Proposal: Generic conditions for 'if' and 'case'
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 ... -- View this message in context: http://haskell.1045720.n5.nabble.com/Proposal-Generic-conditions-for-if-and-... Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.
I didn't test it, but you might want to look into the 'rebindable syntax'
extension and its 'ifThenElse' feature.
Nicolas
On Sep 2, 2013 12:51 AM, "Wvv"
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 ...
-- View this message in context: http://haskell.1045720.n5.nabble.com/Proposal-Generic-conditions-for-if-and-... Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
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
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@
-- View this message in context: http://haskell.1045720.n5.nabble.com/Proposal-Generic-conditions-for-if-and-... Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.
participants (2)
-
Nicolas Trangez -
Wvv