diff --git a/program.ethlang b/program.ethlang
index b43b843..a650c4a 100644
--- a/program.ethlang
+++ b/program.ethlang
@@ -1,13 +1,14 @@
 
 fn add(a: int, b: int) -> int
 {
-    return __add(a, b);
+    return a + b;
 }
 
 fn main()
 {
-    let sum: void = add(2, 3);
+    let sum = add(2, 3);
     print_int(sum);
+    print_int(2 - 3);
 }
 
 
diff --git a/src/ast.ts b/src/ast.ts
index a6fdde5..4bd09da 100644
--- a/src/ast.ts
+++ b/src/ast.ts
@@ -80,6 +80,8 @@ export class Node {
                 return visit();
             case "CallExpr":
                 return visit(k.expr, ...k.args);
+            case "BinaryExpr":
+                return visit(k.left, k.right);
             case "IdentTy":
                 return visit();
         }
@@ -106,8 +108,29 @@ export type NodeKind =
     | { tag: "IdentExpr"; ident: string }
     | { tag: "IntExpr"; value: number }
     | { tag: "CallExpr"; expr: Node; args: Node[] }
+    | { tag: "BinaryExpr"; op: BinaryOp; left: Node; right: Node; tok: string }
     | { tag: "IdentTy"; ident: string };
 
+export type BinaryOp =
+    | "Or"
+    | "And"
+    | "Eq"
+    | "Ne"
+    | "Lt"
+    | "Gt"
+    | "Lte"
+    | "Gte"
+    | "BitOr"
+    | "BitXor"
+    | "BitAnd"
+    | "Shl"
+    | "Shr"
+    | "Add"
+    | "Subtract"
+    | "Multiply"
+    | "Divide"
+    | "Remainder";
+
 export interface Visitor {
     visit(node: Node): void | "break";
 }
diff --git a/src/front.ts b/src/front.ts
index 68f0f7a..cef61b3 100644
--- a/src/front.ts
+++ b/src/front.ts
@@ -1,23 +1,6 @@
 import * as ast from "./ast.ts";
 import { Ty } from "./ty.ts";
 
-const rootSyms = [
-    {
-        id: "print_int",
-        ty: Ty.create("Fn", {
-            params: [Ty.Int],
-            retTy: Ty.Void,
-        }),
-    },
-    {
-        id: "__add",
-        ty: Ty.create("Fn", {
-            params: [Ty.Int, Ty.Int],
-            retTy: Ty.Int,
-        }),
-    },
-];
-
 export class Checker {
     private nodeTys = new Map<number, Ty>();
 
@@ -76,7 +59,7 @@ export class Checker {
                 return this.check(sym.stmt);
             }
             if (sym.tag === "Builtin") {
-                return rootSyms.find((s) => s.id === sym.id)!.ty;
+                return builtins.find((s) => s.id === sym.id)!.ty;
             }
             if (sym.tag === "FnParam") {
                 return this.check(sym.param);
@@ -95,6 +78,25 @@ export class Checker {
             return this.checkCall(node);
         }
 
+        if (node.is("BinaryExpr")) {
+            const left = this.check(node.kind.left);
+            const right = this.check(node.kind.right);
+            const binaryOp = binaryOpPatterns
+                .find((pat) =>
+                    pat.op === node.kind.op &&
+                    left.compatibleWith(pat.left) &&
+                    right.compatibleWith(pat.right)
+                );
+            if (!binaryOp) {
+                this.error(
+                    node.line,
+                    `operator '${node.kind.tok}' cannot be applied to types '${left.pretty()}' and '${right.pretty()}'`,
+                );
+                this.fail();
+            }
+            return binaryOp.result;
+        }
+
         if (node.is("IdentTy")) {
             switch (node.kind.ident) {
                 case "void":
@@ -231,6 +233,18 @@ export class Checker {
     }
 }
 
+type BinaryOpPattern = {
+    op: ast.BinaryOp;
+    left: Ty;
+    right: Ty;
+    result: Ty;
+};
+
+const binaryOpPatterns: BinaryOpPattern[] = [
+    { op: "Add", left: Ty.Int, right: Ty.Int, result: Ty.Int },
+    { op: "Subtract", left: Ty.Int, right: Ty.Int, result: Ty.Int },
+];
+
 export type Sym =
     | { tag: "Error" }
     | { tag: "Builtin"; id: string }
@@ -264,7 +278,7 @@ class ResolverSyms {
     static root(): ResolverSyms {
         return new ResolverSyms(
             new Map(
-                rootSyms.map<[string, Sym]>((sym) => [
+                builtins.map<[string, Sym]>((sym) => [
                     sym.id,
                     { tag: "Builtin", id: sym.id },
                 ]),
@@ -365,6 +379,28 @@ export function resolve(
     return new ResolveMap(resols);
 }
 
+type Builtin = {
+    id: string;
+    ty: Ty;
+};
+
+const builtins: Builtin[] = [
+    {
+        id: "print_int",
+        ty: Ty.create("Fn", {
+            params: [Ty.Int],
+            retTy: Ty.Void,
+        }),
+    },
+    {
+        id: "__add",
+        ty: Ty.create("Fn", {
+            params: [Ty.Int, Ty.Int],
+            retTy: Ty.Int,
+        }),
+    },
+];
+
 export function parse(
     filename: string,
     text: string,
@@ -480,6 +516,57 @@ export class Parser {
     }
 
     parseExpr(): ast.Node {
+        return this.parseBinary();
+    }
+
+    parseBinary(prec = 7): ast.Node {
+        const loc = this.loc();
+        if (prec == 0) {
+            return this.parsePrefixE();
+        }
+        const ops: [Tok["type"], ast.BinaryOp, number][] = [
+            ["or", "Or", 9],
+            ["and", "And", 8],
+            ["==", "Eq", 7],
+            ["!=", "Ne", 7],
+            ["<", "Lt", 7],
+            [">", "Gt", 7],
+            ["<=", "Lte", 7],
+            [">=", "Gte", 7],
+            ["|", "BitOr", 6],
+            ["^", "BitXor", 5],
+            ["&", "BitAnd", 4],
+            ["<<", "Shl", 3],
+            [">>", "Shr", 3],
+            ["+", "Add", 2],
+            ["-", "Subtract", 2],
+            ["*", "Multiply", 1],
+            ["/", "Divide", 1],
+            ["%", "Remainder", 1],
+        ];
+
+        let left = this.parseBinary(prec - 1);
+
+        let should_continue = true;
+        while (should_continue) {
+            should_continue = false;
+            for (const [tok, op, p] of ops) {
+                if (prec >= p && this.eat(tok)) {
+                    const right = this.parseBinary(prec - 1);
+                    left = ast.Node.create(
+                        loc,
+                        "BinaryExpr",
+                        { op, left, right, tok },
+                    );
+                    should_continue = true;
+                    break;
+                }
+            }
+        }
+        return left;
+    }
+
+    parsePrefixE() {
         return this.parsePostfix();
     }
 
@@ -588,13 +675,15 @@ export class Parser {
 
 export type Tok = { type: string; value: string; line: number };
 
-const keywordRegex =
-    /^(?:fn)|(?:return)|(?:let)|(?:if)|(?:else)|(?:while)|(?:break)$/;
+const keywordPattern =
+    /^(?:fn)|(?:return)|(?:let)|(?:if)|(?:else)|(?:while)|(?:break)|(?:or)|(?:and)$/;
+const operatorPattern =
+    /((?:\->)|(?:==)|(?:!=)|(?:<=)|(?:>=)|(?:<<)|(?:>>)|[\n\(\)\{\}\,\.\;\:\!\=\<\>\&\^\|\+\-\*\/\%])/g;
 
 export function tokenize(text: string): Tok[] {
     return text
         .replace(/\/\/[^\n]*/g, "")
-        .replace(/((?:\-\>)|[\n\(\)\{\},.;:])/g, " $1 ")
+        .replace(operatorPattern, " $1 ")
         .split(/[ \t\r]/)
         .filter((value) => value !== "")
         .reduce<[[string, number][], number]>(
@@ -612,7 +701,7 @@ export function tokenize(text: string): Tok[] {
             /^[a-zA-Z_][a-zA-Z0-9_]*$/.test(tok.value)
                 ? {
                     ...tok,
-                    type: keywordRegex.test(tok.value) ? tok.value : "ident",
+                    type: keywordPattern.test(tok.value) ? tok.value : "ident",
                 }
                 : tok
         )
diff --git a/src/main.ts b/src/main.ts
index e00a6f7..9974db1 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -34,7 +34,9 @@ if (!mainFn) {
 const m = new middle.MiddleLowerer(resols, checker);
 const mainMiddleFn = m.lowerFn(mainFn);
 
-console.log(mainMiddleFn.pretty());
+if (!Deno.args.includes("--test")) {
+    console.log(mainMiddleFn.pretty());
+}
 
 const interp = new MirInterpreter();
 interp.eval(mainMiddleFn);
diff --git a/src/middle.ts b/src/middle.ts
index 0bd8d24..186b83e 100644
--- a/src/middle.ts
+++ b/src/middle.ts
@@ -63,6 +63,12 @@ class FnLowerer {
             this.pushInst(Ty.Void, "Return", { source });
             return;
         }
+        if (stmt.is("AssignStmt")) {
+            const source = this.lowerExpr(stmt.kind.expr);
+            const target = this.lowerAssignPlace(stmt.kind.place);
+            this.pushInst(Ty.Void, "LocalStore", { target, source });
+            return;
+        }
         if (stmt.is("ExprStmt")) {
             this.lowerExpr(stmt.kind.expr);
             return;
@@ -70,6 +76,21 @@ class FnLowerer {
         throw new Error(`'${stmt.kind.tag}' not handled`);
     }
 
+    private lowerAssignPlace(place: ast.Node): Inst {
+        if (place.is("IdentExpr")) {
+            const sym = this.resols.get(place);
+            if (sym.tag === "Let") {
+                const local = this.localMap.get(sym.param.id);
+                if (!local) {
+                    throw new Error();
+                }
+                return local;
+            }
+            throw new Error(`'${sym.tag}' not handled`);
+        }
+        throw new Error(`'${place.kind.tag}' not handled`);
+    }
+
     private lowerExpr(expr: ast.Node): Inst {
         if (expr.is("IdentExpr")) {
             const sym = this.resols.get(expr);
@@ -118,6 +139,19 @@ class FnLowerer {
             const callee = this.lowerExpr(expr.kind.expr);
             return this.pushInst(ty, "Call", { callee, args });
         }
+        if (expr.is("BinaryExpr")) {
+            const ty = this.checker.check(expr);
+            const binaryOp = binaryOpPatterns
+                .find((pat) =>
+                    expr.kind.op === pat.op && ty.compatibleWith(pat.ty)
+                );
+            if (!binaryOp) {
+                throw new Error();
+            }
+            const left = this.lowerExpr(expr.kind.left);
+            const right = this.lowerExpr(expr.kind.right);
+            return this.pushInst(ty, binaryOp.tag, { left, right });
+        }
         throw new Error(`'${expr.kind.tag}' not handled`);
     }
 
@@ -138,6 +172,17 @@ class FnLowerer {
     }
 }
 
+type BinaryOpPattern = {
+    op: ast.BinaryOp;
+    ty: Ty;
+    tag: BinaryOp;
+};
+
+const binaryOpPatterns: BinaryOpPattern[] = [
+    { op: "Add", ty: Ty.Int, tag: "Add" },
+    { op: "Subtract", ty: Ty.Int, tag: "Sub" },
+];
+
 export class Fn {
     constructor(
         public stmt: ast.FnStmt,
@@ -237,7 +282,22 @@ export class Inst {
                         return ` ${r(k.target)}, ${r(k.source)}`;
                     case "Return":
                         return ` ${r(k.source)}`;
+                    case "Eq":
+                    case "Ne":
+                    case "Lt":
+                    case "Gt":
+                    case "Lte":
+                    case "Gte":
+                    case "BitOr":
+                    case "BitXor":
+                    case "BitAnd":
+                    case "Shl":
+                    case "Shr":
                     case "Add":
+                    case "Sub":
+                    case "Mul":
+                    case "Div":
+                    case "Rem":
                         return ` ${r(k.left)} ${r(k.right)}`;
                     case "DebugPrint":
                         return ` ${k.args.map(r).join(", ")}`;
@@ -259,5 +319,23 @@ export type InstKind =
     | { tag: "LocalLoad"; source: Inst }
     | { tag: "LocalStore"; target: Inst; source: Inst }
     | { tag: "Return"; source: Inst }
-    | { tag: "Add"; left: Inst; right: Inst }
+    | { tag: BinaryOp; left: Inst; right: Inst }
     | { tag: "DebugPrint"; args: Inst[] };
+
+export type BinaryOp =
+    | "Eq"
+    | "Ne"
+    | "Lt"
+    | "Gt"
+    | "Lte"
+    | "Gte"
+    | "BitOr"
+    | "BitXor"
+    | "BitAnd"
+    | "Shl"
+    | "Shr"
+    | "Add"
+    | "Sub"
+    | "Mul"
+    | "Div"
+    | "Rem";
diff --git a/src/mir_interpreter.ts b/src/mir_interpreter.ts
index e049c4a..792ba8a 100644
--- a/src/mir_interpreter.ts
+++ b/src/mir_interpreter.ts
@@ -57,26 +57,63 @@ export class MirInterpreter {
                     continue;
                 case "Return":
                     return regs.get(k.source)!;
-                case "Add": {
+                case "Eq":
+                case "Ne":
+                case "Lt":
+                case "Gt":
+                case "Lte":
+                case "Gte":
+                case "BitOr":
+                case "BitXor":
+                case "BitAnd":
+                case "Shl":
+                case "Shr":
+                case "Add":
+                case "Sub":
+                case "Mul":
+                case "Div":
+                case "Rem": {
                     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,
-                            }),
-                        );
+                    const lk = left.kind;
+                    const rk = right.kind;
+
+                    if (lk.tag === "Int" && rk.tag === "Int") {
+                        const value = (() => {
+                            switch (k.tag) {
+                                case "Eq":
+                                case "Ne":
+                                case "Lt":
+                                case "Gt":
+                                case "Lte":
+                                case "Gte":
+                                case "BitOr":
+                                case "BitXor":
+                                case "BitAnd":
+                                case "Shl":
+                                case "Shr":
+                                    break;
+                                case "Add":
+                                    return lk.value + rk.value;
+                                case "Sub":
+                                    return lk.value - rk.value;
+                                case "Mul":
+                                case "Div":
+                                case "Rem":
+                                    break;
+                            }
+                            throw new Error(`'${k.tag}' not handled`);
+                        })();
+                        regs.set(inst, new Val({ tag: "Int", value }));
                         continue;
                     }
-                    throw new Error();
+                    throw new Error(`'${k.tag}' not handled`);
                 }
                 case "DebugPrint":
                     console.log(
-                        `debug: ${
-                            k.args.map((a) => regs.get(a)!.pretty()).join(", ")
-                        }`,
+                        k.args
+                            .map((a) => regs.get(a)!.pretty())
+                            .join(", "),
                     );
                     continue;
             }
diff --git a/tests/assign.ethlang b/tests/assign.ethlang
new file mode 100644
index 0000000..1f61661
--- /dev/null
+++ b/tests/assign.ethlang
@@ -0,0 +1,9 @@
+// expect: 456
+
+fn main()
+{
+    let v: int = 123;
+    v = 456;
+    print_int(v);
+}
+
diff --git a/tests/fn_int.ethlang b/tests/fn_int.ethlang
new file mode 100644
index 0000000..d1bdbb2
--- /dev/null
+++ b/tests/fn_int.ethlang
@@ -0,0 +1,10 @@
+
+fn my_int_fn() -> int {
+    return 123;
+}
+
+fn main()
+{
+    my_int_fn();
+}
+
diff --git a/tests/fn_void.ethlang b/tests/fn_void.ethlang
new file mode 100644
index 0000000..7e32da5
--- /dev/null
+++ b/tests/fn_void.ethlang
@@ -0,0 +1,12 @@
+
+fn my_implicit_void() {}
+
+fn my_explicit_void() -> void {}
+
+fn main()
+{
+    my_implicit_void();
+    my_explicit_void();
+}
+
+
diff --git a/tests/let.ethlang b/tests/let.ethlang
new file mode 100644
index 0000000..c702037
--- /dev/null
+++ b/tests/let.ethlang
@@ -0,0 +1,8 @@
+
+fn main()
+{
+    let a = 123;
+    let b: int = 321;
+    let c = b;
+}
+
diff --git a/tests/operators.ethlang b/tests/operators.ethlang
new file mode 100644
index 0000000..ed623bd
--- /dev/null
+++ b/tests/operators.ethlang
@@ -0,0 +1,12 @@
+// expect: 8
+// expect: 2
+
+fn main()
+{
+    let a = 5;
+    let b = 3;
+
+    print_int(a + b);
+    print_int(a - b);
+}
+
diff --git a/tests/test.sh b/tests/test.sh
new file mode 100755
index 0000000..24a0aa3
--- /dev/null
+++ b/tests/test.sh
@@ -0,0 +1,61 @@
+#!/bin/bash
+
+set -e
+
+TEST_DIR=$(dirname $0)
+SRC_DIR=$TEST_DIR/../src
+
+TEST_SRC=$(fd '\.ethlang' $TEST_DIR)
+
+count_total=0
+count_succeeded=0
+
+for test_file in $TEST_SRC
+do
+    echo "- $(basename $test_file)"
+    set +e
+    output=$(deno run -A $SRC_DIR/main.ts $test_file --test)
+    status=$?
+    set -e
+
+    
+    if [[ status -ne 0 ]]
+    then
+        echo "-- failed: exit code $status --"
+    fi
+
+    if [[ status -eq 0 ]]
+    then
+        if grep -q '// expect:' $test_file
+        then
+            expected=$(grep '// expect:' $test_file | sed -E 's/\/\/ expect: (.*?)/\1/g')
+            if [[ $output != $expected ]]
+            then
+                echo "-- failed: incorrect output --"
+                echo "-- expected --"
+                echo "$expected"
+                echo "-- actual --"
+                echo "$output"
+                status=1
+            fi
+        fi
+    fi
+    
+    count_total=$(($count_total + 1))
+    if [[ status -eq 0 ]]
+    then
+        count_succeeded=$(($count_succeeded + 1))
+    else
+        echo "failed"
+    fi
+
+done
+
+if [[ $count_succeeded -eq $count_total ]]
+then
+    echo "=== all tests passed ($count_succeeded/$count_total passed) ===" 
+else
+    echo "=== tests failed ($count_succeeded/$count_total passed) ==="
+fi
+
+