Merge branch 'functor'

functor.hs

@@ -0,0 +1,29 @@
+-- Slide 64
+
+data Expr a = Val a | Inc (Expr a) | Dec (Expr a) | Inv (Expr a) | Neg (Expr a)
+
+evaluate::(Fractional a)=>Expr a -> a
+evaluate (Val x) = x
+evaluate (Inc e) = 1 + evaluate e
+evaluate (Dec e) = (evaluate e) - 1
+evaluate (Inv e) = 1 / (evaluate e)
+evaluate (Neg e) = negate (evaluate e)
+
+-- data Maybe a = Nothing | Just a
+
+isZero::(Num a, Eq a)=>Maybe a -> Bool
+isZero m = case m of 
+	Nothing -> True
+	Just x -> if x == 0 then True else False
+
+mfmap::(Fractional a) => (a -> b) -> Maybe a -> Maybe b
+mfmap f m = case m of
+	Nothing -> Nothing
+	Just x -> Just (f x)
+
+safeval::(Fractional a, Eq a) => Expr a -> Maybe a
+safeval (Val x) = Just x
+safeval (Inc e) = mfmap (+ 1) (safeval e)
+safeval (Dec e) = mfmap (subtract 1) (safeval e)
+safeval (Inv e) = let v = safeval e in if isZero v then Nothing else mfmap (1 /) v
+safeval (Neg e) = mfmap negate (safeval e)