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- -- 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)
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