1 files changed, 8 insertions, 9 deletions
diff --git a/src/HW.hs b/src/HW.hs
index a2678ca..6f4a8fc 100644
--- a/src/HW.hs
+++ b/src/HW.hs
@@ -34,20 +34,19 @@ subst n x (Lam y m)
-- | Take a single step in normal order reduction or return Nothing
normalstep :: Expr -> Maybe Expr
+
-- beta
normalstep (App (Lam x m) n) = Just (subst n x m)
+
-- body
-normalstep (Lam x m) = case normalstep m of
- Just m' -> Just (Lam x m')
- Nothing -> Nothing
+normalstep (Lam x m) = normalstep m >>= return . Lam x
+
-- arg
-normalstep (App m n) | normalstep m == Nothing = case normalstep n of
- Just n' -> Just (App m n')
- Nothing -> Nothing
+normalstep (App m n) | normalstep m == Nothing = normalstep n >>= return . App m
+
-- func
-normalstep (App m n) = case normalstep m of
- Just m' -> Just (App m' n)
- Nothing -> Nothing
+normalstep (App m n) = normalstep m >>= return . (`App` n)
+
-- No further reductions
normalstep _ = Nothing
|
