Re: [Haskell-cafe] Proposal: Generic conditions for 'if' and 'case'
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
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Re: [Haskell-cafe] Proposal: Generic conditions for 'if' and 'case'
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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[Haskell-cafe] 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-case-tp5735366.html 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
Re: [Haskell-cafe] Proposal: Generic conditions for 'if' and 'case'
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 vite...@rambler.ru 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 ... -- View this message in context: http://haskell.1045720.n5.nabble.com/Proposal-Generic-conditions-for-if-and-case-tp5735366.html 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
