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

2 Sep 2013


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

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