[Haskell-cafe] Ada-style ranges

[Steffen Schuldenzucker]

On 04/26/2010 12:50 PM, haskell at kudling.de wrote:
> 
> 
> Hi list,
> 
>  
> 
> how would you describe Ada's ranges in Haskell's typesystem?
> 
> http://en.wikibooks.org/wiki/Ada_Programming/Types/range

Hi Lenny,

can non-constant expressions be given as arguments to 'range'? If not, then
what about a opaque wrapper type?

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

module Range1 (Range1, fromRange1, mkBounded, mkRange1) where

newtype Range1 = Range1 { fromRange1 :: Integer }
    deriving (Eq, Num, Ord, Show)

instance Bounded Range1 where
    minBound = Range1 $ -5
    maxBound = Range1 $ 10

mkBounded :: (Bounded a, Ord a) => (b -> a) -> b -> Maybe a
mkBounded f x = case f x of
    y | minBound <= y && y <= maxBound -> Just y
      | otherwise                      -> Nothing

mkRange1 ::  Integer -> Maybe Range1
mkRange1 = mkBounded Range1

-- Steffen