diff --git a/src/ast.ts b/src/ast.ts
index 5a2e946..daa0a79 100644
--- a/src/ast.ts
+++ b/src/ast.ts
@@ -95,11 +95,16 @@ export type NodeWithKind<
     Tag extends NodeKind["tag"],
 > = Node & { kind: { tag: Tag } };
 
-export function assertNodeWithKind<Tag extends NodeKind["tag"]>(
-    node: Node,
-    tag: Tag,
-): asserts node is NodeWithKind<Tag> {
+export function assertNodeWithKind<
+    Tag extends NodeKind["tag"],
+>(node: Node, tag: Tag): asserts node is NodeWithKind<Tag> {
     if (node.kind.tag !== tag) {
         throw new Error();
     }
 }
+
+export function isNodeWithKind<
+    Tag extends NodeKind["tag"],
+>(node: Node, tag: Tag): node is NodeWithKind<Tag> {
+    return node.kind.tag === tag;
+}
diff --git a/src/front.ts b/src/front.ts
index be275c3..eb68cea 100644
--- a/src/front.ts
+++ b/src/front.ts
@@ -24,10 +24,52 @@ export class Checker {
     private checkNode(node: ast.Node): Ty {
         const k = node.kind;
 
-        if (k.tag === "FnStmt") {
+        if (ast.isNodeWithKind(node, "FnStmt")) {
+            return this.checkFnStmt(node);
         }
 
-        throw new Error("not checked");
+        if (ast.isNodeWithKind(node, "IdentTy")) {
+            switch (node.kind.ident) {
+                case "int":
+                    return Ty.Int;
+                default:
+                    this.error(node.line, `unknown type '${node.kind.ident}'`);
+            }
+        }
+
+        throw new Error(`'${k.tag}' not checked`);
+    }
+
+    private checkFnStmt(stmt: ast.NodeWithKind<"FnStmt">): Ty {
+        const k = stmt.kind;
+
+        const params = k.params.map((param) => this.check(param));
+        const retTy = k.retTy ? this.check(k.retTy) : Ty.Void;
+
+        k.body.visit({
+            visit: (node) => {
+                if (ast.isNodeWithKind(node, "ReturnStmt")) {
+                    if (node.kind.expr) {
+                        const ty = this.check(node.kind.expr);
+                    } else {
+                    }
+                }
+            },
+        });
+
+        const ty = Ty.create("Fn", { params, retTy });
+        return Ty.create("FnStmt", { stmt, ty });
+    }
+
+    private typesCompatible(a: Ty, b: Ty): boolean {
+        const ak = a.kind;
+        const bk = b.kind;
+        if (ak.tag === "Error") {
+            return false;
+        }
+        if (ak.tag === "Void") {
+            return bk.tag === "Void";
+        }
     }
 
     private error(line: number, message: string): never {
diff --git a/src/ty.ts b/src/ty.ts
index f2b474f..37bec0b 100644
--- a/src/ty.ts
+++ b/src/ty.ts
@@ -33,6 +33,26 @@ export class Ty {
         public id: number,
         public kind: TyKind,
     ) {}
+
+    isKind<
+        Tag extends TyKind["tag"],
+    >(tag: Tag): this is Ty & { kind: { tag: Tag } } {
+        return this.kind.tag === tag;
+    }
+
+    compatibleWith(other: Ty): boolean {
+        if (this.isKind("Error")) {
+            return false;
+        }
+        if (this.isKind("Void")) {
+            return other.isKind("Void");
+        }
+        if (this.isKind("Int")) {
+            return other.isKind("Int");
+        }
+        if (this.isKind("Fn")) {
+        }
+    }
 }
 
 export type TyKind =