diff --git a/src/ast.ts b/src/ast.ts
index ea45c26..e98fd66 100644
--- a/src/ast.ts
+++ b/src/ast.ts
@@ -1,5 +1,6 @@
 import { ErrorEmitted, LoadedFile, Span, unreachable } from "./error";
 import { LitIntType } from "./lexer";
+import { Ty, TyFn, TypeckResults } from "./types";
 import { ComplexMap } from "./utils";
 import { Instr, ValType } from "./wasm/defs";
 
@@ -507,161 +508,6 @@ export type LocalInfo = {
 
 // types
 
-export type TyString = {
-  kind: "string";
-};
-
-export type TyInt = {
-  kind: "int";
-};
-
-export type TyI32 = {
-  kind: "i32";
-};
-
-export type TyBool = {
-  kind: "bool";
-};
-
-export type TyTuple = {
-  kind: "tuple";
-  elems: Ty[];
-};
-
-export type TyUnit = {
-  kind: "tuple";
-  elems: [];
-};
-
-export type TyFn = {
-  kind: "fn";
-  params: Ty[];
-  returnTy: Ty;
-};
-
-export type TyVar = {
-  kind: "var";
-  index: number;
-};
-
-export type TyStruct = {
-  kind: "struct";
-  itemId: ItemId;
-  params: string[];
-  genericArgs: Ty[];
-  _name: string;
-  fields_no_subst: [string, Ty][];
-};
-
-export type TyRawPtr = {
-  kind: "rawptr";
-  inner: Ty;
-};
-
-export type TyNever = {
-  kind: "never";
-};
-
-export type TyParam = {
-  kind: "param";
-  /**
-   * The index of the type parameter of the parent.
-   * If the parent is `type A[T, U] = U;`
-   * then `U` will have index 1.
-   */
-  idx: number;
-  name: string;
-};
-
-export type TyAlias = {
-  kind: "alias";
-  actual: Ty;
-  genericArgs: Ty[];
-  params: string[];
-};
-
-export type TyError = {
-  kind: "error";
-  err: ErrorEmitted;
-};
-
-export type Ty =
-  | TyString
-  | TyInt
-  | TyI32
-  | TyBool
-  | TyTuple
-  | TyFn
-  | TyVar
-  | TyStruct
-  | TyRawPtr
-  | TyNever
-  | TyParam
-  | TyAlias
-  | TyError;
-
-export function tyIsUnit(ty: Ty): ty is TyUnit {
-  return ty.kind === "tuple" && ty.elems.length === 0;
-}
-
-export const TY_UNIT: Ty = { kind: "tuple", elems: [] };
-export const TY_STRING: Ty = { kind: "string" };
-export const TY_BOOL: Ty = { kind: "bool" };
-export const TY_INT: Ty = { kind: "int" };
-export const TY_I32: Ty = { kind: "i32" };
-export const TY_NEVER: Ty = { kind: "never" };
-
-export type TypeckResults = {
-  main: Resolution | undefined;
-};
-
-export function structFieldsSubstituted(ty: TyStruct): [string, Ty][] {
-  const args = ty.genericArgs;
-  return ty.fields_no_subst.map(([name, type]) => [
-    name,
-    substituteTy(args, type),
-  ]);
-}
-
-// Substitute the parameter of a type. We are only able to handle one
-// level of generic definitions, for example for fields the struct def or for exprs the function generics.
-export function substituteTy(genericArgs: Ty[], ty: Ty): Ty {
-  const subst = (ty: Ty) => substituteTy(genericArgs, ty);
-  switch (ty.kind) {
-    case "param":
-      if (ty.idx >= genericArgs.length) {
-        throw new Error(
-          `substitution out of range, param index ${ty.idx} of param ${ty.name} out of range for length ${genericArgs.length}`,
-        );
-      }
-      return genericArgs[ty.idx];
-    case "tuple":
-      return { ...ty, elems: ty.elems.map(subst) };
-    case "fn":
-      return {
-        ...ty,
-        returnTy: subst(ty.returnTy),
-        params: ty.params.map(subst),
-      };
-    case "struct":
-    case "alias":
-      return {
-        ...ty,
-        genericArgs: ty.genericArgs.map(subst),
-      };
-    case "rawptr":
-      return { ...ty, inner: subst(ty.inner) };
-    // Primitives
-    case "var":
-    case "string":
-    case "int":
-    case "i32":
-    case "bool":
-    case "never":
-    case "error":
-      return ty;
-  }
-}
 
 // folders
 
diff --git a/src/codegen.test.ts b/src/codegen.test.ts
index e71e1c4..97804d6 100644
--- a/src/codegen.test.ts
+++ b/src/codegen.test.ts
@@ -1,5 +1,6 @@
-import { ItemId, TY_I32, TY_INT, TyStruct } from "./ast";
+import { ItemId } from "./ast";
 import { layoutOfStruct } from "./codegen";
+import { TY_I32, TY_INT, TyStruct } from "./types";
 
 it("should compute struct layout correctly", () => {
   const ty: TyStruct = {
diff --git a/src/codegen.ts b/src/codegen.ts
index d16bcf6..705205d 100644
--- a/src/codegen.ts
+++ b/src/codegen.ts
@@ -10,22 +10,17 @@ import {
   ItemId,
   LoopId,
   Resolution,
-  Ty,
-  TyFn,
-  TyStruct,
-  TyTuple,
   Typecked,
   mkDefaultFolder,
   superFoldExpr,
   superFoldItem,
   varUnreachable,
-  TyRawPtr,
   paramUnreachable as codegenUnreachableTy,
-  structFieldsSubstituted,
 } from "./ast";
 import { GlobalContext } from "./context";
 import { unreachable } from "./error";
 import { printTy } from "./printer";
+import { Ty, TyFn, TyRawPtr, TyStruct, TyTuple, structFieldsSubstituted } from "./types";
 import { ComplexMap, encodeUtf8, unwrap } from "./utils";
 import * as wasm from "./wasm/defs";
 
diff --git a/src/printer.ts b/src/printer.ts
index 408dafc..ca7534f 100644
--- a/src/printer.ts
+++ b/src/printer.ts
@@ -9,12 +9,10 @@ import {
   ItemMod,
   Resolution,
   StringLiteral,
-  Ty,
   Type,
   ItemType,
-  tyIsUnit,
-  substituteTy,
 } from "./ast";
+import { Ty, substituteTy, tyIsUnit } from "./types";
 
 export function printAst(ast: Pkg<AnyPhase>): string {
   return ast.rootItems.map(printItem).join("\n");
diff --git a/src/typeck/base.ts b/src/typeck/base.ts
index eb42073..83d0a62 100644
--- a/src/typeck/base.ts
+++ b/src/typeck/base.ts
@@ -1,6 +1,7 @@
-import { ItemId, Pkg, Resolved, Ty } from "../ast";
+import { ItemId, Pkg, Resolved } from "../ast";
 import { GlobalContext } from "../context";
 import { CompilerError, ErrorEmitted } from "../error";
+import { Ty } from "../types";
 import { ComplexMap } from "../utils";
 
 export type TypeckCtx = {
diff --git a/src/typeck/expr.ts b/src/typeck/expr.ts
index a9eb8cc..d502ff0 100644
--- a/src/typeck/expr.ts
+++ b/src/typeck/expr.ts
@@ -13,14 +13,6 @@ import {
   Resolution,
   Resolved,
   StructLiteralField,
-  TY_BOOL,
-  TY_I32,
-  TY_INT,
-  TY_NEVER,
-  TY_STRING,
-  TY_UNIT,
-  Ty,
-  TyFn,
   Type,
   Typecked,
   mkDefaultFolder,
@@ -28,6 +20,7 @@ import {
 } from "../ast";
 import { CompilerError, ErrorEmitted, Span, unreachable } from "../error";
 import { printTy } from "../printer";
+import { TY_BOOL, TY_I32, TY_INT, TY_NEVER, TY_STRING, TY_UNIT, Ty, TyFn } from "../types";
 import { INSTRS, Instr, VALTYPES, ValType } from "../wasm/defs";
 import { TypeckCtx, emitError, mkTyFn, tyError, tyErrorFrom } from "./base";
 import { InferContext } from "./infer";
diff --git a/src/typeck/index.ts b/src/typeck/index.ts
index 0b996f3..8c313e6 100644
--- a/src/typeck/index.ts
+++ b/src/typeck/index.ts
@@ -5,17 +5,13 @@ import {
   Item,
   ItemId,
   Resolved,
-  TY_I32,
-  TY_INT,
-  Ty,
-  TyFn,
   Typecked,
   foldAst,
   mkDefaultFolder,
-  tyIsUnit,
 } from "../ast";
 import { GlobalContext } from "../context";
 import { CompilerError, ErrorEmitted, Span } from "../error";
+import { TY_I32, TY_INT, Ty, TyFn, tyIsUnit } from "../types";
 import { ComplexMap } from "../utils";
 import { emitError } from "./base";
 import { checkBody, exprError } from "./expr";
diff --git a/src/typeck/infer.test.ts b/src/typeck/infer.test.ts
index 4453411..49d4a5d 100644
--- a/src/typeck/infer.test.ts
+++ b/src/typeck/infer.test.ts
@@ -1,5 +1,5 @@
-import { TY_INT, TY_STRING, TY_UNIT } from "../ast";
 import { Emitter, ErrorHandler, Span } from "../error";
+import { TY_INT, TY_STRING, TY_UNIT } from "../types";
 import { InferContext } from "./infer";
 
 const SPAN: Span = Span.startOfFile({ content: "" });
diff --git a/src/typeck/infer.ts b/src/typeck/infer.ts
index 9f75947..5a4fa8e 100644
--- a/src/typeck/infer.ts
+++ b/src/typeck/infer.ts
@@ -1,6 +1,6 @@
-import { Ty } from "../ast";
 import { CompilerError, ErrorHandler, Span } from "../error";
 import { printTy } from "../printer";
+import { Ty } from "../types";
 
 type TyVarRes =
   | {
diff --git a/src/typeck/item.ts b/src/typeck/item.ts
index bb5cd15..ef8ea1c 100644
--- a/src/typeck/item.ts
+++ b/src/typeck/item.ts
@@ -1,18 +1,11 @@
 import {
   ItemId,
   Resolved,
-  Ty,
-  TY_BOOL,
-  TY_I32,
-  TY_INT,
-  TY_NEVER,
-  TY_STRING,
-  TY_UNIT,
   Type,
-  substituteTy,
 } from "../ast";
 import { CompilerError, Span } from "../error";
 import { printTy } from "../printer";
+import { TY_BOOL, TY_I32, TY_INT, TY_NEVER, TY_STRING, TY_UNIT, Ty, substituteTy } from "../types";
 import { TypeckCtx, tyError, tyErrorFrom } from "./base";
 
 function builtinAsTy(cx: TypeckCtx, name: string, span: Span): Ty {
diff --git a/src/types.ts b/src/types.ts
new file mode 100644
index 0000000..0b65819
--- /dev/null
+++ b/src/types.ts
@@ -0,0 +1,158 @@
+import { ItemId, Resolution } from "./ast";
+import { ErrorEmitted } from "./error";
+
+export type TyString = {
+  kind: "string";
+};
+
+export type TyInt = {
+  kind: "int";
+};
+
+export type TyI32 = {
+  kind: "i32";
+};
+
+export type TyBool = {
+  kind: "bool";
+};
+
+export type TyTuple = {
+  kind: "tuple";
+  elems: Ty[];
+};
+
+export type TyUnit = {
+  kind: "tuple";
+  elems: [];
+};
+
+export type TyFn = {
+  kind: "fn";
+  params: Ty[];
+  returnTy: Ty;
+};
+
+export type TyVar = {
+  kind: "var";
+  index: number;
+};
+
+export type TyStruct = {
+  kind: "struct";
+  itemId: ItemId;
+  params: string[];
+  genericArgs: Ty[];
+  _name: string;
+  fields_no_subst: [string, Ty][];
+};
+
+export type TyRawPtr = {
+  kind: "rawptr";
+  inner: Ty;
+};
+
+export type TyNever = {
+  kind: "never";
+};
+
+export type TyParam = {
+  kind: "param";
+  /**
+   * The index of the type parameter of the parent.
+   * If the parent is `type A[T, U] = U;`
+   * then `U` will have index 1.
+   */
+  idx: number;
+  name: string;
+};
+
+export type TyAlias = {
+  kind: "alias";
+  actual: Ty;
+  genericArgs: Ty[];
+  params: string[];
+};
+
+export type TyError = {
+  kind: "error";
+  err: ErrorEmitted;
+};
+
+export type Ty =
+  | TyString
+  | TyInt
+  | TyI32
+  | TyBool
+  | TyTuple
+  | TyFn
+  | TyVar
+  | TyStruct
+  | TyRawPtr
+  | TyNever
+  | TyParam
+  | TyAlias
+  | TyError;
+
+export function tyIsUnit(ty: Ty): ty is TyUnit {
+  return ty.kind === "tuple" && ty.elems.length === 0;
+}
+
+export const TY_UNIT: Ty = { kind: "tuple", elems: [] };
+export const TY_STRING: Ty = { kind: "string" };
+export const TY_BOOL: Ty = { kind: "bool" };
+export const TY_INT: Ty = { kind: "int" };
+export const TY_I32: Ty = { kind: "i32" };
+export const TY_NEVER: Ty = { kind: "never" };
+
+export type TypeckResults = {
+  main: Resolution | undefined;
+};
+
+export function structFieldsSubstituted(ty: TyStruct): [string, Ty][] {
+  const args = ty.genericArgs;
+  return ty.fields_no_subst.map(([name, type]) => [
+    name,
+    substituteTy(args, type),
+  ]);
+}
+
+// Substitute the parameter of a type. We are only able to handle one
+// level of generic definitions, for example for fields the struct def or for exprs the function generics.
+export function substituteTy(genericArgs: Ty[], ty: Ty): Ty {
+  const subst = (ty: Ty) => substituteTy(genericArgs, ty);
+  switch (ty.kind) {
+    case "param":
+      if (ty.idx >= genericArgs.length) {
+        throw new Error(
+          `substitution out of range, param index ${ty.idx} of param ${ty.name} out of range for length ${genericArgs.length}`,
+        );
+      }
+      return genericArgs[ty.idx];
+    case "tuple":
+      return { ...ty, elems: ty.elems.map(subst) };
+    case "fn":
+      return {
+        ...ty,
+        returnTy: subst(ty.returnTy),
+        params: ty.params.map(subst),
+      };
+    case "struct":
+    case "alias":
+      return {
+        ...ty,
+        genericArgs: ty.genericArgs.map(subst),
+      };
+    case "rawptr":
+      return { ...ty, inner: subst(ty.inner) };
+    // Primitives
+    case "var":
+    case "string":
+    case "int":
+    case "i32":
+    case "bool":
+    case "never":
+    case "error":
+      return ty;
+  }
+}
diff --git a/ui-tests/type/generics/generics_structs_in_args.stderr b/ui-tests/type/generics/generics_structs_in_args.stderr
index f4499a9..a668fbc 100644
--- a/ui-tests/type/generics/generics_structs_in_args.stderr
+++ b/ui-tests/type/generics/generics_structs_in_args.stderr
@@ -14,4 +14,4 @@ Error: substitution out of range, param index 0 of param T out of range for leng
     at Array.map (<anonymous>)
     at foldAst (/home/nils/projects/riverdelta/target/ast.js:164:34)
 
-Node.js v18.18.2
+Node.js v20.10.0
diff --git a/ui-tests/type/generics/structs.stderr b/ui-tests/type/generics/structs.stderr
index f4499a9..a668fbc 100644
--- a/ui-tests/type/generics/structs.stderr
+++ b/ui-tests/type/generics/structs.stderr
@@ -14,4 +14,4 @@ Error: substitution out of range, param index 0 of param T out of range for leng
     at Array.map (<anonymous>)
     at foldAst (/home/nils/projects/riverdelta/target/ast.js:164:34)
 
-Node.js v18.18.2
+Node.js v20.10.0
diff --git a/ui-tests/type/generics/wrong_amount.stderr b/ui-tests/type/generics/wrong_amount.stderr
index f4499a9..a668fbc 100644
--- a/ui-tests/type/generics/wrong_amount.stderr
+++ b/ui-tests/type/generics/wrong_amount.stderr
@@ -14,4 +14,4 @@ Error: substitution out of range, param index 0 of param T out of range for leng
     at Array.map (<anonymous>)
     at foldAst (/home/nils/projects/riverdelta/target/ast.js:164:34)
 
-Node.js v18.18.2
+Node.js v20.10.0