diff --git a/compile.phi b/compile.phi
index 953b456..69755b5 100644
--- a/compile.phi
+++ b/compile.phi
@@ -1,9 +1,15 @@
 
-(fn Emitter (ast) (do
+(fn Emitter (ast filename) (do
     (let output ())
 
+    (let (enter_scope leave_scope define_sym get_sym) (call Syms))
+
     (fn generate () (do
         (call emit "#!/usr/bin/env node\n")
+        (call emit "import { Runtime } from \"./runtime.js\"\n")
+        (call emit "const runtime = new Runtime(\"")
+        (call emit filename)
+        (call emit "\")\n")
         (call emit_exprs ast)
         (return (call strings_join output))
     ))
@@ -11,7 +17,7 @@
     (fn emit_exprs (exprs) (do
         (for expr exprs (do
             (call emit_expr expr)
-            (call emit "\n")
+            (call emit ";\n")
         ))
     ))
 
@@ -21,13 +27,33 @@
             (call emit_list expr)
         ) (if (== ty "int") (
             (let (_ _ value) expr)
-            (call emit (call format "%" value))
+            (call emit (call format "{ type: \"int\", value: % }" value))
         ) (if (== ty "string") (
             (let (_ _ value) expr)
-            (call emit (call format "\"%\"" (call string_escape value)))
+            (call emit (call format "{ type: \"string\", value: \"%\" }" (call string_escape value)))
         ) (if (== ty "ident") (
             (let (_ _ value) expr)
-            (call emit (call format "_%" value))
+
+            (let sym (call get_sym value))
+            (if (== sym null) (do
+                (call panic "undefined symbol '%'" value)
+            ))
+
+            (let (sym_ty) sym)
+            (if (== sym_ty "builtin") (do
+                (let (_ id) sym)
+                (if (== id "println") (do
+                    (call emit (call format 
+                        "((...args) => (runtime.setLine(%), runtime.builtinPrintln(...args)))"
+                     line))
+                ) (do
+                    (call panic "unknown builtin '%'" id)
+                ))
+            ) (if (== sym_ty "fn") (do
+                (call emit (call format "_%" value))
+            ) (do
+                (call panic "not implemented '%'" sym_ty)
+            )))
         ) (do
             (call panic "unknown expr type '%' on line %" ty line)
         )))))
@@ -36,7 +62,7 @@
     (fn emit_list (expr) (do
         (let (ty line s) expr)
         (if (== (call len s) 0) (do
-            (call emit "[]")
+            (call emit "{ type: \"list\", values: [] }")
             (return)
         ))
         (let ((_ _ id)) s)
@@ -53,6 +79,9 @@
 
                 (call emit (call format "_%" name))
             ))
+
+            (call define_sym name ("fn" name line))
+
             (call emit (call format ") {\n" name))
             (call emit_expr body)
             (call emit "}")
@@ -85,7 +114,7 @@
 
             (call emit ")")
         ) (do
-            (call emit "[")
+            (call emit "{ type: \"list\", values: [")
             (let first true)
             (for e s (do
                 (if (not first) (do
@@ -95,7 +124,7 @@
 
                 (call emit_expr e)
             ))
-            (call emit "]")
+            (call emit "] }")
         )))))
     ))
 
@@ -106,6 +135,65 @@
     (return (generate))
 ))
 
+(fn Syms () (do
+    (let syms (null (
+        ("println" ("builtin" "println"))
+    )))
+
+    (fn enter_scope () (do
+        (= syms (syms ()))
+    ))
+
+    (fn leave_scope () (do
+        (let (parent _) syms)
+        (= syms parent)
+    ))
+
+    (fn define (ident sym) (do
+        (let (_ syms) syms)
+        (let i 0)
+        (loop (do
+            (if (>= i (call len syms)) (break))
+            (let (s_ident s_sym) (call at syms i))
+            (if (== ident s_ident) (do
+                (call set s_syms i sym)
+                (return)
+            ))
+            (+= i 1)
+        ))
+        (call push syms (ident sym))
+    ))
+
+    (fn find_sym (syms ident) (do
+        (let (parent map) syms)
+        (let i 0)
+        (loop (do
+            (if (>= i (call len map)) (break))
+            (let (s_ident s_sym) (call at map i))
+            (if (== ident s_ident) (do
+                (return s_sym)
+            ))
+            (+= i 1)
+        ))
+        (if (!= parent null) (do
+            (return (call find_syms parent ident))
+        ))
+        (return null)
+    ))
+
+    (fn get (ident) (do
+        (return (call find_sym syms ident))
+    ))
+    
+    (return (
+        enter_scope
+        leave_scope
+        define
+        get
+    ))
+))
+
+
 (fn string_escape (str) (do
     (let str_len (call len str))
     (let i 0)
@@ -357,8 +445,10 @@
 ))
 
 
+(let input_filename "program.phi")
+
 (call println "reading file...")
-(let text (call read_text_file "program.phi"))
+(let text (call read_text_file input_filename))
 
 //(call println "=== text ===")
 //(call println text)
@@ -382,7 +472,7 @@
 //))
 
 (call println "emitting...")
-(let emitter (call Emitter ast))
+(let emitter (call Emitter ast input_filename))
 (let (emit) emitter)
 (let js_code (call emit))
 
diff --git a/phi.js b/phi.js
index e8f0f6f..9f446aa 100644
--- a/phi.js
+++ b/phi.js
@@ -1,7 +1,8 @@
 "use strict";
 
-import * as fs from "node:fs";
+import fs from "node:fs";
 import process from "node:process";
+import { Runtime } from "./runtime.js";
 
 function main() {
     const filename = process.argv[2];
@@ -20,7 +21,7 @@ function main() {
         lastValue = result.value;
     }
     if (lastValue !== null) {
-        console.log(valueToJs(lastValue));
+        console.log(Runtime.valueToJs(lastValue));
     }
 }
 
@@ -28,8 +29,8 @@ class Evaluator {
     constructor(filename) {
         this.syms = { parent: undefined, map: new Map(this.builtins) };
         this.currentLine = 0;
-        this.callStack = [{ name: "<file>", line: 0 }];
         this.filename = filename;
+        this.runtime = new Runtime(filename);
     }
 
     /**
@@ -37,9 +38,13 @@ class Evaluator {
      */
     eval(expr) {
         if (!expr) {
-            console.log(this.callStack)
-            throw new Error()
+            this.runtime.printPanic(
+                "expression could not be evaluated",
+                this.currentLine,
+            );
+            throw new Error("expression could not be evaluated");
         }
+
         this.currentLine = expr.line;
         if (expr.type === "list") {
             return this.evalList(expr, expr.line);
@@ -129,8 +134,13 @@ class Evaluator {
         } else if (id in this.artithmeticOps) {
             const left = this.evalToValue(s[1]);
             const right = this.evalToValue(s[2]);
-            if (id === "+" && left.type === "string" && right.type === "string") {
-                return { type: "value", value: { type: "string", value: left.value + right.value } };
+            if (
+                id === "+" && left.type === "string" && right.type === "string"
+            ) {
+                return {
+                    type: "value",
+                    value: { type: "string", value: left.value + right.value },
+                };
             }
             return {
                 type: "value",
@@ -139,6 +149,26 @@ class Evaluator {
                     value: this.artithmeticOps[id](left.value, right.value),
                 },
             };
+        } else if (id === "==") {
+            const left = this.evalToValue(s[1]);
+            const right = this.evalToValue(s[2]);
+            return {
+                type: "value",
+                value: {
+                    type: "bool",
+                    value: this.runtime.eq(left, right),
+                },
+            };
+        } else if (id === "!=") {
+            const left = this.evalToValue(s[1]);
+            const right = this.evalToValue(s[2]);
+            return {
+                type: "value",
+                value: {
+                    type: "bool",
+                    value: !this.runtime.eq(left, right),
+                },
+            };
         } else if (id in this.comparisonOps) {
             const left = this.evalToValue(s[1]);
             const right = this.evalToValue(s[2]);
@@ -235,7 +265,7 @@ class Evaluator {
             }
         }
         this.syms = outerSyms;
-        return { type: "value", value: { type: "null" } }
+        return { type: "value", value: { type: "null" } };
     }
 
     evalDo(expr) {
@@ -268,21 +298,15 @@ class Evaluator {
             throw new Error("cannot call non-function");
         }
 
-        if (this.callStack.length > 100) {
-            this.panic("stack overflow")
-            this.printCallStack()
-            process.exit(1);
-        }
-
-        this.callStack.push({ name: fnValue.name, line: expr.line });
+        this.runtime.pushCall(fnValue.name, expr.line);
         const callerSyms = this.syms;
         this.syms = {
             parent: fnValue.syms,
             map: new Map(),
         };
         if (fnValue.params.length !== args.length) {
-            throw new Error(
-                `incorrect amount of arguments for function '${fnValue.name}' on line ${expr.line}`,
+            this.panic(
+                `incorrect amount of arguments for function '${fnValue.name}'`,
             );
         }
         for (let i = 0; i < fnValue.params.length; ++i) {
@@ -299,7 +323,7 @@ class Evaluator {
         }
 
         this.syms = callerSyms;
-        this.callStack.pop()
+        this.runtime.popCall();
         return { type: "value", value: returnValue };
     }
 
@@ -342,10 +366,10 @@ class Evaluator {
             });
         } else if (pattern.type === "list") {
             if (value.type !== "list") {
-                throw new Error(`expected list on line ${pattern.line}`);
+                this.panic(`expected list`);
             }
             for (const [i, p] of pattern.values.entries()) {
-                this.assignPattern(p, value.values[i] ?? { type: "null" })
+                this.assignPattern(p, value.values[i] ?? { type: "null" });
             }
         } else {
             throw new Error(`cannot assign to pattern on line ${pattern.line}`);
@@ -363,89 +387,8 @@ class Evaluator {
     }
 
     panic(msg) {
-        console.error(`\x1b[1;91mpanic\x1b[1;97m: ${msg}\x1b[0m"`);
-        this.printCallStack();
-    }
-
-    printCallStack() {
-        const last = this.callStack[this.callStack.length - 1];
-        console.error(`    \x1b[90mat \x1b[37m${last.name} \x1b[90m(${this.filename}:${this.currentLine})\x1b[0m`);
-        for (let i = this.callStack.length - 2; i >= 0; --i) {
-            const name = this.callStack[i].name;
-            const line = this.callStack[i + 1].line;
-            console.error(`    \x1b[90mat \x1b[37m${name} \x1b[90m(${this.filename}:${line})\x1b[0m`);
-        }
-    }
-
-    builtinFormat(msg, ...args) {
-        let value = valueToPrint(msg);
-        for (const arg of args) {
-            value = value.replace("%", valueToPrint(arg));
-        }
-        return { type: "string", value };
-    }
-    builtinPrintln(msg, ...args) {
-        let text = valueToPrint(msg);
-
-        for (const arg of args) {
-            text = text.replace("%", valueToPrint(arg));
-        }
-
-        console.log(text);
-        return { type: "null" };
-    }
-    builtinPanic(msg, ...args) {
-        let text = valueToPrint(msg);
-
-        for (const arg of args) {
-            text = text.replace("%", valueToPrint(arg));
-        }
-
-        this.panic(text);
-        process.exit(1);
-        return { type: "null" };
-    }
-    builtinReadTextFile(path) {
-        const text = fs.readFileSync(path.value).toString();
-        return { type: "string", value: text };
-    }
-    builtinWriteTextFile(path, text) {
-        fs.writeFileSync(path.value, text.value);
-        return { type: "null" };
-    }
-    builtinPush(list, value) {
-        if (list.type === "string") {
-            list.value += value.value;
-            return list;
-        }
-        list.values.push(value);
-        return list;
-    }
-    builtinAt(value, index) {
-        if (value.type === "string") {
-            return { type: "string", value: value.value[index.value] };
-        }
-        return value.values[index.value];
-    }
-    builtinLen(value) {
-        if (value.type === "string") {
-            return { type: "int", value: value.value.length };
-        }
-        return { type: "int", value: value.values.length };
-    }
-    builtinStringToInt(value) {
-        return { type: "int", value: Number(value.value) };
-    }
-    builtinCharCode(value) {
-        return { type: "int", value: value.value.charCodeAt(0) };
-    }
-    builtinStringsJoin(value) {
-        return {
-            type: "string",
-            value: value.values
-                .map(value => value.value)
-                .join("")
-        };
+        this.runtime.setLine(this.currentLine);
+        this.runtime.panic(msg);
     }
 
     artithmeticOps = {
@@ -453,8 +396,6 @@ class Evaluator {
         "-": (left, right) => right - left,
     };
     comparisonOps = {
-        "==": (left, right) => left === right,
-        "!=": (left, right) => left !== right,
         "<": (left, right) => left < right,
         ">": (left, right) => left > right,
         "<=": (left, right) => left <= right,
@@ -463,24 +404,33 @@ class Evaluator {
     assignOps = {
         "=": (_, right) => right,
         "+=": (left, right) => ({
-            type: left.type === "string" && left.type === right.type ? "string" : "int",
-            value: left.value + right.value
+            type: left.type === "string" && left.type === right.type
+                ? "string"
+                : "int",
+            value: left.value + right.value,
+        }),
+        "-=": (left, right) => ({
+            type: "int",
+            value: left.value - right.value,
         }),
-        "-=": (left, right) => ({ type: "int", value: left.value - right.value }),
     };
 
     builtinFns = {
-        "format": (...args) => this.builtinFormat(...args),
-        "println": (...args) => this.builtinPrintln(...args),
-        "panic": (...args) => this.builtinPanic(...args),
-        "read_text_file": (...args) => this.builtinReadTextFile(...args),
-        "write_text_file": (...args) => this.builtinWriteTextFile(...args),
-        "push": (...args) => this.builtinPush(...args),
-        "at": (...args) => this.builtinAt(...args),
-        "len": (...args) => this.builtinLen(...args),
-        "string_to_int": (...args) => this.builtinStringToInt(...args),
-        "char_code": (...args) => this.builtinCharCode(...args),
-        "strings_join": (...args) => this.builtinStringsJoin(...args),
+        "format": (...args) => this.runtime.builtinFormat(...args),
+        "print": (...args) => this.runtime.builtinPrint(...args),
+        "println": (...args) => this.runtime.builtinPrintln(...args),
+        "panic": (...args) =>
+            this.runtime.builtinPanic(this.currentLine, ...args),
+        "read_text_file": (...args) =>
+            this.runtime.builtinReadTextFile(...args),
+        "write_text_file": (...args) =>
+            this.runtime.builtinWriteTextFile(...args),
+        "push": (...args) => this.runtime.builtinPush(...args),
+        "at": (...args) => this.runtime.builtinAt(...args),
+        "len": (...args) => this.runtime.builtinLen(...args),
+        "string_to_int": (...args) => this.runtime.builtinStringToInt(...args),
+        "char_code": (...args) => this.runtime.builtinCharCode(...args),
+        "strings_join": (...args) => this.runtime.builtinStringsJoin(...args),
     };
 
     consts = {
@@ -496,54 +446,6 @@ class Evaluator {
     ];
 }
 
-function valueToPrint(value) {
-    if (value.type === "null") {
-        return "null";
-    } else if (value.type === "bool") {
-        return `${value.value}`;
-    } else if (value.type === "int") {
-        return `${value.value}`;
-    } else if (value.type === "string") {
-        return `${value.value}`;
-    } else if (value.type === "list") {
-        return `(${value.values.map((v) => valueToString(v)).join(" ")})`;
-    } else {
-        throw new Error(`unknown value type ${value.type}`);
-    }
-}
-
-function valueToString(value) {
-    if (value.type === "null") {
-        return "null";
-    } else if (value.type === "bool") {
-        return `${value.value}`;
-    } else if (value.type === "int") {
-        return `${value.value}`;
-    } else if (value.type === "string") {
-        return `"${value.value}"`;
-    } else if (value.type === "list") {
-        return `(${value.values.map((v) => valueToString(v)).join(" ")})`;
-    } else {
-        throw new Error(`unknown value type ${value.type}`);
-    }
-}
-
-function valueToJs(value) {
-    if (value.type === "null") {
-        return null;
-    } else if (value.type === "bool") {
-        return value.value;
-    } else if (value.type === "int") {
-        return value.value;
-    } else if (value.type === "string") {
-        return value.value;
-    } else if (value.type === "list") {
-        return value.values.map((v) => valueToJs(v));
-    } else {
-        throw new Error(`unknown value type ${value.type}`);
-    }
-}
-
 /**
  * @param {Expr} expr
  * @returns {string}
diff --git a/runtime.js b/runtime.js
new file mode 100644
index 0000000..99d1b1a
--- /dev/null
+++ b/runtime.js
@@ -0,0 +1,210 @@
+import fs from "node:fs";
+import process from "node:process";
+
+export class Runtime {
+    constructor(filename) {
+        this.syms = { parent: undefined, map: new Map(this.builtins) };
+        this.callStack = [{ name: "<entry>", line: 0 }];
+        this.filename = filename;
+        this.currentLine = 0;
+    }
+
+    setLine(line) {
+        this.currentLine = line;
+    }
+
+    pushCall(name, line) {
+        this.callStack.push({ name, line });
+    }
+
+    popCall() {
+        this.callStack.pop();
+    }
+
+    panic(msg) {
+        this.printPanic(msg);
+        process.exit(1);
+    }
+
+    printPanic(msg) {
+        console.error(`\x1b[1;91mpanic\x1b[1;97m: ${msg}\x1b[0m`);
+        this.printCallStack();
+    }
+
+    printCallStack() {
+        const last = this.callStack[this.callStack.length - 1];
+        console.error(
+            `    \x1b[90mat \x1b[37m${last.name} \x1b[90m(${this.filename}:${this.currentLine})\x1b[0m`,
+        );
+        for (let i = this.callStack.length - 2; i >= 0; --i) {
+            const name = this.callStack[i].name;
+            const line = this.callStack[i + 1].line;
+            console.error(
+                `    \x1b[90mat \x1b[37m${name} \x1b[90m(${this.filename}:${line})\x1b[0m`,
+            );
+        }
+    }
+
+    format(msg, ...args) {
+        let value = Runtime.valueToPrint(msg);
+        for (const arg of args) {
+            value = value.replace("%", Runtime.valueToPrint(arg));
+        }
+        return value;
+    }
+
+    eq(left, right) {
+        if (left.type === "null") {
+            return right.type === "null";
+        }
+        if (right.type === "null") {
+            return left.type === "null";
+        }
+
+        if (left.type !== right.type) {
+            this.panic(`cannot compare ${left.type} with ${right.type}`);
+        }
+        const type = left.type;
+        if (["bool", "int", "string"].includes(type)) {
+            return left.value === right.value;
+        } else if (type === "list") {
+            if (left.values.length !== right.values.length) {
+                return false;
+            }
+            for (let i = 0; i < left.values.length; ++i) {
+                if (!this.eq(left.values[i], right.values[i])) {
+                    return false;
+                }
+            }
+            return true;
+        } else {
+            this.panic(`equality not implemented for ${type}`);
+        }
+    }
+
+    builtinFormat(msg, ...args) {
+        return { type: "string", value: this.format(msg, ...args) };
+    }
+
+    builtinPrint(msg, ...args) {
+        process.stdout.write(this.format(msg, ...args));
+        return { type: "null" };
+    }
+
+    builtinPrintln(msg, ...args) {
+        console.log(this.format(msg, ...args));
+        return { type: "null" };
+    }
+
+    builtinPanic(currentLine, msg, ...args) {
+        this.panic(this.format(msg, ...args), currentLine);
+        return { type: "null" };
+    }
+
+    builtinReadTextFile(path) {
+        const text = fs.readFileSync(path.value).toString();
+        return { type: "string", value: text };
+    }
+
+    builtinWriteTextFile(path, text) {
+        fs.writeFileSync(path.value, text.value);
+        return { type: "null" };
+    }
+
+    builtinPush(list, value) {
+        if (list.type === "string") {
+            list.value += value.value;
+            return list;
+        }
+        list.values.push(value);
+        return list;
+    }
+
+    builtinAt(value, index) {
+        if (value.type === "string") {
+            return { type: "string", value: value.value[index.value] };
+        }
+        return value.values[index.value] ?? { type: "null" };
+    }
+
+    builtinSet(subject, index, value) {
+        subject.values[index.value] = value;
+        return { type: "null" };
+    }
+
+    builtinLen(value) {
+        if (value.type === "string") {
+            return { type: "int", value: value.value.length };
+        }
+        return { type: "int", value: value.values.length };
+    }
+
+    builtinStringToInt(value) {
+        return { type: "int", value: Number(value.value) };
+    }
+
+    builtinCharCode(value) {
+        return { type: "int", value: value.value.charCodeAt(0) };
+    }
+
+    builtinStringsJoin(value) {
+        return {
+            type: "string",
+            value: value.values
+                .map((value) => value.value)
+                .join(""),
+        };
+    }
+
+    static valueToPrint(value) {
+        if (value.type === "null") {
+            return "null";
+        } else if (value.type === "bool") {
+            return `${value.value}`;
+        } else if (value.type === "int") {
+            return `${value.value}`;
+        } else if (value.type === "string") {
+            return `${value.value}`;
+        } else if (value.type === "list") {
+            return `(${
+                value.values.map((v) => Runtime.valueToString(v)).join(" ")
+            })`;
+        } else {
+            throw new Error(`unknown value type ${value.type}`);
+        }
+    }
+
+    static valueToString(value) {
+        if (value.type === "null") {
+            return "null";
+        } else if (value.type === "bool") {
+            return `${value.value}`;
+        } else if (value.type === "int") {
+            return `${value.value}`;
+        } else if (value.type === "string") {
+            return `"${value.value}"`;
+        } else if (value.type === "list") {
+            return `(${
+                value.values.map((v) => Runtime.valueToString(v)).join(" ")
+            })`;
+        } else {
+            throw new Error(`unknown value type ${value.type}`);
+        }
+    }
+
+    static valueToJs(value) {
+        if (value.type === "null") {
+            return null;
+        } else if (value.type === "bool") {
+            return value.value;
+        } else if (value.type === "int") {
+            return value.value;
+        } else if (value.type === "string") {
+            return value.value;
+        } else if (value.type === "list") {
+            return value.values.map((v) => Runtime.valueToJs(v));
+        } else {
+            throw new Error(`unknown value type ${value.type}`);
+        }
+    }
+}