[Haskell-cafe] Fwd: constraint inference

[Dmitry Olshansky]

Sorry, I didn't copy to cafe...

---------- Forwarded message ----------
From: Dmitry Olshansky <olshanskydr at gmail.com>
Date: 2017-07-07 9:24 GMT+03:00
Subject: Re: [Haskell-cafe] constraint inference
To: Ivan Lazar Miljenovic <ivan.miljenovic at gmail.com>


I am not sure why but when I didn't set RankNTypes I got an appropriate
error. So I specified it.

But if I write
  let { f :: (forall n. KnownSymbol n => Proxy n) -> String; f p =
symbolVal p }
then nothing is changed.




2017-07-07 9:11 GMT+03:00 Ivan Lazar Miljenovic <ivan.miljenovic at gmail.com>:

> You've specified RankNTypes but aren't using it; did you mean "(forall
> n. KnownSymbol n => Proxy n)" ?
>
> Typically in these situations you also can't partially apply it, so
> you might need to make it "f p = symbolVal p".
>
> On 7 July 2017 at 15:50, Dmitry Olshansky <olshanskydr at gmail.com> wrote:
> > Hello, cafe!
> >
> > If we have many constraints then compiling is sometimes too slow.
> Especially
> > if there are some type calculations.
> > And I don't know how to divide this time among different source files
> > because all constraints are checked only when we call a function.
> >
> > I tried to add constraint info into argument definition instead of the
> > function signature but this trick is not working (see below).
> >
> > Is it (at least theoretically) possible to infer the constraint inside a
> > function from the type of function's argument?
> >
> > ------------------------------------
> > $ stack ghci
> >  ---- GHCi, version 8.0.2:
> > Prelude> import Data.Proxy
> > Prelude Data.Proxy> import GHC.TypeLits
> > Prelude Data.Proxy GHC.TypeLits> :set -XRankNTypes
> > Prelude Data.Proxy GHC.TypeLits> let { f :: (KnownSymbol n => Proxy n) ->
> > String; f = symbolVal }
> >
> > <interactive>:4:54: error:
> >     • No instance for (KnownSymbol n) arising from a use of ‘symbolVal’
> >       Possible fix:
> >         add (KnownSymbol n) to the context of
> >           the type signature for:
> >             f :: (KnownSymbol n => Proxy n) -> String
> >     • In the expression: symbolVal
> >       In an equation for ‘f’: f = symbolVal
> > ------------------------------------
> >
> > Best regards,
> > Dmitry
> >
> >
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>
>
> --
> Ivan Lazar Miljenovic
> Ivan.Miljenovic at gmail.com
> http://IvanMiljenovic.wordpress.com
>
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