[Haskell-cafe] constraint inference

[Ivan Lazar Miljenovic]

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