Sql Servant

sql-servant.hs

-- | playing around with a possible postgresql-simple plugin for servant

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}

module Galene.Common.Update where

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

data Field (f :: Symbol) (t :: *) = Field

data Update (t :: Symbol) (d :: *) = Update

data (path :: k) :> a = Proxy path :> a
infixr 9 :>

-- | in the real verison `Show` would be replaced with `ToRow`
data SqlQuery where
    SqlQuery :: (Show a) => [a] -> SqlQuery

class HasSQL fields where
    type Query fields :: *
    update :: Proxy fields -> Query fields -> IO ()

instance KnownSymbol t => HasSQL (Update t result) where
    type Query (Update t result) = SqlQuery
    update _ _ = putStrLn "testing"

instance (HasSQL sub, Show t) => HasSQL (Field c t :> sub) where
    type Query (Field sym t :> sub) = t -> Query sub
    update _ _ = putStrLn "testing"

-- | user code

data Product = Product Int String

type MyUpdate = Field "id" Int :> Update "product" Product


f :: Int -> SqlQuery
f i = SqlQuery [i]

myQuery :: Query MyUpdate
myQuery = f

query :: HasSQL fields => Proxy fields -> Query fields -> IO ()
query = update

x :: IO ()
x = query (Proxy :: Proxy MyUpdate) myQuery