From bf6f745595570a3ebaad34b8c4a8a8c827e15178 Mon Sep 17 00:00:00 2001
From: Kjetil Orbekk <kjetil.orbekk@gmail.com>
Date: Fri, 11 Mar 2016 05:20:55 +0100
Subject: Finish parser chapter.

Adds support for a very simple language (described by K.Ast).
---
 src/K/Eval.hs | 40 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)
 create mode 100644 src/K/Eval.hs

(limited to 'src/K/Eval.hs')

diff --git a/src/K/Eval.hs b/src/K/Eval.hs
new file mode 100644
index 0000000..a7ebcba
--- /dev/null
+++ b/src/K/Eval.hs
@@ -0,0 +1,40 @@
+module K.Eval where
+
+import K.Ast
+import K.Parsing
+import Data.Maybe
+
+isNum Zero = True
+isNum (Succ t) = isNum t
+isNum _ = False
+
+isVal :: Expr -> Bool
+isVal Tru = True
+isVal Fals = True
+isVal t | isNum t = True
+isVal _ = False
+
+eval' x = case x of
+    IsZero Zero               -> Just Tru
+    IsZero (Succ t) | isNum t -> Just Fals
+    IsZero t                  -> IsZero <$> eval' t
+
+    Succ t                    -> Succ <$> eval' t
+
+    Pred Zero                 -> Just Zero
+    Pred (Succ t) | isNum t   -> Just t
+    Pred t                    -> Pred <$> eval' t
+
+    If Tru t _                -> Just t
+    If Fals _ f               -> Just f
+    If c t f                  -> (\c' -> If c' t f) <$> eval' c
+
+    _                         -> Nothing
+
+nf x = fromMaybe x (nf <$> eval' x)
+
+eval :: Expr -> Maybe Expr
+eval t = case nf t of
+    nft | isVal nft -> Just nft
+        | otherwise -> Nothing
+    
-- 
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