Re: [Haskell-cafe] Re: Polymorphic record field?

On Sunday 26 September 2010 14:00:38, Kevin Jardine 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?

Ah, yes, forgot about that. As GHC says, using getMyValue would let the quantified type variable escape, the type would be getMyValue :: exists a. MyStruct -> a (not allowed in Haskell). You can only use myField per pattern matching foo :: MyStruct -> whatever foo (MyStruct field) = methodOfMyTypeClass field

On Sep 26, 1:09 pm, Daniel Fischer 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 #-}