    return super_fold_item(item, this);
+  },
+  expr(expr) {
+    return super_fold_expr(expr, this);
+  },
+  ident(ident) {
+    return ident;
+  },
+  type(type) {
+    return super_fold_type(type, this);
+  },
+};
+
+export function fold_ast(ast: Ast, folder: Folder): Ast {
+  return ast.map((item) => folder.item(item));
+}
+
+export function super_fold_item(item: Item, folder: Folder): Item {
+  switch (item.kind) {
+    case "function": {
+      const args = item.node.args.map(({ name, type, span }) => ({
+        name,
+        type: folder.type(type),
+        span,
+      }));
+
+      return {
+        kind: "function",
+        span: item.span,
+        node: {
+          name: item.node.name,
+          args,
+          body: folder.expr(item.node.body),
+          returnType: item.node.returnType && folder.type(item.node.returnType),
+        },
+      };
+    }
+  }
+}
+
+export function super_fold_expr(expr: Expr, folder: Folder): Expr {
+  const span = expr.span;
+  switch (expr.kind) {
+    case "empty": {
+      return { kind: "empty", span };
+    }
+    case "let": {
+      return {
+        kind: "let",
+        name: expr.name,
+        type: expr.type && folder.type(expr.type),
+        rhs: folder.expr(expr.rhs),
+        after: folder.expr(expr.after),
+        span,
+      };
+    }
+    case "block": {
+      return {
+        kind: "block",
+        exprs: expr.exprs.map((expr) => folder.expr(expr)),
+        span,
+      };
+    }
+    case "literal": {
+      return { kind: "literal", value: expr.value, span };
+    }
+    case "ident": {
+      return { kind: "ident", value: folder.ident(expr.value), span };
+    }
+    case "binary": {
+      return {
+        kind: "binary",
+        binaryKind: expr.binaryKind,
+        lhs: folder.expr(expr.lhs),
+        rhs: folder.expr(expr.rhs),
+        span,
+      };
+    }
+    case "unary": {
+      return {
+        kind: "unary",
+        unaryKind: expr.unaryKind,
+        rhs: folder.expr(expr.rhs),
+        span,
+      };
+    }
+    case "call": {
+      return {
+        kind: "call",
+        lhs: folder.expr(expr.lhs),
+        args: expr.args.map((expr) => folder.expr(expr)),
+        span,
+      };
+    }
+    case "if": {
+      return {
+        kind: "if",
+        cond: folder.expr(expr.cond),
+        then: folder.expr(expr.then),
+        else: expr.else && folder.expr(expr.else),
+        span,
+      };
+    }
+  }
+}
+
+export function super_fold_type(type: Type, folder: Folder): Type {
+  const span = type.span;
+  switch (type.kind) {
+    case "ident": {
+      return {
+        kind: "ident",
+        value: folder.ident(type.value),
+        span,
+      };
+    }
+    case "list": {
+      return {
+        kind: "list",
+        elem: folder.type(type.elem),
+        span,
+      };
+    }
+    case "tuple": {
+      return {
+        kind: "tuple",
+        elems: type.elems.map((type) => folder.type(type)),
+        span,
+      };
+    }
+  }
+}
diff --git a/src/error.ts b/src/error.ts
index 8754d3f..cba03b1 100644
--- a/src/error.ts
+++ b/src/error.ts
@@ -76,6 +76,6 @@ export function lines(input: string): Span[] {
   return lines;
 }
 
-export function todo(msg: string, span: Span): never {
-  throw new CompilerError(`TODO: ${msg}`, span);
+export function todo(msg: string): never {
+  throw new CompilerError(`TODO: ${msg}`, { start: 0, end: 0 });
 }
diff --git a/src/index.ts b/src/index.ts
index fe9fe11..9698fe6 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -2,15 +2,20 @@ import { withErrorHandler } from "./error";
 import { tokenize } from "./lexer";
 import { parse } from "./parser";
 import { printAst } from "./printer";
+import { resolve } from "./resolve";
 
 const input = `
-function main(argv: [String]): uwu = (
-  print("Hello, world!");
-  let a: [String] = 0 in
-  let b = 1 in
-  if 0 then 0 else (
-    if 1 == 1 then 1 else "what" 
-  ;"meow")
+function main(argv: [String]): () = (
+  print(argv);
+  if 1 then (
+    print("AAAAAAAAAAAAAAAAAAAA");
+    let a = 0 in
+    a;
+  ) else (
+    print("AAAAAAAAAAAAAAAAAAAAAA");
+    let b = 0 in
+    b;
+  )
 );
 `;
 
@@ -22,13 +27,17 @@ function main() {
 
     const ast = parse(tokens);
     console.log("-----AST------");
-    
-    console.dir(ast, { depth: 10 });
+
+    console.dir(ast, { depth: 50 });
 
     const printed = printAst(ast);
     console.log("-----AST pretty------");
     console.log(printed);
-    
+
+    const resolved = resolve(ast);
+    console.log("-----AST resolved------");
+    const resolvedPrinted = printAst(resolved);
+    console.log(resolvedPrinted);
   });
 }
 
diff --git a/src/parser.ts b/src/parser.ts
index cf8d2e9..f5cfdad 100644
--- a/src/parser.ts
+++ b/src/parser.ts
@@ -1,6 +1,7 @@
 import {
   ARITH_FACTOR_KINDS,
   ARITH_TERM_KINDS,
+  Ast,
   BinaryKind,
   COMPARISON_KINDS,
   Expr,
@@ -17,7 +18,7 @@ import { BaseToken, Token, TokenIdent } from "./lexer";
 
 type Parser<T> = (t: Token[]) => [Token[], T];
 
-export function parse(t: Token[]): Item[] {
+export function parse(t: Token[]): Ast {
   const items: Item[] = [];
 
   while (t.length > 0) {
@@ -242,6 +243,7 @@ function parseExprCall(t: Token[]): [Token[], Expr] {
 
 function parseExprAtom(startT: Token[]): [Token[], Expr] {
   let [t, tok] = next(startT);
+  const span = tok.span;
 
   if (tok.kind === "(") {
     let expr: Expr;
@@ -255,7 +257,7 @@ function parseExprAtom(startT: Token[]): [Token[], Expr] {
     }
     [t] = expectNext(t, ")");
 
-    return [t, { kind: "block", span: tok.span, exprs }];
+    return [t, { kind: "block", span, exprs }];
   }
 
   if (tok.kind === "lit_string") {
@@ -263,7 +265,7 @@ function parseExprAtom(startT: Token[]): [Token[], Expr] {
       t,
       {
         kind: "literal",
-        span: tok.span,
+        span,
         value: { kind: "str", value: tok.value },
       },
     ];
@@ -274,33 +276,48 @@ function parseExprAtom(startT: Token[]): [Token[], Expr] {
       t,
       {
         kind: "literal",
-        span: tok.span,
+        span,
         value: { kind: "int", value: tok.value },
       },
     ];
   }
 
   if (tok.kind === "identifier") {
-    return [t, { kind: "ident", span: tok.span, value: tok.ident }];
+    return [
+      t,
+      {
+        kind: "ident",
+        span,
+        value: { name: tok.ident, span },
+      },
+    ];
   }
 
   // Parse nothing at all.
-  return [startT, { kind: "empty", span: tok.span }];
+  return [startT, { kind: "empty", span }];
 }
 
 function parseType(t: Token[]): [Token[], Type] {
   let tok;
   [t, tok] = next(t);
+  const span = tok.span;
 
   switch (tok.kind) {
     case "identifier": {
-      return [t, { kind: "ident", value: tok.ident, span: tok.span }];
+      return [
+        t,
+        {
+          kind: "ident",
+          value: { name: tok.ident, span },
+          span,
+        },
+      ];
     }
     case "[": {
       let elem;
       [t, elem] = parseType(t);
       [t] = expectNext(t, "]");
-      return [t, { kind: "list", elem, span: tok.span }];
+      return [t, { kind: "list", elem, span }];
     }
     case "(": {
       let first = true;
@@ -316,12 +333,12 @@ function parseType(t: Token[]): [Token[], Type] {
       }
       [t] = expectNext(t, ")");
 
-      return [t, { kind: "tuple", elems, span: tok.span }];
+      return [t, { kind: "tuple", elems, span }];
     }
     default: {
       throw new CompilerError(
         `unexpected token: \`${tok.kind}\`, expected type`,
-        tok.span
+        span
       );
     }
   }
diff --git a/src/printer.ts b/src/printer.ts
index bfd9c82..ec330dd 100644
--- a/src/printer.ts
+++ b/src/printer.ts
@@ -1,6 +1,14 @@
-import { Expr, FunctionDef, Item, Type } from "./ast";
+import {
+  Ast,
+  Expr,
+  FunctionDef,
+  Identifier,
+  Item,
+  Resolution,
+  Type,
+} from "./ast";
 
-export function printAst(ast: Item[]): string {
+export function printAst(ast: Ast): string {
   return ast.map(printItem).join("\n");
 }
 
@@ -67,7 +75,7 @@ function printExpr(expr: Expr, indent: number): string {
       }
     }
     case "ident": {
-      return expr.value;
+      return printIdent(expr.value);
     }
     case "binary": {
       return `${printExpr(expr.lhs, indent)} ${expr.binaryKind} ${printExpr(
@@ -107,7 +115,7 @@ function printExpr(expr: Expr, indent: number): string {
 function printType(type: Type): string {
   switch (type.kind) {
     case "ident":
-      return type.value;
+      return printIdent(type.value);
     case "list":
       return `[${printType(type.elem)}]`;
     case "tuple":
@@ -115,6 +123,22 @@ function printType(type: Type): string {
   }
 }
 
+function printIdent(ident: Identifier): string {
+  const printRes = (res: Resolution): string => {
+    switch (res.kind) {
+      case "local":
+        return `#${res.index}`;
+      case "item":
+        return `#G${res.index}`;
+      case "builtin": {
+        return `#B`;
+      }
+    }
+  };
+  const res = ident.res ? printRes(ident.res) : "";
+  return `${ident.name}${res}`;
+}
+
 function linebreak(indent: number): string {
   return `\n${ind(indent)}`;
 }
diff --git a/src/resolve.ts b/src/resolve.ts
new file mode 100644
index 0000000..a351a62
--- /dev/null
+++ b/src/resolve.ts
@@ -0,0 +1,130 @@
+import {
+  Ast,
+  DEFAULT_FOLDER,
+  Folder,
+  Identifier,
+  Resolution,
+  fold_ast,
+  super_fold_expr,
+  super_fold_item,
+} from "./ast";
+import { CompilerError } from "./error";
+
+const BUILTINS = new Set<string>(["print", "String"]);
+
+export function resolve(ast: Ast): Ast {
+  const items = new Map<string, number>();
+
+  for (let i = 0; i < ast.length; i++) {
+    const item = ast[i];
+    const existing = items.get(item.node.name);
+    if (existing !== undefined) {
+      throw new CompilerError(
+        `item \`${item.node.name}\` has already been declared`,
+        item.span
+      );
+    }
+    items.set(item.node.name, i);
+  }
+
+  const scopes: string[] = [];
+
+  const popScope = (expected: string) => {
+    const popped = scopes.pop();
+    if (popped !== expected) {
+      throw new Error(
+        `Scopes corrupted, wanted to pop ${name} but popped ${popped}`
+      );
+    }
+  };
+
+  const resolveIdent = (ident: Identifier): Resolution => {
+    const lastIdx = scopes.length - 1;
+    for (let i = lastIdx; i >= 0; i--) {
+      const candidate = scopes[i];
+      if (candidate === ident.name) {
+        const index = lastIdx - i;
+        return {
+          kind: "local",
+          index,
+        };
+      }
+    }
+
+    const item = items.get(ident.name);
+    if (item !== undefined) {
+      return {
+        kind: "item",
+        index: item,
+      };
+    }
+
+    if (BUILTINS.has(ident.name)) {
+      return { kind: "builtin" };
+    }
+    
+    throw new CompilerError(`cannot find ${ident.name}`, ident.span);
+  };
+
+  const resolver: Folder = {
+    ...DEFAULT_FOLDER,
+    item(item) {
+      switch (item.kind) {
+        case "function": {
+          const args = item.node.args.map(({ name, span, type }) => ({
+            name,
+            span,
+            type: this.type(type),
+          }));
+          const returnType =
+            item.node.returnType && this.type(item.node.returnType);
+
+          item.node.args.forEach(({ name }) => scopes.push(name));
+          const body = super_fold_expr(item.node.body, this);
+          item.node.args.forEach(({ name }) => popScope(name));
+
+          return {
+            kind: "function",
+            span: item.span,
+            node: {
+              name: item.node.name,
+              args,
+              returnType,
+              body,
+            },
+          };
+        }
+      }
+
+      return super_fold_item(item, this);
+    },
+    expr(expr) {
+      if (expr.kind === "let") {
+        const rhs = this.expr(expr.rhs);
+        const type = expr.type && this.type(expr.type);
+        scopes.push(expr.name);
+        const after = this.expr(expr.after);
+        popScope(expr.name);
+
+        return {
+          kind: "let",
+          name: expr.name,
+          rhs,
+          after,
+          type,
+          span: expr.span,
+        };
+      }
+
+      return super_fold_expr(expr, this);
+    },
+    ident(ident) {
+      const res = resolveIdent(ident);
+      return { name: ident.name, span: ident.span, res };
+    },
+  };
+
+  const resolved = fold_ast(ast, resolver);
+
+  return resolved;
+}