diff --git a/src/wasm.ts b/src/wasm.ts
new file mode 100644
index 0000000..cf619b6
--- /dev/null
+++ b/src/wasm.ts
@@ -0,0 +1,363 @@
+// Type definitions and encoding of Wasm.
+// See https://webassembly.github.io/spec/core.
+
+// Base types.
+
+export type Vec<T> = T[];
+export type u32 = number;
+export type u64 = number;
+export type f32 = number;
+export type f64 = number;
+export type VecByte = Uint8Array;
+export type Name = string;
+
+// types
+
+export type Numtype = "i32" | "i64" | "f32" | "f64";
+
+export type Vectype = "v128";
+
+export type Reftype = "funcref" | "externref";
+
+export type Valtype = Numtype | Vectype | Reftype;
+
+export type ResultType = Vec<Valtype>;
+
+export type Functype = {
+  params: ResultType;
+  returns: ResultType;
+};
+
+export type Limits = {
+  min: u32;
+  max: u32;
+};
+
+export type Memtype = Limits;
+
+export type Tabletype = {
+  limits: Limits;
+  reftype: Reftype;
+};
+
+export type GlobalType = {
+  mut: Mut;
+  type: Valtype;
+};
+
+export type Mut = "const" | "var";
+
+export type Externtype =
+  | {
+      kind: "func";
+      type: Functype;
+    }
+  | {
+      kind: "table";
+      type: Tabletype;
+    }
+  | {
+      kind: "mem";
+      type: Memtype;
+    }
+  | {
+      kind: "global";
+      type: GlobalType;
+    };
+
+// instructions
+
+// . numeric
+
+export type BitWidth = "32" | "64";
+
+export type Sign = "u" | "s";
+
+export type NumericInstr =
+  | { kind: "i32.const"; imm: u32 }
+  | { kind: "i64.const"; imm: u64 }
+  | { kind: "f32.const"; imm: f32 }
+  | { kind: "f64.const"; imm: f64 }
+  | { kind: `i${BitWidth}.${IUnOp}` }
+  | { kind: `f${BitWidth}.${FUnOp}` }
+  | { kind: `i${BitWidth}.${IBinOp}` }
+  | { kind: `f${BitWidth}.${FBinOp}` }
+  | { kind: ITestOp }
+  | { kind: IRelOp }
+  | { kind: FRelOp }
+  | { kind: `i${BitWidth}.extend8_s` }
+  | { kind: `i${BitWidth}.extend16_s` }
+  | { kind: `i64.extend32_s` }
+  | { kind: "i32.wrap_i64" }
+  | { kind: `i64.extend_i32_${Sign}` }
+  | { kind: `i${BitWidth}.trunc_f${BitWidth}_${Sign}` }
+  | { kind: `i${BitWidth}.trunc_sat_f${BitWidth}_${Sign}` }
+  | { kind: "f32.demote_f64" }
+  | { kind: "f64.promote_f32" }
+  | { kind: `f${BitWidth}.convert_i${BitWidth}_${Sign}` }
+  | { kind: "i32.reinterpret_f32" | "i64.reinterpret_f64" }
+  | { kind: "f32.reinterpret_i32" | "f64.reinterpret_i64" };
+
+export type IUnOp = "clz" | "ctz" | "popcnt";
+
+export type IBinOp =
+  | "add"
+  | "sub"
+  | "mul"
+  | `div_${Sign}`
+  | `rem_${Sign}`
+  | "and"
+  | "or"
+  | "xor"
+  | "shl"
+  | `shr_${Sign}`
+  | "rotl"
+  | "rotr";
+
+export type FUnOp =
+  | "abs"
+  | "neg"
+  | "sqrt"
+  | "ceil"
+  | "floor"
+  | "trunc"
+  | "nearest";
+
+export type FBinOp = "add" | "sub" | "mul" | "div" | "min" | "max" | "copysign";
+
+export type ITestOp = "eqz";
+
+export type IRelOp =
+  | "eq"
+  | "ne"
+  | `lt_${Sign}`
+  | `gt_${Sign}`
+  | `le_${Sign}`
+  | `ge_${Sign}`;
+
+export type FRelOp = "eq" | "ne" | "lt" | "gt" | "le" | "ge";
+
+// . vectors
+
+export type VectorInstr = never;
+
+// . reference
+
+export type ReferenceInstr =
+  | { kind: "ref.null"; imm: Reftype }
+  | { kind: "ref.is_null" }
+  | { kind: "ref.func"; imm: FuncIdx };
+
+// . parametric
+
+export type ParametricInstr =
+  | { kind: "drop" }
+  | { kind: "select"; type?: Valtype[] };
+
+// . variable
+
+export type VariableInstr =
+  | {
+      kind: `local.${"get" | "set" | "tee"}`;
+      imm: LocalIdx;
+    }
+  | {
+      kind: `global.${"get" | "set"}`;
+      imm: LocalIdx;
+    };
+
+// . table
+
+export type TableInstr =
+  | {
+      kind: `table.${"get" | "set" | "size" | "grow" | "fill"}`;
+      imm: TableIdx;
+    }
+  | {
+      kind: "table.copy";
+      imm1: TableIdx;
+      imm2: TableIdx;
+    }
+  | {
+      kind: "table.init";
+      imm1: TableIdx;
+      imm2: ElemIdx;
+    }
+  | {
+      kind: "elem.drop";
+      imm: ElemIdx;
+    };
+
+// . memory
+
+export type MemArg = {
+  offset: u32;
+  align: u32;
+};
+
+export type MemoryInstr =
+  | {
+      kind: `${`${"i" | "f"}${BitWidth}` | "v128"}.${"load" | "store"}`;
+      imm: MemArg;
+    }
+  | {
+      kind: `i${BitWidth}.load${"8" | "16"}_${Sign}`;
+      imm: MemArg;
+    }
+  | { kind: `i64.load32_${Sign}`; imm: MemArg }
+  | {
+      kind: `i${BitWidth}.store${"8" | "16"}`;
+      imm: MemArg;
+    }
+  | { kind: "i64.store32"; imm: MemArg }
+  | {
+      kind: `memory.${"size" | "grow" | "fill" | "copy"}`;
+    }
+  | {
+      kind: "memory.init";
+      imm: DataIdx;
+    }
+  | { kind: "data.drop"; imm: DataIdx };
+
+// . control
+
+export type Blocktype =
+  | { kind: "typeidx"; idx: TypeIdx }
+  | { kind: "valtype"; type?: Valtype };
+
+export type ControlInstr =
+  | {
+      kind: "nop" | "unreachable";
+    }
+  | {
+      kind: "block" | "loop";
+      type: Blocktype;
+      instr: Instr[];
+    }
+  | {
+      kind: "if";
+      type: Blocktype;
+      then: Instr[];
+      else: Instr[];
+    }
+  | {
+      kind: "br" | "br_if";
+      label: LabelIdx;
+    }
+  | {
+      kind: "br_table";
+      labels: LabelIdx[];
+      label: LabelIdx;
+    }
+  | {
+      kind: "return";
+    }
+  | {
+      kind: "call";
+      func: FuncIdx;
+    }
+  | {
+      kind: "call_indirect";
+      table: TableIdx;
+      type: TypeIdx;
+    };
+
+// . final
+
+export type Instr =
+  | NumericInstr
+  | VectorInstr
+  | ReferenceInstr
+  | ParametricInstr
+  | VariableInstr
+  | TableInstr
+  | MemoryInstr
+  | ControlInstr;
+
+export type Expr = Instr[];
+
+// Modules
+
+export type Module = {
+  types: Vec<Functype>;
+  funcs: Vec<Func>;
+  tables: Vec<Table>;
+  mems: Vec<Mem>;
+  globals: Vec<Global>;
+  elems: Vec<Elem>;
+  datas: Vec<Data>;
+  start?: Start;
+  imports: Vec<Import>;
+  exports: Vec<Export>;
+};
+
+export type TypeIdx = u32;
+export type FuncIdx = u32;
+export type TableIdx = u32;
+export type MemIdx = u32;
+export type GlobalIdx = u32;
+export type ElemIdx = u32;
+export type DataIdx = u32;
+export type LocalIdx = u32;
+export type LabelIdx = u32;
+
+export type Func = {
+  type: TypeIdx;
+  locals: Vec<Valtype>;
+  body: Expr;
+};
+
+export type Table = {
+  type: Tabletype;
+};
+
+export type Mem = {
+  type: Memtype;
+};
+
+export type GLobal = {
+  type: GlobalType;
+  init: Expr;
+};
+
+export type Elem = unknown;
+
+export type Data = {
+  init: VecByte;
+  mode: Datamode;
+};
+
+export type Datamode =
+  | { kind: "passive" }
+  | { kind: "active"; memory: MemIdx; offset: Expr };
+
+export type Start = {
+  func: FuncIdx;
+};
+
+export type Export = {
+  name: Name;
+  desc: ExportDesc;
+};
+
+export type ExportDesc =
+  | {
+      kind: "func";
+      idx: FuncIdx;
+    }
+  | { kind: "table"; idx: TableIdx }
+  | { kind: "mem"; idx: MemIdx | { kind: "global"; idx: GlobalIdx } };
+
+export type Import = {
+  module: Name;
+  name: Name;
+  desc: ImportDesc;
+};
+
+export type ImportDesc =
+  | {
+      kind: "func";
+      idx: FuncIdx;
+    }
+  | { kind: "table"; idx: TableIdx }
+  | { kind: "mem"; idx: MemIdx | { kind: "global"; idx: GlobalIdx } };