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

------------------------------------