Thanks for your very prompt reply Brent.
I just now gave it a try, and it works just like you said.
Thank you!
On Thu, Apr 12, 2012 at 12:31 AM, Brent Yorgey
Hi Ramin,
What you need is Rank2Types:
{-# LANGUAGE Rank2Types #-}
module ABC where
...
apply :: (forall a. TClass a => (a -> a -> a)) -> T -> T -> T apply fn x y = case (x, y) of (TA x, TA y) -> TA (fn x y) (TB x, TB y) -> TB (fn x y) (TC x, TC y) -> TC (fn x y) _ -> error "T constructors do not match"
The problem with the old type signature of apply:
apply :: TClass a => (a -> a -> a) -> T -> T -> T
is that this type means that the *caller* of apply gets to choose the type 'a', and may provide some function which only works for some particular type a (which must be an instance of TClass). But that's not what you want. The new type:
apply :: (forall a. TClass a => (a -> a -> a)) -> T -> T -> T
means that the caller must provide a function which is guaranteed to work for *any* instance of TClass. (Note the 'forall' and the extra parentheses.)
-Brent
On Thu, Apr 12, 2012 at 12:06:28AM +0900, Ramin Honary wrote:
Hi everyone,
I am using GHC 7.0.4. I boiled-down my problem into a simple program which I attached to this message. I don't know how to get the program to compile.
I need to pass arbitrary class member functions as a parameter to a very large, complex, polymorphic function called "apply", and have this class member function applied to abitrary fields of data constructors of a very large, complex data type.
What I expect to happen is this: For each case alternative, GHC will infer the type of the polymorphic function from the matched pattern in the case alternative, and use that inference to select the correct instance function for that type. So for case alternative (TA x, TA y) -> TA (fn x y), the correct instance for "fn" is selected based on the type of "x" and "y" which are determined by the pattern match, and by the type of "TA". If that case alternative does not match, then it should try the next alternative (TB x, TB y) -> TB (fn x y), and since "fn" is polymorphic, it will still be able to select the correct instance for the type given by the pattern match. So I can pass "f1" or "f2" or "f3" to "apply" as the "fn" parameter, and have the correct instance of "f1" or "f2" or "f3" be selected depending on where in the "case" expression "fn" is used.
What is actually happening is: GHC arbitrarily selects one of the case alternatives (usually the last case alternative, sometimes the first) and decides that from now on this one type is the type for "fn" for every case alternative. So for every other case alternative, it decides that "fn" is of the wrong type and rejects the program, rather than trying to infer a different type for "fn" specific to this case alternative. Said another way, the type inference algorithm is being too strict, and I would like it to be more lazy, computing the type once per each case alternative, rather than computing the type only once for the entire case statement. Is there a language option for that?
I have tried using language options, like -XNoMonoPatBinds -XNoMonomorphismRestriction -XMonoLocalBinds -XPolymorphicComponents -XImpredicativeTypes, but none of those help. My current solution is to use Template Haskell and make the "apply" function a splice, so the whole case statement is copied to every place it is used (which takes a very, very long for it time to comple). Is there a way to do this without Template Haskell? Can I get GHC to behave as I expected it to?
Thanks, Ramin
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