[Haskell-cafe] weird type signature in Arrow Notation
Sebastian Fischer
fischer at nii.ac.jp
Fri Aug 5 03:43:36 CEST 2011
I created a ticket with a slightly further simplified program:
http://hackage.haskell.org/trac/ghc/ticket/5380
On Fri, Aug 5, 2011 at 10:10 AM, Sebastian Fischer <fischer at nii.ac.jp>wrote:
> here is a reduced program that still segfaults:
>
> {-# LANGUAGE Arrows #-}
>
> import Control.Arrow
>
> main :: IO ()
> main = print segfault
>
> segfault :: [()]
> segfault = anythingYouWant ()
>
> anythingYouWant :: a
> anythingYouWant = testB False (const ()) ()
>
> testB :: ArrowChoice arrow
> => bool -> arrow () () -> arrow () anything
> testB bool arrow =
> proc () ->
> do if bool then arrow -< ()
> else arrow -< ()
>
> Sebastian
>
> On Fri, Aug 5, 2011 at 6:20 AM, Brent Yorgey <byorgey at seas.upenn.edu>wrote:
>
>> On Tue, Aug 02, 2011 at 05:08:33PM -0400, bob zhang wrote:
>> > hi, all
>> > testB :: (ArrowChoice t1, Num a1, Num a) => (a -> a1 -> t2) -> t1 a t3
>> > -> t1 a1 t3 -> t1 (a, a1) t
>> > testB f g h = proc (x,y) -> do
>> > if (f x y)then g -< x + 1 else h -< y + 2
>> >
>> > it's very strange that the type of _f_ is (a->a1->t2) which I thought
>> > should be a -> a1 -> Bool,
>> >
>> > btw, is there any way to get the output of preprocessing using -XArrow
>> > extensions,
>> >
>> > Thanks a lot
>> > best, bob
>>
>> Congratulations, you have definitely found a GHC bug! Note there are
>> actually two things wrong with testB's type signature: first, t2 ought
>> to be Bool, as you note. But even worse, notice that the return type
>> of the result arrow, t, has nothing to do with any of the other types!
>> This means that we can use testB along with the (->) instance for
>> Arrow to construct elements of arbitrary types:
>>
>> ghci> let anythingYouWant = testB (\x y -> False) (const 3) (const 2)
>> (2,2)
>> ghci> :t anythingYouWant
>> anythingYouWant :: t
>> ghci> anythingYouWant :: Integer
>> 2
>> ghci> anythingYouWant :: Int
>> 2
>> ghci> anythingYouWant :: Double
>> 1.0e-323
>> ghci> anythingYouWant :: Char
>> '\STX'
>> ghci> (anythingYouWant :: (Double -> Double) -> [Double]) sqrt
>> [
>> [1] 17391 segmentation fault ghci
>>
>> whoops!
>>
>> I'm using GHC 7.0.3, but Daniel Wagner and I also tried it (with the
>> same results) on GHC 7.2.0rc1 and GHC HEAD.
>>
>> I wasn't able to find a ticket for this on the GHC bug tracker, I
>> guess we should file one!
>>
>> I tried to find a way to get the output of preprocessing using -XArrow
>> but wasn't able to find one (other than -ddump-ds which gives you the
>> unoptimized *GHC core* output, which is quite hard to read).
>>
>> -Brent
>>
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>>
>
>
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