diff --git a/src-haskell/Interpreter.hs b/src-haskell/Interpreter.hs
index d69677a..88e7699 100644
--- a/src-haskell/Interpreter.hs
+++ b/src-haskell/Interpreter.hs
@@ -1,5 +1,8 @@
+{-# LANGUAGE LambdaCase #-}
+
 {-# OPTIONS_GHC -Wno-unused-imports #-} -- Turn off unused import warning off in stub
 {-# OPTIONS_GHC -Wno-unused-matches #-} -- Turn off unused binding warning off in stub
+{-# OPTIONS_GHC -Wno-unused-top-binds #-} -- Turn off unused binding warning off in stub
 
 -- | Interpreter for lambda-calculus with if, +, -, <.
 --
@@ -16,6 +19,8 @@ import Data.Functor
 import Data.Map (Map)
 import qualified Data.Map as Map
 
+import Debug.Trace
+
 import Fun.Abs
 import Fun.Print
 
@@ -24,6 +29,7 @@ import Fun.Print
 data Strategy
   = CallByName
   | CallByValue
+  deriving (Eq, Show)
 
 -- | Error monad.
 
@@ -33,4 +39,64 @@ type Err = Except String
 
 interpret :: Strategy -> Program -> Err Integer
 interpret strategy (Prog defs (DMain mainExp)) = do
-  throwError $ "TODO: implement interpreter"
+
+    when (strategy == CallByName) $ todo $ "call-by-name"
+
+    -- Run the interpreter.
+    v <- eval cxt mainExp
+
+    -- Return the result.
+    case v of
+      _ -> todo "extract result"
+
+  where
+
+    cxt = todo "initial context"
+
+---------------------------------------------------------------------------
+-- * Data structures for the interpreter.
+---------------------------------------------------------------------------
+
+-- | Context.
+
+data Cxt = Cxt
+  { cxtStrategy :: Strategy  -- ^ Evaluation strategy (fixed).
+  -- TODO environment
+  }
+
+-- | Values.
+data Value = Value_TODO
+
+---------------------------------------------------------------------------
+-- * Interpreter.
+---------------------------------------------------------------------------
+
+-- | Evaluation.
+eval :: Cxt -> Exp -> Err Value
+eval cxt = \case
+
+  -- Call-by-value evaluation rules:
+  --
+  -- Variable:
+  --
+  --      ------------
+  --      γ ⊢ x ⇓ γ(x)
+  --
+  -- Lambda:
+  --
+  --      --------------------------------
+  --      γ ⊢ (λx → f) ⇓ (let γ in λx → f)
+  --
+  -- Application:
+  --
+  --      γ      ⊢ e₁ ⇓ let δ in λx → f
+  --      γ      ⊢ e₂ ⇓ v₂
+  --      δ,x=v₂ ⊢ f ⇓ v
+  --      -----------------------------
+  --      γ ⊢ e₁ e₂ ⇓ v
+
+  e -> error $ unwords [ "Not yet implemented: evaluation of", printTree e ]
+
+-- | TODOs.
+todo :: String -> a
+todo msg = error $ unwords [ "Not yet implemented:", msg ]