** this is literate haskell
hoping someone can help me. What I am trying to do is
class Ba1 a
ba1 :: Ba1 a => a -> IO ()
ba1 x = print "helllo"
what I wish to do is declare another function
class Foo a
proxy :: Foo a => a -> IO ()
so that I can do something like
proxy x = ba1 x
this does not work in hugs and gives
ERROR "test.lhs":19 - Cannot justify constraints in explicitly typed binding
*** Expression : proxy
*** Type : Foo a => a -> IO ()
*** Given context : Foo a
*** Constraints : Ba1 a
which is entirely expected. The trick is to tell hugs that Foo and Ba1 are
the same
so how do I do that ? I have tried
instance Ba1 a => Foo a
instance Foo a => Ba1 a
which should define equality but to no avail ??
Can this be done ?
I thought the problem might be drawing the inference Foo == Ba1
and to try and force the inference i did:
class Foo a
proxy1 :: (Foo a,Ba1 a) => a -> IO ()
proxy1 x = ba1 x
proxy2 :: Foo a => a -> IO ()
proxy2 x = proxy x
This typed checked but
Main> :load test
Main> proxy2
ERROR -
*** The type checker has reached the cutoff limit while trying to
*** determine whether:
*** Foo a
*** can be deduced from:
*** ()
*** This may indicate that the problem is undecidable. However,
*** you may still try to increase the cutoff limit using the -c
*** option and then try again. (The current setting is -c40)
any ideas out there ??
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