gistfile1.hs

module Test where

data Range
    = FromTo Integer Integer
    deriving (Eq, Ord)

instance Show Range where
    show (FromTo x y) = show x ++ " --> " ++ show y

x --> y
    | x <= y    = FromTo x y
    | otherwise = error "Invalid range, begin > end"

instance Num Range where
    (+) = rBinOp (+)
    (*) = rBinOp (*)

    x - y = x + negate y

    negate      x = rMap negate $ rangeFlip x
    abs         x = rMap abs x
    signum      x = rMap signum x
    fromInteger x = x --> x

rMap f (FromTo    beg     end)
      = FromTo (f beg) (f end)

pairFlip (beg, end)
       = (end, beg)

rangeFlip (FromTo beg end)
         = FromTo end beg

rBinOp
    binOp
    (FromTo beg1 end1)
    (FromTo beg2 end2)
    = beg --> end
    where
    beg = beg1 `binOp` beg2
    end = end1 `binOp` end2

x =  1 --> 10
y = 10 --> 20
k = 4

--- ghci output ---

*Test> x * (k --> k)
4 --> 40
*Test> x * k

<interactive>:1:4:
    Couldn't match expected type `Range'
           against inferred type `Integer'
    In the second argument of `(*)', namely `k'
    In the expression: x * k
    In the definition of `it': it = x * k
*Test> x * fromInteger k
4 --> 40
*Test>