[Haskell-cafe] Proposal: Generic conditions for 'if' and 'case'

[Wvv]

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
> 
> 
> _______________________________________________
> Haskell-Cafe mailing list

> Haskell-Cafe@

> http://www.haskell.org/mailman/listinfo/haskell-cafe





--
View this message in context: http://haskell.1045720.n5.nabble.com/Proposal-Generic-conditions-for-if-and-case-tp5735366p5735424.html
Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.