[Haskell-cafe] Re: Polymorphic record field?

[Kevin Jardine]

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... at 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... 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 #-}
> > _______________________________________________
> > Haskell-Cafe mailing list
> > Haskell-C... at haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe
>
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-C... at haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe