[Haskell-cafe] Re: Polymorphic record field?

[Daniel Fischer]

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 <daniel.is.fisc... at 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 #-}