[Haskell-cafe] Re: Polymorphic record field?

[Kevin Jardine]

Hi Michael,

Yes, that does compile!

Much less ugly and does not expose the internal bits of MyStruct.

Thanks.

Kevin

On Sep 26, 2:26 pm, Michael Snoyman <mich... at snoyman.com> wrote:
> How about
>
> f myS@(MyStruct { myField = value })
>
> ?
>
>
>
> On Sun, Sep 26, 2010 at 2:14 PM, Kevin Jardine <kevinjard... at gmail.com> wrote:
> > 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 #-}
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