Method contexts

[Marcin 'Qrczak' Kowalczyk]

Sun, 28 Jan 2001 19:41:15 +0100, Hannah Schroeter <uk1o at rz.uni-karlsruhe.de> pisze:

> Why can't you write:
> 
> class Eq k => Member c k where
>   member :: c -> k -> Bool
> 
> > class Member c a => Seq c a
> 
> instance Eq a => Member [a] a where
>   ...
> 
> > instance Seq [a] a

Because the last instance is then illegal: requires Eq a in the
context, which I obviously don't want.

BTW, there is a bug in the ghc's CVS HEAD. The following program:

class Eq k => Member c k where
    member :: c -> k -> Bool
class Member c a => Seq c a where
    list :: c -> [a]
instance Eq a => Member [a] a where
    member = flip elem
instance Seq [a] a where
    list = id
type F = Int -> Int
test:: [F] -> F -> Bool
test = member
main:: IO ()
main = print ((id :: F) == id)

does compile. When the definition of test is removed, there is a
correst error about missing instance Eq (Int -> Int). The use of
member:: [F] -> F -> Bool magically creates this instance!

Unfortunately it does not really allow to compare functions and the
answer is <<loop>> :-)

-- 
 __("<  Marcin Kowalczyk * qrczak at knm.org.pl http://qrczak.ids.net.pl/
 \__/
  ^^                      SYGNATURA ZASTÊPCZA
QRCZAK