OverloadedLabels

ghci session

> #a a
"alfa"
>
> #a b

<interactive>:29:1: error:
    • Non type-variable argument
        in the constraint: IsLabel "a" (M.Map [Char] Int -> t)
      (Use FlexibleContexts to permit this)
    • When checking the inferred type
        it :: forall t. IsLabel "a" (M.Map [Char] Int -> t) => t

> :t #a a
#a a :: [Char]

> :t #a b
#a b :: IsLabel "a" (IntMap -> t) => t

OverloadedLabelsTest.hs

{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE OverloadedLabels #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
module Main where

import Data.Proxy

import qualified Data.Map as M

import GHC.TypeLits
import GHC.OverloadedLabels

--

type StringMap = M.Map String String

a :: StringMap
a = M.fromList [("a", "alpha"), ("b", "beta")]

instance (KnownSymbol s, x ~ String) => IsLabel s (StringMap -> x) where
  fromLabel m = m M.! symbolVal (Proxy @s)
  
--

type IntMap = M.Map String Int

b :: IntMap
b = M.fromList [("a", 1), ("b", 2)]

instance (KnownSymbol s) => IsLabel s (IntMap -> Int) where
  fromLabel m = m M.! symbolVal (Proxy @s)