[Haskell-cafe] known, I know: class contexts and mutual recursion
Conor McBride
ctm at cs.nott.ac.uk
Wed Nov 29 13:14:56 EST 2006
Hi folks
I just tripped over the "Contexts differ in length" error message. I
know it's not a new problem, but I thought I'd enquire as to its status.
For those of you who haven't seen it, here's an example, contrived but
compact.
> data Thing
> = Val Int
> | Grok Thing (Maybe Int -> Int)
> eval :: Monad m => Thing -> m Int
> eval (Val i) = return i
> eval (Grok t f) = return (f (eval t))
My eval function compiles ok. See? The recursive call to eval targets
the Maybe monad, so I get
*Mmm> eval (Grok (Val 5) (maybe 0 (1 +))) :: Maybe Int
Just 6
However, when I try to decompose eval as a pair of mutually recursive
functions, namely
> foo :: Monad m => Thing -> m Int
> foo (Val i) = return i
> foo (Grok t f) = return (goo t f)
> goo :: Thing -> (Maybe Int -> Int) -> Int
> goo t f = f (foo t)
I get
Mmm.lhs:15:1:
Contexts differ in length
When matching the contexts of the signatures for
foo :: forall (m :: * -> *). (Monad m) => Thing -> m Int
goo :: Thing -> (Maybe Int -> Int) -> Int
The signature contexts in a mutually recursive group should all be
identical
Poking about on the web, I got the impression that this was a known
infelicity in ghc 6.4 (which I'm using), due to be ironed out. However,
an early-adopting colleague with 6.6 alleges that foo-goo is still
poisonous. I'm wondering what the story is. I mean, is there some nasty
problem lurking here which prevents the lifting of this peculiar
restriction?
I'm not in a panic about this. I have a workaround for the problem as I
encountered it in practice. Mind you, it's the sort of thing that's
likely to happen more often, the more you localise the effects you tend
to use. In the above, goo doesn't throw exceptions; rather, because goo
has a handler, it can offer a /local/ exception-throwing capability to foo.
Curious
Conor
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