[Haskell-cafe] Fwd: constraint inference

7 Jul 2017

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

---------- Forwarded message ----------
From: Dmitry Olshansky 
Date: 2017-07-07 9:24 GMT+03:00
Subject: Re: [Haskell-cafe] constraint inference
To: Ivan Lazar Miljenovic 

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 :
...
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  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@gmail.com
http://IvanMiljenovic.wordpress.com