diff --git a/compiler/Checker.ts b/compiler/Checker.ts
new file mode 100644
index 0000000..28b780b
--- /dev/null
+++ b/compiler/Checker.ts
@@ -0,0 +1,229 @@
+import { EType, Expr } from "./ast.ts";
+import { Pos } from "./Token.ts";
+import { VType, VTypeParam, vtypesEqual, vtypeToString } from "./vtypes.ts";
+
+export class Checker {
+    public report(msg: string, pos: Pos) {
+        console.error(`${msg} at ${pos.line}:${pos.col}`);
+    }
+    public checkEType(etype: EType): VType {
+        const pos = etype.pos;
+        if (etype.kind.type === "ident") {
+            if (etype.kind.value === "null") {
+                return { type: "null" };
+            }
+            if (etype.kind.value === "int") {
+                return { type: "int" };
+            }
+            if (etype.kind.value === "bool") {
+                return { type: "bool" };
+            }
+            if (etype.kind.value === "string") {
+                return { type: "string" };
+            }
+            this.report(`undefined type '${etype.kind.value}'`, pos);
+            return { type: "error" };
+        }
+        if (etype.kind.type === "array") {
+            const inner = this.checkEType(etype.kind.inner);
+            return { type: "array", inner };
+        }
+        if (etype.kind.type === "struct") {
+            const noTypeTest = etype.kind.fields.reduce(
+                (acc, param) => [acc[0] || !param.etype, param.ident],
+                [false, ""],
+            );
+            if (noTypeTest[0]) {
+                this.report(
+                    `field '${noTypeTest[1]}' declared without type`,
+                    pos,
+                );
+                return { type: "error" };
+            }
+            const declaredTwiceTest = etype.kind.fields.reduce<
+                [boolean, string[], string]
+            >(
+                (acc, curr) => {
+                    if (acc[0]) {
+                        return acc;
+                    }
+                    if (acc[1].includes(curr.ident)) {
+                        return [true, acc[1], curr.ident];
+                    }
+                    return [false, [...acc[1], curr.ident], ""];
+                },
+                [false, [], ""],
+            );
+            if (
+                declaredTwiceTest[0]
+            ) {
+                this.report(`field ${declaredTwiceTest[2]} defined twice`, pos);
+                return { type: "error" };
+            }
+            const fields = etype.kind.fields.map((param): VTypeParam => ({
+                ident: param.ident,
+                vtype: this.checkEType(param.etype!),
+            }));
+            return { type: "struct", fields };
+        }
+        throw new Error(`unknown explicit type ${etype.kind.type}`);
+    }
+
+    public checkExpr(expr: Expr): VType {
+        const pos = expr.pos;
+        const vtype = ((): VType => {
+            switch (expr.kind.type) {
+                case "error":
+                    throw new Error("error in AST");
+                case "ident":
+                    throw new Error("ident expr in AST");
+                case "sym":
+                    return this.checkSymExpr(expr);
+                case "null":
+                    return { type: "null" };
+                case "int":
+                    return { type: "int" };
+                case "bool":
+                    return { type: "bool" };
+                case "string":
+                    return { type: "string" };
+                case "binary":
+                    return this.checkBinaryExpr(expr);
+                case "group":
+                    return this.checkExpr(expr.kind.expr);
+                case "field": {
+                    const subject = this.checkExpr(expr.kind.subject);
+                    if (subject.type !== "struct") {
+                        this.report("cannot use field on non-struct", pos);
+                        return { type: "error" };
+                    }
+                    const value = expr.kind.value;
+                    const found = subject.fields.find((param) =>
+                        param.ident === value
+                    );
+                    if (!found) {
+                        this.report(
+                            `no field named '${expr.kind.value}' on struct`,
+                            pos,
+                        );
+                        return { type: "error" };
+                    }
+                    return found.vtype;
+                }
+                case "index": {
+                    const subject = this.checkExpr(expr.kind.subject);
+                    if (subject.type !== "array") {
+                        this.report("cannot index on non-array", pos);
+                        return { type: "error" };
+                    }
+                    return subject.inner;
+                }
+                case "call": {
+                    const subject = this.checkExpr(expr.kind.subject);
+                    if (subject.type !== "fn") {
+                        this.report("cannot call non-fn", pos);
+                        return { type: "error" };
+                    }
+                    const args = expr.kind.args.map((arg) =>
+                        this.checkExpr(arg)
+                    );
+                    if (args.length !== subject.params.length) {
+                        this.report(
+                            `incorrect number of arguments` +
+                                `, expected ${subject.params.length}`,
+                            pos,
+                        );
+                    }
+                    for (let i = 0; i < args.length; ++i) {
+                        if (!vtypesEqual(args[i], subject.params[i].vtype)) {
+                            this.report(
+                                `incorrect argument ${i} '${
+                                    subject.params[i].ident
+                                }'` +
+                                    `, expected ${
+                                        vtypeToString(subject.params[i].vtype)
+                                    }` +
+                                    `, got ${vtypeToString(args[i])}`,
+                                pos,
+                            );
+                            break;
+                        }
+                    }
+                    return subject.returnType;
+                }
+                case "unary":
+                case "if":
+                case "loop":
+                case "block":
+            }
+            throw new Error(`unhandled type ${expr.kind.type}`);
+        })();
+        expr.vtype = vtype;
+        throw new Error(`unknown expression ${expr.kind.type}`);
+    }
+
+    public checkSymExpr(expr: Expr): VType {
+        const pos = expr.pos;
+        if (expr.kind.type !== "sym") {
+            throw new Error();
+        }
+    }
+
+    public checkBinaryExpr(expr: Expr): VType {
+        const pos = expr.pos;
+        if (expr.kind.type !== "binary") {
+            throw new Error();
+        }
+        const left = this.checkExpr(expr.kind.left);
+        const right = this.checkExpr(expr.kind.right);
+        for (const operation of simpleBinaryOperations) {
+            if (operation.binaryType !== expr.kind.binaryType) {
+                continue;
+            }
+            if (!vtypesEqual(operation.operand, left)) {
+                continue;
+            }
+            if (!vtypesEqual(left, right)) {
+                continue;
+            }
+            return operation.result ?? operation.operand;
+        }
+        this.report(
+            `cannot apply binary operation '${expr.kind.binaryType}' ` +
+                `on types '${vtypeToString(left)}' and '${
+                    vtypeToString(right)
+                }'`,
+            pos,
+        );
+        return { type: "error" };
+    }
+}
+
+const simpleBinaryOperations: {
+    binaryType: string;
+    operand: VType;
+    result?: VType;
+}[] = [
+    // arithmetic
+    { binaryType: "+", operand: { type: "int" } },
+    { binaryType: "-", operand: { type: "int" } },
+    { binaryType: "*", operand: { type: "int" } },
+    { binaryType: "/", operand: { type: "int" } },
+    // logical
+    { binaryType: "and", operand: { type: "bool" } },
+    { binaryType: "or", operand: { type: "bool" } },
+    // equality
+    { binaryType: "==", operand: { type: "null" }, result: { type: "bool" } },
+    { binaryType: "==", operand: { type: "int" }, result: { type: "bool" } },
+    { binaryType: "==", operand: { type: "string" }, result: { type: "bool" } },
+    { binaryType: "==", operand: { type: "bool" }, result: { type: "bool" } },
+    { binaryType: "!=", operand: { type: "null" }, result: { type: "bool" } },
+    { binaryType: "!=", operand: { type: "int" }, result: { type: "bool" } },
+    { binaryType: "!=", operand: { type: "string" }, result: { type: "bool" } },
+    { binaryType: "!=", operand: { type: "bool" }, result: { type: "bool" } },
+    // comparison
+    { binaryType: "<", operand: { type: "int" }, result: { type: "bool" } },
+    { binaryType: ">", operand: { type: "int" }, result: { type: "bool" } },
+    { binaryType: "<=", operand: { type: "int" }, result: { type: "bool" } },
+    { binaryType: ">=", operand: { type: "int" }, result: { type: "bool" } },
+];
diff --git a/compiler/Lexer.ts b/compiler/Lexer.ts
index 1752ee4..f7813dd 100644
--- a/compiler/Lexer.ts
+++ b/compiler/Lexer.ts
@@ -5,16 +5,17 @@ export class Lexer {
     private line = 1;
     private col = 1;
 
-
-    public constructor (private text: string) {}
+    public constructor(private text: string) {}
 
     public next(): Token | null {
-        if (this.done())
+        if (this.done()) {
             return null;
+        }
         const pos = this.pos();
         if (this.test(/[ \t\n\r]/)) {
-            while (!this.done() && this.test(/[ \t\n\r]/))
+            while (!this.done() && this.test(/[ \t\n\r]/)) {
                 this.step();
+            }
             return this.next();
         }
 
@@ -39,6 +40,8 @@ export class Lexer {
                     return this.token("if", pos);
                 case "else":
                     return this.token("else", pos);
+                case "struct":
+                    return this.token("struct", pos);
                 default:
                     return { ...this.token("ident", pos), identValue: value };
             }
@@ -53,25 +56,26 @@ export class Lexer {
         }
 
         if (this.test("0")) {
-            this.step()
+            this.step();
             if (!this.done() && this.test(/[0-9]/)) {
                 console.error(
-                    `Lexer: invalid number`
-                        + ` at ${pos.line}:${pos.col}`,
+                    `Lexer: invalid number` +
+                        ` at ${pos.line}:${pos.col}`,
                 );
                 return this.token("error", pos);
             }
-            return { ...this.token("int", pos), intValue: 0};
+            return { ...this.token("int", pos), intValue: 0 };
         }
 
-        if (this.test("\"")) {
+        if (this.test('"')) {
             this.step();
             let value = "";
-            while (!this.done() && !this.test("\"")) {
+            while (!this.done() && !this.test('"')) {
                 if (this.test("\\")) {
                     this.step();
-                    if (this.done())
+                    if (this.done()) {
                         break;
+                    }
                     value += {
                         "n": "\n",
                         "t": "\t",
@@ -82,10 +86,10 @@ export class Lexer {
                 }
                 this.step();
             }
-            if (this.done() || !this.test("\"")) {
+            if (this.done() || !this.test('"')) {
                 console.error(
-                    `Lexer: unclosed/malformed string`
-                        + ` at ${pos.line}:${pos.col}`,
+                    `Lexer: unclosed/malformed string` +
+                        ` at ${pos.line}:${pos.col}`,
                 );
                 return this.token("error", pos);
             }
@@ -113,7 +117,7 @@ export class Lexer {
                     return this.token("->", pos);
                 }
                 if (this.test("=")) {
-                    this.step()
+                    this.step();
                     return this.token("-=", pos);
                 }
             }
@@ -125,16 +129,21 @@ export class Lexer {
                 this.step();
                 return this.token("+=", pos);
             }
+            if (first === "-" && !this.done() && this.test(">")) {
+                this.step();
+                return this.token("->", pos);
+            }
             return this.token(first, pos);
         }
         if (this.test("/")) {
-            this.step()
+            this.step();
             if (this.test("/")) {
-                while (!this.done() && !this.test("\n"))
+                while (!this.done() && !this.test("\n")) {
                     this.step();
-                return this.next()
+                }
+                return this.next();
             }
-            return this.token("/", pos)
+            return this.token("/", pos);
         }
         if (this.test("false")) {
             this.step();
@@ -180,20 +189,29 @@ export class Lexer {
             this.step();
             return this.token("return", pos);
         }
-        console.error(`Lexer: illegal character '${this.current()}' at ${pos.line}:${pos.col}`);
+        console.error(
+            `Lexer: illegal character '${this.current()}' at ${pos.line}:${pos.col}`,
+        );
         this.step();
-        return this.next();    
+        return this.next();
     }
-    
-    private done(): boolean { return this.index >= this.text.length; }
-    
-    private current(): string { return this.text[this.index]; }
 
-    public currentPos(): Pos { return this.pos(); }
+    private done(): boolean {
+        return this.index >= this.text.length;
+    }
+
+    private current(): string {
+        return this.text[this.index];
+    }
+
+    public currentPos(): Pos {
+        return this.pos();
+    }
 
     private step() {
-        if (this.done())
+        if (this.done()) {
             return;
+        }
         if (this.current() === "\n") {
             this.line += 1;
             this.col = 1;
@@ -207,8 +225,8 @@ export class Lexer {
         return {
             index: this.index,
             line: this.line,
-            col: this.col
-        }
+            col: this.col,
+        };
     }
 
     private token(type: string, pos: Pos): Token {
@@ -217,13 +235,10 @@ export class Lexer {
     }
 
     private test(pattern: RegExp | string): boolean {
-        if (typeof pattern === "string")
+        if (typeof pattern === "string") {
             return this.current() === pattern;
-        else
+        } else {
             return pattern.test(this.current());
+        }
     }
-
-
-
-
 }
diff --git a/compiler/Parser.ts b/compiler/Parser.ts
index 7d50c6b..ddf61e1 100644
--- a/compiler/Parser.ts
+++ b/compiler/Parser.ts
@@ -1,4 +1,13 @@
-import { Expr, ExprKind, Param, Stmt, StmtKind, BinaryType} from "./ast.ts";
+import {
+    BinaryType,
+    EType,
+    ETypeKind,
+    Expr,
+    ExprKind,
+    Param,
+    Stmt,
+    StmtKind,
+} from "./ast.ts";
 import { Lexer } from "./Lexer.ts";
 import { Pos, Token } from "./Token.ts";
 
@@ -10,12 +19,19 @@ export class Parser {
         this.currentToken = lexer.next();
     }
 
-    private step() { this.currentToken = this.lexer.next() }
-    public done(): boolean { return this.currentToken == null; }
-    private current(): Token { return this.currentToken!; }
+    private step() {
+        this.currentToken = this.lexer.next();
+    }
+    public done(): boolean {
+        return this.currentToken == null;
+    }
+    private current(): Token {
+        return this.currentToken!;
+    }
     private pos(): Pos {
-        if (this.done())
+        if (this.done()) {
             return this.lexer.currentPos();
+        }
         return this.current().pos;
     }
 
@@ -25,13 +41,18 @@ export class Parser {
 
     private report(msg: string, pos = this.pos()) {
         console.log(`Parser: ${msg} at ${pos.line}:${pos.col}`);
-        class ReportNotAnError extends Error { constructor() { super("ReportNotAnError"); }  }
+        class ReportNotAnError extends Error {
+            constructor() {
+                super("ReportNotAnError");
+            }
+        }
         try {
             throw new ReportNotAnError();
         } catch (error) {
-            if (!(error instanceof ReportNotAnError))
+            if (!(error instanceof ReportNotAnError)) {
                 throw error;
-            console.log(error)
+            }
+            console.log(error);
         }
     }
 
@@ -47,33 +68,47 @@ export class Parser {
         return { kind, pos, id };
     }
 
+    private etype(kind: ETypeKind, pos: Pos): EType {
+        const id = this.nextNodeId;
+        this.nextNodeId += 1;
+        return { kind, pos, id };
+    }
+
     private parseMultiLineBlockExpr(): Expr {
         const pos = this.pos();
-        if (this.test("{")) 
+        if (this.test("{")) {
             return this.parseBlock();
-        if (this.test("if"))
+        }
+        if (this.test("if")) {
             return this.parseIf();
-        if (this.test("loop"))
+        }
+        if (this.test("loop")) {
             return this.parseLoop();
+        }
         this.report("expected expr");
         return this.expr({ type: "error" }, pos);
     }
 
     private parseSingleLineBlockStmt(): Stmt {
         const pos = this.pos();
-        if (this.test("let"))
+        if (this.test("let")) {
             return this.parseLet();
-        if (this.test("return"))
+        }
+        if (this.test("return")) {
             return this.parseReturn();
-        if (this.test("break")) 
+        }
+        if (this.test("break")) {
             return this.parseBreak();
+        }
         this.report("expected stmt");
         return this.stmt({ type: "error" }, pos);
     }
 
     private eatSemicolon() {
         if (!this.test(";")) {
-            this.report(`expected ';', got '${this.currentToken?.type ?? "eof"}'`);
+            this.report(
+                `expected ';', got '${this.currentToken?.type ?? "eof"}'`,
+            );
             return;
         }
         this.step();
@@ -91,11 +126,12 @@ export class Parser {
             if (this.test("}")) {
                 this.step();
                 return this.expr({ type: "block", stmts }, pos);
-            } else if (this.test("return") || this.test("break") || this.test("let")) {
+            } else if (
+                this.test("return") || this.test("break") || this.test("let")
+            ) {
                 stmts.push(this.parseSingleLineBlockStmt());
                 this.eatSemicolon();
-            }
-            else if (this.test("fn")) {
+            } else if (this.test("fn")) {
                 stmts.push(this.parseSingleLineBlockStmt());
                 stmts.push(this.parseFn());
             } else if (this.test("{") || this.test("if") || this.test("loop")) {
@@ -111,7 +147,12 @@ export class Parser {
                     this.step();
                     const value = this.parseExpr();
                     this.eatSemicolon();
-                    stmts.push(this.stmt({ type: "assign", subject: expr, value }, pos));
+                    stmts.push(
+                        this.stmt(
+                            { type: "assign", subject: expr, value },
+                            pos,
+                        ),
+                    );
                 } else if (this.test(";")) {
                     this.step();
                     stmts.push(this.stmt({ type: "expr", expr }, expr.pos));
@@ -133,13 +174,15 @@ export class Parser {
         while (!this.done()) {
             if (this.test("fn")) {
                 stmts.push(this.parseFn());
-            } else if (this.test("let") || this.test("return") || this.test("break")) {
+            } else if (
+                this.test("let") || this.test("return") || this.test("break")
+            ) {
                 stmts.push(this.parseSingleLineBlockStmt());
                 this.eatSemicolon();
             } else if (this.test("{") || this.test("if") || this.test("loop")) {
                 const expr = this.parseMultiLineBlockExpr();
                 stmts.push(this.stmt({ type: "expr", expr }, expr.pos));
-            } else  {
+            } else {
                 stmts.push(this.parseAssign());
                 this.eatSemicolon();
             }
@@ -161,12 +204,20 @@ export class Parser {
             return this.stmt({ type: "error" }, pos);
         }
         const params = this.parseFnParams();
+        let returnType: EType | null = null;
+        if (this.test("->")) {
+            this.step();
+            returnType = this.parseEType();
+        }
         if (!this.test("{")) {
             this.report("expected block");
             return this.stmt({ type: "error" }, pos);
         }
         const body = this.parseBlock();
-        return this.stmt({ type: "fn", ident, params, body }, pos);
+        if (returnType === null) {
+            return this.stmt({ type: "fn", ident, params, body }, pos);
+        }
+        return this.stmt({ type: "fn", ident, params, returnType, body }, pos);
     }
 
     public parseFnParams(): Param[] {
@@ -175,18 +226,21 @@ export class Parser {
             this.step();
             return [];
         }
-        let params: Param[] = [];
+        const params: Param[] = [];
         const paramResult = this.parseParam();
-        if (!paramResult.ok)
+        if (!paramResult.ok) {
             return [];
+        }
         params.push(paramResult.value);
         while (this.test(",")) {
             this.step();
-            if (this.test(")"))
+            if (this.test(")")) {
                 break;
+            }
             const paramResult = this.parseParam();
-            if (!paramResult.ok)
+            if (!paramResult.ok) {
                 return [];
+            }
             params.push(paramResult.value);
         }
         if (!this.test(")")) {
@@ -197,11 +251,15 @@ export class Parser {
         return params;
     }
 
-    public parseParam(): { ok: true, value: Param } | { ok: false } {
+    public parseParam(): { ok: true; value: Param } | { ok: false } {
         const pos = this.pos();
         if (this.test("ident")) {
             const ident = this.current().identValue!;
             this.step();
+            if (this.test(":")) {
+                const etype = this.parseEType();
+                return { ok: true, value: { ident, etype, pos } };
+            }
             return { ok: true, value: { ident, pos } };
         }
         this.report("expected param");
@@ -212,8 +270,9 @@ export class Parser {
         const pos = this.pos();
         this.step();
         const paramResult = this.parseParam();
-        if (!paramResult.ok)
+        if (!paramResult.ok) {
             return this.stmt({ type: "error" }, pos);
+        }
         const param = paramResult.value;
         if (!this.test("=")) {
             this.report("expected '='");
@@ -297,10 +356,25 @@ export class Parser {
             const subject = this.parsePrefix();
             return this.expr({ type: "unary", unaryType: "not", subject }, pos);
         }
-        for (const binaryType of ["+", "*", "==", "-", "/", "!=", "<", ">", "<=", ">=", "or", "and"]) {
-            const subject = this.parseBinary(binaryType as BinaryType, pos)
+        for (
+            const binaryType of [
+                "+",
+                "*",
+                "==",
+                "-",
+                "/",
+                "!=",
+                "<",
+                ">",
+                "<=",
+                ">=",
+                "or",
+                "and",
+            ]
+        ) {
+            const subject = this.parseBinary(binaryType as BinaryType, pos);
             if (subject !== null) {
-                return subject
+                return subject;
             }
         }
         return this.parsePostfix();
@@ -312,8 +386,8 @@ export class Parser {
             const left = this.parsePrefix();
             const right = this.parsePrefix();
             return this.expr({ type: "binary", binaryType, left, right }, pos);
-        }  
-        return null
+        }
+        return null;
     }
 
     public parsePostfix(): Expr {
@@ -349,8 +423,9 @@ export class Parser {
                     args.push(this.parseExpr());
                     while (this.test(",")) {
                         this.step();
-                        if (this.test(")"))
+                        if (this.test(")")) {
                             break;
+                        }
                         args.push(this.parseExpr());
                     }
                 }
@@ -394,7 +469,7 @@ export class Parser {
         }
         if (this.test("null")) {
             this.step();
-            return this.expr({ type: "null"}, pos);
+            return this.expr({ type: "null" }, pos);
         }
         if (this.test("(")) {
             this.step();
@@ -406,16 +481,78 @@ export class Parser {
             this.step();
             return this.expr({ type: "group", expr }, pos);
         }
-        if (this.test("{"))
+        if (this.test("{")) {
             return this.parseBlock();
-        if (this.test("if"))
+        }
+        if (this.test("if")) {
             return this.parseIf();
-        if (this.test("loop"))
+        }
+        if (this.test("loop")) {
             return this.parseLoop();
+        }
 
         this.report("expected expr", pos);
         this.step();
         return this.expr({ type: "error" }, pos);
     }
 
+    public parseEType(): EType {
+        const pos = this.pos();
+        if (this.test("ident")) {
+            const ident = this.current().identValue!;
+            return this.etype({ type: "ident", value: ident }, pos);
+        }
+        if (this.test("[")) {
+            this.step();
+            const inner = this.parseEType();
+            if (!this.test("]")) {
+                this.report("expected ']'", pos);
+                return this.etype({ type: "error" }, pos);
+            }
+            this.step();
+            return this.etype({ type: "array", inner }, pos);
+        }
+        if (this.test("struct")) {
+            this.step();
+            if (!this.test("{")) {
+                this.report("expected '{'");
+                return this.etype({ type: "error" }, pos);
+            }
+            const fields = this.parseETypeStructFields();
+            return this.etype({ type: "struct", fields }, pos);
+        }
+        this.report("expected type");
+        return this.etype({ type: "error" }, pos);
+    }
+
+    public parseETypeStructFields(): Param[] {
+        this.step();
+        if (this.test("}")) {
+            this.step();
+            return [];
+        }
+        const params: Param[] = [];
+        const paramResult = this.parseParam();
+        if (!paramResult.ok) {
+            return [];
+        }
+        params.push(paramResult.value);
+        while (this.test(",")) {
+            this.step();
+            if (this.test("}")) {
+                break;
+            }
+            const paramResult = this.parseParam();
+            if (!paramResult.ok) {
+                return [];
+            }
+            params.push(paramResult.value);
+        }
+        if (!this.test("}")) {
+            this.report("expected '}'");
+            return params;
+        }
+        this.step();
+        return params;
+    }
 }
diff --git a/compiler/ast.ts b/compiler/ast.ts
index 464ff14..4a27113 100644
--- a/compiler/ast.ts
+++ b/compiler/ast.ts
@@ -1,59 +1,97 @@
 import { Pos } from "./Token.ts";
+import { VType } from "./vtypes.ts";
 
 export type UnaryType = "not";
-export type BinaryType = "+" | "*" | "==" | "-" | "/" | "!=" | "<" | ">" | "<=" | ">=" | "or" | "and";
+export type BinaryType =
+    | "+"
+    | "*"
+    | "=="
+    | "-"
+    | "/"
+    | "!="
+    | "<"
+    | ">"
+    | "<="
+    | ">="
+    | "or"
+    | "and";
 
 export type Param = {
-    ident: string,
-    pos: Pos,
+    ident: string;
+    etype?: EType;
+    pos: Pos;
+    vtype?: VType;
 };
 
 export type Stmt = {
-    kind: StmtKind,
-    pos: Pos,
-    id: number,
+    kind: StmtKind;
+    pos: Pos;
+    vtype?: VType;
+    id: number;
 };
 
 export type StmtKind =
     | { type: "error" }
-    | { type: "break", expr?: Expr }
-    | { type: "return", expr?: Expr }
-    | { type: "fn", ident: string, params: Param[], body: Expr }
-    | { type: "let", param: Param, value: Expr }
-    | { type: "assign", subject: Expr, value: Expr }
-    | { type: "expr", expr: Expr }
-    ;
+    | { type: "break"; expr?: Expr }
+    | { type: "return"; expr?: Expr }
+    | {
+        type: "fn";
+        ident: string;
+        params: Param[];
+        returnType?: EType;
+        body: Expr;
+    }
+    | { type: "let"; param: Param; value: Expr }
+    | { type: "assign"; subject: Expr; value: Expr }
+    | { type: "expr"; expr: Expr };
 
 export type Expr = {
-    kind: ExprKind,
-    pos: Pos,
-    id: number,
+    kind: ExprKind;
+    pos: Pos;
+    vtype?: VType;
+    id: number;
 };
 
 export type ExprKind =
     | { type: "error" }
-    | { type: "int", value: number }
-    | { type: "string", value: string }
-    | { type: "ident", value: string }
-    | { type: "group", expr: Expr }
-    | { type: "field", subject: Expr, value: string }
-    | { type: "index", subject: Expr, value: Expr }
-    | { type: "call", subject: Expr, args: Expr[] }
-    | { type: "unary", unaryType: UnaryType, subject: Expr }
-    | { type: "binary", binaryType: BinaryType, left: Expr, right: Expr }
-    | { type: "if", cond: Expr, truthy: Expr, falsy?: Expr }
-    | { type: "bool", value: boolean}
-    | { type: "null"}
-    | { type: "loop", body: Expr }
-    | { type: "block", stmts: Stmt[], expr?: Expr }
-    | { type: "sym", ident: string, defType: "let" | "fn" | "fn_param" | "builtin", stmt?: Stmt, param?: Param }
-    ;
+    | { type: "int"; value: number }
+    | { type: "string"; value: string }
+    | { type: "ident"; value: string }
+    | { type: "group"; expr: Expr }
+    | { type: "field"; subject: Expr; value: string }
+    | { type: "index"; subject: Expr; value: Expr }
+    | { type: "call"; subject: Expr; args: Expr[] }
+    | { type: "unary"; unaryType: UnaryType; subject: Expr }
+    | { type: "binary"; binaryType: BinaryType; left: Expr; right: Expr }
+    | { type: "if"; cond: Expr; truthy: Expr; falsy?: Expr }
+    | { type: "bool"; value: boolean }
+    | { type: "null" }
+    | { type: "loop"; body: Expr }
+    | { type: "block"; stmts: Stmt[]; expr?: Expr }
+    | {
+        type: "sym";
+        ident: string;
+        defType: "let" | "fn" | "fn_param" | "builtin";
+        stmt?: Stmt;
+        param?: Param;
+    };
 
 export type Sym = {
-    ident: string,
-    type: "let" | "fn" | "fn_param" | "builtin",
-    pos?: Pos,
-    stmt?: Stmt,
-    param?: Param,
-}
+    ident: string;
+    type: "let" | "fn" | "fn_param" | "builtin";
+    pos?: Pos;
+    stmt?: Stmt;
+    param?: Param;
+};
 
+export type EType = {
+    kind: ETypeKind;
+    pos: Pos;
+    id: number;
+};
+
+export type ETypeKind =
+    | { type: "error" }
+    | { type: "ident"; value: string }
+    | { type: "array"; inner: EType }
+    | { type: "struct"; fields: Param[] };
diff --git a/compiler/vtypes.ts b/compiler/vtypes.ts
new file mode 100644
index 0000000..9f617ed
--- /dev/null
+++ b/compiler/vtypes.ts
@@ -0,0 +1,63 @@
+export type VType =
+    | { type: "" }
+    | { type: "error" }
+    | { type: "unknown" }
+    | { type: "null" }
+    | { type: "int" }
+    | { type: "string" }
+    | { type: "bool" }
+    | { type: "array"; inner: VType }
+    | { type: "struct"; fields: VTypeParam[] }
+    | { type: "fn"; params: VTypeParam[]; returnType: VType };
+
+export type VTypeParam = {
+    ident: string;
+    vtype: VType;
+};
+
+export function vtypesEqual(a: VType, b: VType): boolean {
+    if (a.type !== b.type) {
+        return false;
+    }
+    if (
+        ["error", "unknown", "null", "int", "string", "bool", "struct"]
+            .includes(a.type)
+    ) {
+        return true;
+    }
+    if (a.type === "array" && b.type === "array") {
+        return vtypesEqual(a.inner, b.inner);
+    }
+    if (a.type === "fn" && b.type === "fn") {
+        if (a.params.length !== b.params.length) {
+            return false;
+        }
+        for (let i = 0; i < a.params.length; ++i) {
+            if (!vtypesEqual(a.params[i].vtype, b.params[i].vtype)) {
+                return false;
+            }
+        }
+        return vtypesEqual(a.returnType, b.returnType);
+    }
+    return false;
+}
+
+export function vtypeToString(vtype: VType): string {
+    if (
+        ["error", "unknown", "null", "int", "string", "bool", "struct"]
+            .includes(vtype.type)
+    ) {
+        return vtype.type;
+    }
+    if (vtype.type === "array") {
+        return `[${vtypeToString(vtype.inner)}]`;
+    }
+    if (vtype.type === "fn") {
+        const paramString = vtype.params.map((param) =>
+            `${param.ident}: ${vtypeToString(param.vtype)}`
+        )
+            .join(", ");
+        return `fn (${paramString}) -> ${vtypeToString(vtype.returnType)}`;
+    }
+    throw new Error(`unhandled vtype '${vtype.type}'`);
+}