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