diff --git a/program.ethlang b/program.ethlang
index 8ad6b2d..b43b843 100644
--- a/program.ethlang
+++ b/program.ethlang
@@ -6,9 +6,8 @@ fn add(a: int, b: int) -> int
 
 fn main()
 {
-    let sum = add(2, 3);
+    let sum: void = add(2, 3);
     print_int(sum);
 }
 
-// vim: syntax=rust
 
diff --git a/src/main.ts b/src/main.ts
index 9461669..e00a6f7 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -1,6 +1,7 @@
 import * as ast from "./ast.ts";
 import * as front from "./front.ts";
 import * as middle from "./middle.ts";
+import { MirInterpreter } from "./mir_interpreter.ts";
 
 const filename = Deno.args[0];
 const text = await Deno.readTextFile(filename);
@@ -34,3 +35,6 @@ const m = new middle.MiddleLowerer(resols, checker);
 const mainMiddleFn = m.lowerFn(mainFn);
 
 console.log(mainMiddleFn.pretty());
+
+const interp = new MirInterpreter();
+interp.eval(mainMiddleFn);
diff --git a/src/middle.ts b/src/middle.ts
index 0768ac7..0bd8d24 100644
--- a/src/middle.ts
+++ b/src/middle.ts
@@ -34,6 +34,7 @@ class FnLowerer {
     lower(): Fn {
         const ty = this.checker.check(this.stmt);
         this.lowerBlock(this.stmt.kind.body.as("Block"));
+        this.pushInst(Ty.Void, "Return", { source: this.makeVoid() });
         return new Fn(this.stmt, ty, this.bbs);
     }
 
diff --git a/src/mir_interpreter.ts b/src/mir_interpreter.ts
new file mode 100644
index 0000000..e049c4a
--- /dev/null
+++ b/src/mir_interpreter.ts
@@ -0,0 +1,113 @@
+import * as mir from "./middle.ts";
+
+export class MirInterpreter {
+    constructor() {}
+
+    eval(fn: mir.Fn) {
+        this.evalFn(fn, []);
+    }
+
+    private evalFn(fn: mir.Fn, args: Val[]): Val {
+        const regs = new Map<mir.Inst, Val>();
+        const locals: (Val | null)[] = [];
+        const localMap = new Map<mir.Inst, number>();
+
+        let bb = fn.bbs[0];
+        for (const inst of bb.insts) {
+            const k = inst.kind;
+            switch (k.tag) {
+                case "Error":
+                    throw new Error();
+                case "Void":
+                case "Int":
+                case "Fn":
+                    regs.set(inst, new Val(k));
+                    continue;
+                case "Param":
+                    regs.set(inst, args[k.idx]);
+                    continue;
+                case "Call": {
+                    const fn = regs.get(k.callee);
+                    if (!fn || fn.kind.tag !== "Fn") {
+                        throw new Error();
+                    }
+                    const args = k.args.map((arg) => regs.get(arg)!);
+                    const val = this.evalFn(fn.kind.fn, args);
+                    regs.set(inst, val);
+                    continue;
+                }
+                case "AllocLocal":
+                    localMap.set(inst, locals.length);
+                    locals.push(null);
+                    continue;
+                case "LocalLoad":
+                    if (!localMap.has(k.source)) {
+                        throw new Error();
+                    }
+                    if (locals[localMap.get(k.source)!] === null) {
+                        throw new Error();
+                    }
+                    regs.set(inst, locals[localMap.get(k.source)!]!);
+                    continue;
+                case "LocalStore":
+                    if (!localMap.has(k.target)) {
+                        throw new Error();
+                    }
+                    locals[localMap.get(k.target)!] = regs.get(k.source)!;
+                    continue;
+                case "Return":
+                    return regs.get(k.source)!;
+                case "Add": {
+                    const left = regs.get(k.left)!;
+                    const right = regs.get(k.right)!;
+                    if (left.kind.tag === "Int" && right.kind.tag == "Int") {
+                        regs.set(
+                            inst,
+                            new Val({
+                                tag: "Int",
+                                value: left.kind.value + right.kind.value,
+                            }),
+                        );
+                        continue;
+                    }
+                    throw new Error();
+                }
+                case "DebugPrint":
+                    console.log(
+                        `debug: ${
+                            k.args.map((a) => regs.get(a)!.pretty()).join(", ")
+                        }`,
+                    );
+                    continue;
+            }
+            const _: never = k;
+        }
+        return Val.Void;
+    }
+}
+
+class Val {
+    constructor(
+        public kind: ValKind,
+    ) {}
+
+    static Void = new Val({ tag: "Void" });
+
+    pretty() {
+        const k = this.kind;
+        switch (k.tag) {
+            case "Void":
+                return "void";
+            case "Int":
+                return `${k.value}`;
+            case "Fn":
+                return `<${k.fn.ty.pretty()}>`;
+        }
+        const _: never = k;
+    }
+}
+
+type ValKind =
+    | { tag: "Void" }
+    | { tag: "Int"; value: number }
+    | { tag: "Fn"; fn: mir.Fn };