[Haskell-cafe] A type signature inferred by GHCi that is rejected
when written explicitly
Pablo Nogueira
pablo at babel.ls.fi.upm.es
Tue Jul 8 04:03:21 EDT 2008
> I myselft don't understand why GHCi doesn't accept the type it
> infered as an explicit signature ...
I think it has to do with the following:
Looking at the type errors, they seem to indicate that the type
checker is being general and does not assume the |From| and |To|
"relations" are between
a type |t| and (s (t x) x)| but, in general, between |t| and |s (t' x) x|.
Given that
from :: (From a1 c1 x) => a1 x -> c1 x
to :: (To a2 c2 y) => c2 y -> a2 y
bimap :: Bifunctor s => (t1 -> t3) -> (t2 -> t4) -> s t1 t2 -> s t3 t4
During type checking the following equations spring up:
c2 y = s t3 t4
c1 x = s t1 t2
t2 = x
t4 = y
t1 = a1 x
t3 = a2 y
That'd give the same type as that inferred, but somehow new variables
|a11| and |a12| appear.
> caused by a lack of functional dependencies.
> class From a c x | a -> c where
> class To a c y | c -> a where
> ... hushes GHCi. The question now is, of course, if the new
> dependencies are too restrictive for your problem.
They are of little avail given the instances I define:
instance Bifunctor s => From (Fix s) (s (Fix s x)) x where
from = out
instance Bifunctor s => To (Fix s) (s (Fix s y)) y where
to = In
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