StrMapRecord.hs

{-# LANGUAGE DataKinds              #-}
{-# LANGUAGE FlexibleInstances      #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE KindSignatures         #-}
{-# LANGUAGE ScopedTypeVariables    #-}
{-# LANGUAGE TypeOperators          #-}
{-# LANGUAGE UndecidableInstances   #-}

module StrMapRecord where

import GHC.TypeLits (Symbol, symbolVal, KnownSymbol)
import Data.Proxy (Proxy(..))

class Build (xs :: [(Symbol, *)]) a b | xs a -> b where
  build :: Proxy xs -> [(String, String)] -> a -> b

instance Build '[] t t where
  build _ _ = id

instance (Read t, KnownSymbol name, Build xs a b) => Build ('(name, t) ': xs) (t -> a) b where
  build _ values ta = build (Proxy :: Proxy xs) values (ta (read val))
    where Just val = lookup (symbolVal (Proxy :: Proxy name)) values

--------------------------------------------------------------------------------
-- * Example

data MyRecord
  = MyRecord
    { foo :: Int
    , bar :: String
    } deriving Show

type Fields = '[ '("foo", Int), '("bar", String)]

myRecord = build (Proxy :: Proxy Fields) [("foo", "10"), ("bar", "\"hello\"")] MyRecord