Hi,
You probably want to read up on:
Monomorphism restriction (I don't think this applies here, but I'm
never too sure!):
http://www.haskell.org/hawiki/MonomorphismRestriction
Defaulting:
http://www.haskell.org/onlinereport/decls.html#sect4.3.4
I don't understand either to any great degree, but when points free
starts changing things, its usually one of those two kicking in.
And as a side note, haskell@ tends to be used more for announcements,
and haskell-cafe@ is used more for asking questions etc.
Thanks
Neil
On 11/3/06, Dan Weston
Help! One of two things is going on:
1) I don't understand what I'm doing 2) GHC is inferring different types for pointed and point-free function definition.
I wanted to define Haskell equivalents to the C ternary operator. Basically, predicate ??? doIfTrue ||| doIfFalse
For some reason, though, in GHC 6.4 and 6.7, the types inferred for abs1 and ab2 (defined below) are different:
*Main> :t abs1 abs1 :: (Num a, Ord a) => a -> a
*Main> :t abs2 abs2 :: Integer -> Integer
How is abs2 less general just because I use point-free notation instead of pointed notation? They should have the same type, no? I'm stumped...
import Control.Arrow import Data.Either
infix 1 ?, ??, ???, ????
(?) True = Left (?) False = Right
(??) True = Left . fst (??) False = Right . snd
p ??? q = (p &&& arr id) >>> uncurry (?) >>> q p ???? q = (p &&& arr id) >>> uncurry (??) >>> q
abs1 x = (< 0) ??? negate ||| id $ x abs2 = (< 0) ??? negate ||| id
checks :: [Bool] checks = [ (True ? 3 ) == (Left 3), (False ? 4 ) == (Right 4), (True ?? (3,4)) == (Left 3), (False ?? (3,4)) == (Right 4), (even ??? (`div` 2) ||| (+1).(*3) $ 3 ) == ( 10), (even ??? (`div` 2) ||| (+1).(*3) $ 4 ) == ( 2), (uncurry (==) ???? (+1) ||| (*3) $ (3,3)) == ( 4), (uncurry (==) ???? (+1) ||| (*3) $ (3,4)) == ( 12), (uncurry (==) ???? (+1) +++ (*3) $ (3,3)) == (Left 4), (uncurry (==) ???? (+1) +++ (*3) $ (3,4)) == (Right 12), (abs1 5 ) == ( 5), (abs1 (-5) ) == ( 5), (abs2 5 ) == ( 5), (abs2 (-5) ) == ( 5) ]
main = print (and checks)
_______________________________________________ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell