Thanks! It is a good toy for testing!
Nicolas Trangez wrote
> Here's an example implementing your proposal:
>
> {-# LANGUAGE RebindableSyntax #-}
>
> import Prelude
>
> class Boolean a where
> toBool :: a -> Bool
>
> instance Boolean Bool where
> toBool = id
>
> instance Boolean [a] where
> toBool = not . null
>
> instance Boolean (Maybe a) where
> toBool = maybe False (const True)
>
> instance Boolean Int where
> toBool = (/= 0)
>
> ifThenElse :: Boolean a => a -> b -> b -> b
> ifThenElse i t e = case toBool i of
> True -> t
> False -> e
>
> main :: IO ()
> main = do
> test False
> test ([] :: [Int])
> test [1]
> test (Nothing :: Maybe Int)
> test (Just 1 :: Maybe Int)
> test (0 :: Int)
> test (1 :: Int)
> {- test 'c' fails to type-check: no instance Boolean Char defined!
> -}
> where
> test v = putStrLn $ show v ++ " is " ++ (if v then "true" else
> "false")
>
> which outputs
>
> False is false
> [] is false
> [1] is true
> Nothing is false
> Just 1 is true
> 0 is false
> 1 is true
>
> Using RebindableSyntax, 'if I then T else E' is rewritten into
> 'ifThenElse I T E' by the compiler, for whatever 'ifThenElse' is in
> scope.
>
> Nicolas
>
>
> _______________________________________________
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