diff --git a/src/component/app/App.tsx b/src/component/app/App.tsx
index 15919f3..2509f1f 100644
--- a/src/component/app/App.tsx
+++ b/src/component/app/App.tsx
@@ -137,8 +137,10 @@ export function App() {
 
   const currentState = appState.history.at(-1)!;
 
-  // infer types for entire app state
+  // static evalution
   const typeInfo = useMemo(() => inferType(currentState, extendedEnv), [currentState]);
+  // dynamic evalutions
+  const evalResult = evalExpr(currentState, extendedEnv);
 
   return (
     <>
@@ -186,7 +188,7 @@ export function App() {
           />
           =
           <Value dynamic={{
-            i: evalExpr(currentState, extendedEnv),
+            i: evalResult.val,
             t: typeInfo.type,
           }}/>
           ::
diff --git a/src/component/expr/ExprBlock.tsx b/src/component/expr/ExprBlock.tsx
index 1990538..32d7d80 100644
--- a/src/component/expr/ExprBlock.tsx
+++ b/src/component/expr/ExprBlock.tsx
@@ -11,6 +11,7 @@ import { LambdaBlock, type LambdaBlockProps, type LambdaBlockState } from "./Lam
 import { LetInBlock, type LetInBlockProps, type LetInBlockState } from "./LetInBlock";
 
 import "./ExprBlock.css";
+import { evalExpr } from "../../eval/eval";
 
 export type ExprBlockState =
     InputBlockState
@@ -49,12 +50,13 @@ export function ExprBlock(props: ExprBlockProps) {
   
   const actions = getActions(globalContext, props.setState);
   const extraHandlers = Object.fromEntries(Object.entries(actions).map(([shortcut, action]) =>
-    [shortcut, (e) => { e.preventDefault(); action(); }]))
-  return <span className={"editor" + (props.typeInfo.err ? " error" : "")}>
+    [shortcut, (e) => { e.preventDefault(); action(); }]));
+  const err = props.typeInfo.err || evalExpr(props.state, env).err;
+  return <span className={"editor" + (err ? " error" : "")}>
     {renderBlock[props.state.kind]()}
-    {(props.typeInfo.err !== undefined) &&
+    {(err !== undefined) &&
       (<div className="errorMessage">
-        {props.typeInfo.err.message.trim()}
+        {err.message.trim()}
       </div>)}
     <Input
       placeholder="<c>"
diff --git a/src/eval/eval.ts b/src/eval/eval.ts
index 86d9578..1268f8f 100644
--- a/src/eval/eval.ts
+++ b/src/eval/eval.ts
@@ -9,7 +9,10 @@ export interface DynamicEnvironment {
   names: any;
 }
 
-export type EvalResult = any;
+export interface EvalResult {
+  val?: any;
+  err?: Error;
+};
 
 export function evalExpr(s: ExprBlockState, env: DynamicEnvironment): EvalResult {
   if (s.kind === "input") {
@@ -28,44 +31,65 @@ export function evalExpr(s: ExprBlockState, env: DynamicEnvironment): EvalResult
 
 export function evalInput(s: InputBlockState, env: DynamicEnvironment): EvalResult {
   if (s.value.kind === "literal") {
+    if (s.text === '') {
+      return {
+        err: new Error('cannot parse empty string as '+s.value.type)
+      };
+    }
     const ctor = {
       Int: BigInt,
       Double: Number,
     }[s.value.type] as (s: string) => any;
-    return ctor(s.text);
+    return {
+      val: ctor(s.text)
+    };
   }
   else if (s.value.kind === "name") {
     const found = trie.get(env.names)(s.text);
     if (found) {
       if (found.recursive) {
+        // dirty
         return found.i();
       }
-      return found.i;
+      return {val: found.i};
     }
   }
+  return {
+    err: new Error(`'${s.text}' not found`),
+  }
 }
 
 export function evalCall(s: CallBlockState, env: DynamicEnvironment): EvalResult {
   const fn = evalExpr(s.fn, env);
   const input = evalExpr(s.input, env);
-  if (fn !== undefined && input !== undefined) {
+  if (fn.val !== undefined && input.val !== undefined) {
     try {
-      return fn(input);
+      const result = fn.val(input.val)
+      return { val: result } ;
+    }
+    catch (e: any) {
+      return { err: e };
     }
-    catch {}
   }
+  return {};
 }
 
 export function evalLet(s: LetInBlockState, env: DynamicEnvironment): EvalResult {
   const valueEnv = {
     names: trie.insert(env.names)(s.name)({
       recursive: true,
-      i: () => { try { return value; } catch (e) {} },
+      i: () => {
+        try {
+          return { val };
+        } catch (e) {
+          return { err: e };
+        }
+      },
     }),
   };
-  const value = evalExpr(s.value, valueEnv);
+  const {val} = evalExpr(s.value, valueEnv);
   const innerEnv = {
-    names: trie.insert(env.names)(s.name)({i: value}),
+    names: trie.insert(env.names)(s.name)({i: val}),
   }
   return evalExpr(s.inner, innerEnv);
 }
@@ -75,7 +99,8 @@ export function evalLambda(s: LambdaBlockState, env: DynamicEnvironment): EvalRe
     const innerEnv = {
       names: trie.insert(env.names)(s.paramName)({i: x})
     };
-    return evalExpr(s.expr, innerEnv);
+    const result = evalExpr(s.expr, innerEnv);
+    return result.val;
   };
-  return fn;
+  return {val: fn};
 }