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
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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