小编Nic*_*gez的帖子

试图设计一个类型驱动的API,我一直试图得到类似下面的工作(使用更复杂的代码/尝试,这被剥离到澄清我正在寻找的最低要求):

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE KindSignatures #-}

module Main where

import Data.Proxy
import GHC.TypeLits

type Printer (s :: Symbol) = IO ()

concrete :: Printer "foo"
concrete = generic

generic :: KnownSymbol s => Printer s
generic = putStrLn (symbolVal (Proxy :: Proxy s))

main :: IO ()
main = concrete

该程序将打印'foo',但不会:

Could not deduce (KnownSymbol s0)
  arising from the ambiguity check for ‘generic’
from the context (KnownSymbol s)
  bound by the type signature for
             generic :: KnownSymbol …