OK, I have a solution. Ugly, but it compiles. MyStruct actually has quite a few fields but I only need to access the polymorphic field 4 times. So for the functions that needed that I wrote: f myS@(MyStruct value _ _ _ ) and then could use value. I could then use the usual record accessors on myS to get the other (non-polymorphic) data. I guess that is what GHC was hinting at when it suggested pattern matching. That is, pattern matching could extract a value that no functions are apparently allowed to touch. All head spinning to me. Thanks for the (as always) fast and useful advice! Kevin On Sep 26, 2:00 pm, Kevin Jardine <kevinjard...@gmail.com> wrote:
OK, thanks for this advice.
The type definition compiles, but when I try to actually access myField, the compiler says:
Cannot use record selector `myField' as a function due to escaped type variables Probable fix: use pattern-matching syntax instead
So I took the hint and wrote a new pattern matching accessor function:
getMyField (MyStruct value) = value
and I get:
Inferred type is less polymorphic than expected Quantified type variable `a' escapes When checking an existential match that binds value :: a
Any further suggestions?
On Sep 26, 1:09 pm, Daniel Fischer <daniel.is.fisc...@web.de> wrote:> On Sunday 26 September 2010 12:53:46, Michael Snoyman wrote:
data MyStruct = forall a. MyTypeClass a => MyStruct {myField :: a}
Note that that requires {-# LANGUAGE ExistentialQuantification #-} _______________________________________________ Haskell-Cafe mailing list Haskell-C...@haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe
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