On Sun, 2013-09-01 at 15:51 -0700, Wvv wrote: > I think it is an old idea, but nevertheless. > Now we have next functions: > > if (a :: Bool) then x else y > > case b of > a1 :: Bool -> x1 > a2 :: Bool -> x2 > ... > > Let we have generic conditions for 'if' and 'case': > > class Boolean a where > toBool :: a -> Bool > > instance Boolean Bool where > toBool = id > > instance Boolean [a] where > toBool [] = False > toBool _ = True > > instance Boolean (Maybe a) where > toBool Nothing = False > toBool _ = True > > instance Boolean Int where > toBool 0 = False > toBool _ = True > > if' (a :: Boolean b) then x else y > > case' d of > a1 :: Boolean b1 -> x1 > a2 :: Boolean b2 -> x2 > ... > > > It is very easy to implement to desugar: > if' a then ... == if toBool ( a ) then ...
I wasn't at my computer when I sent my previous reply, so here's a more full-fledged answer: This is possible using the RebindableSyntax extension. Make sure to read the documentation of the extension before using it, it might have some unexpected implications. Be careful when using this scheme as well... I'd think lots of Haskell'ers would frown upon this kind of implicit conversions (they remind me of Python and its __nonzero__ stuff). 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@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe