post link to transformers
[lambda.git] / code / ski_evaluator.hs
index 26f1c0e..348b216 100644 (file)
@@ -1,24 +1,24 @@
-data Term = I | S | K | App Term Term deriving (Eq, Show)      
-                                                              
-skomega = (App (App (App S I) I) (App (App S I) I))                      
+data Term = I | S | K | App Term Term deriving (Eq, Show)
+
+skomega = (App (App (App S I) I) (App (App S I) I))
 test = (App (App K I) skomega)
 test = (App (App K I) skomega)
-                                                              
-reduce_one_step :: Term -> Term                                      
-reduce_one_step t = case t of                                      
-  App I a -> a                                                      
-  App (App K a) b -> a                                              
-  App (App (App S a) b) c -> App (App a c) (App b c)                      
-  _ -> t                                                      
-                                                              
-is_redex :: Term -> Bool                                      
-is_redex t = not (t == reduce_one_step t)                      
-                                                              
-reduce :: Term -> Term                                              
-reduce t = case t of                                              
-  I -> I                                                      
-  K -> K                                                      
-  S -> S                                                      
-  App a b ->                                                       
-    let t' = App (reduce a) (reduce b) in                      
-      if (is_redex t') then reduce (reduce_one_step t')      
-                       else t'                                
+
+reduce_if_redex :: Term -> Term
+reduce_if_redex t = case t of
+  App I a -> a
+  App (App K a) b -> a
+  App (App (App S a) b) c -> App (App a c) (App b c)
+  _ -> t
+
+is_redex :: Term -> Bool
+is_redex t = not (t == reduce_if_redex t)
+
+reduce_try2 :: Term -> Term
+reduce_try2 t = case t of
+  I -> I
+  K -> K
+  S -> S
+  App a b ->
+    let t' = App (reduce_try2 a) (reduce_try2 b) in
+      if (is_redex t') then reduce_try2 (reduce_if_redex t')
+                       else t'