diff --git a/src-haskell/Interpreter.hs b/src-haskell/Interpreter.hs
index 88e7699..2b03c56 100644
--- a/src-haskell/Interpreter.hs
+++ b/src-haskell/Interpreter.hs
@@ -47,11 +47,20 @@ interpret strategy (Prog defs (DMain mainExp)) = do
 
     -- Return the result.
     case v of
-      _ -> todo "extract result"
+      VInt i -> return i
+      VAbs{} -> throwError "main should return an integer"
 
   where
 
-    cxt = todo "initial context"
+    cxt = Cxt
+      { cxtStrategy  = strategy
+      , cxtSig       = Map.fromList $ map (\ (DDef f xs e) -> (f, mkDef xs e)) defs
+      , cxtEnv       = Map.empty
+      }
+
+    -- | Turn @f x1 ... xn = e@  into @f = \ x1 -> ... \ xn -> e@.
+    mkDef :: [Ident] -> Exp -> Exp
+    mkDef xs e = foldr EAbs e xs
 
 ---------------------------------------------------------------------------
 -- * Data structures for the interpreter.
@@ -61,11 +70,19 @@ interpret strategy (Prog defs (DMain mainExp)) = do
 
 data Cxt = Cxt
   { cxtStrategy :: Strategy  -- ^ Evaluation strategy (fixed).
-  -- TODO environment
+  , cxtSig      :: Signature
+  , cxtEnv      :: Env
   }
 
+type Signature = Map Ident Exp
+
+type Env = Map Ident Value
+
 -- | Values.
-data Value = Value_TODO
+data Value
+  = VInt Integer
+  | VAbs Env Ident Exp -- ^ Function closure
+
 
 ---------------------------------------------------------------------------
 -- * Interpreter.
@@ -75,18 +92,29 @@ data Value = Value_TODO
 eval :: Cxt -> Exp -> Err Value
 eval cxt = \case
 
+  EInt i -> return $ VInt i
+
   -- Call-by-value evaluation rules:
   --
   -- Variable:
   --
   --      ------------
   --      γ ⊢ x ⇓ γ(x)
+
+  EVar x -> do
+     case Map.lookup x (cxtEnv cxt) of
+       Just v  -> return v
+       Nothing -> case Map.lookup x (cxtSig cxt) of
+         Just e -> eval (cxt { cxtEnv = Map.empty }) e
+         Nothing -> throwError $ unwords ["unbound variable", printTree x]
+
   --
   -- Lambda:
   --
   --      --------------------------------
   --      γ ⊢ (λx → f) ⇓ (let γ in λx → f)
-  --
+  EAbs x e -> return $ VAbs (cxtEnv cxt) x e
+
   -- Application:
   --
   --      γ      ⊢ e₁ ⇓ let δ in λx → f
@@ -94,6 +122,12 @@ eval cxt = \case
   --      δ,x=v₂ ⊢ f ⇓ v
   --      -----------------------------
   --      γ ⊢ e₁ e₂ ⇓ v
+  EApp e1 e2 -> do
+    f <- eval cxt e1
+    v <- eval cxt e2
+    case f of
+      VInt{} -> throwError $ unwords [ "cannot apply an integer" ]
+      VAbs delta x e -> eval (cxt { cxtEnv = Map.insert x v delta }) e
 
   e -> error $ unwords [ "Not yet implemented: evaluation of", printTree e ]