[Haskell-cafe] Re: Polymorphic record field?

26 Sep 2010

      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  wrote:
...
How about
f myS@(MyStruct { myField = value })
?
On Sun, Sep 26, 2010 at 2:14 PM, Kevin Jardine  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  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  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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