Enum.hs

{-|
Module      :  Data.Aeson.Schema.TH.Enum
Maintainer  :  Brandon Chinn <brandon@leapyear.io>
Stability   :  experimental
Portability :  portable

Template Haskell functions for Enum types.
-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE TemplateHaskell #-}

module Data.Aeson.Schema.TH.Enum
  ( genFromJSONEnum
  , mkEnum
  ) where

import Control.Monad (forM, unless)
import Data.Aeson (FromJSON(..), Value(..))
import Data.Char (toLower)
import Data.Maybe (mapMaybe)
import qualified Data.Text as Text
import Language.Haskell.TH

-- | Make an enum type with the given constructors, that can be parsed from JSON.
--
-- The 'FromJSON' instance will match to a string value matching the constructor name,
-- case-insensitive.
--
-- @
-- mkEnum "State" ["OPEN", "CLOSED"]
--
-- main = print
--   [ decode \"open" :: Maybe State
--   , decode \"OPEN" :: Maybe State
--   , decode \"closed" :: Maybe State
--   , decode \"CLOSED" :: Maybe State
--   ]
-- @
mkEnum :: String -> [String] -> Q [Dec]
mkEnum name vals = do
  (:) <$> dataDec <*> mkFromJSON name' vals'
  where
    name' = mkName name
    vals' = map mkName vals
    dataDec = dataD (pure []) name' [] Nothing (map toCon vals') [derivClause Nothing deriveClasses]
    deriveClasses =
      [ [t| Eq |]
      , [t| Ord |]
      , [t| Show |]
      , [t| Enum |]
      ]
    toCon val = normalC val []

-- | Generate an instance of 'FromJSON' for the given data type.
--
-- Prefer using 'mkEnum'; this function is useful for data types in which you want greater control
-- over the actual data type.
--
-- The 'FromJSON' instance will match to a string value matching the constructor name,
-- case-insensitive.
--
-- @
-- data State = OPEN | CLOSED deriving (Show,Enum)
-- genFromJSONEnum ''State
--
-- main = print
--   [ decode \"open" :: Maybe State
--   , decode \"OPEN" :: Maybe State
--   , decode \"closed" :: Maybe State
--   , decode \"CLOSED" :: Maybe State
--   ]
-- @
genFromJSONEnum :: Name -> Q [Dec]
genFromJSONEnum name = do
  -- check if 'name' is an Enum
  ClassI _ instances <- reify ''Enum
  let instanceNames = flip mapMaybe instances $ \case
        InstanceD _ _ (AppT _ (ConT n)) _ -> Just n
        _ -> Nothing
  unless (name `elem` instanceNames) $ fail $ "Not an Enum type: " ++ show name

  -- extract constructor names
  cons <- reify name >>= \case
    TyConI (DataD _ _ _ _ cons _) -> forM cons $ \case
      NormalC con [] -> return con
      con -> fail $ "Invalid constructor: " ++ show con
    info -> fail $ "Invalid data type: " ++ show info

  mkFromJSON name cons

{- Helpers -}

mkFromJSON :: Name -> [Name] -> Q [Dec]
mkFromJSON name cons = do
  let toPattern = litP . stringL . map toLower . nameBase
      toMatch con = match (toPattern con) (normalB [| pure $(conE con) |]) []

  t <- newName "t"
  let parseEnum = caseE [| Text.unpack $ Text.toLower $(varE t) |] $
        map toMatch cons ++ [match wildP (normalB $ appE badParse $ varE t) []]

  [d|
    instance FromJSON $(conT name) where
      parseJSON (String $(varP t)) = $parseEnum
      parseJSON v = $badParse v
    |]
  where
    badParse =
      let prefix = litE $ stringL $ "Bad " ++ nameBase name ++ ": "
      in [| fail . ($prefix ++) . show |]