Getter.hs

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

Template Haskell functions for getter functions.
-}
{-# LANGUAGE RecordWildCards #-}
{-# LANGUAGE TemplateHaskell #-}

module Data.Aeson.Schema.TH.Getter where

import Control.Monad (unless)
import Data.Aeson.Schema.Internal (Object)
import Data.Maybe (isNothing)
import Language.Haskell.TH

import Data.Aeson.Schema.TH.Get (generateGetterExp)
import Data.Aeson.Schema.TH.Parse (GetterExp(..), parseGetterExp)
import Data.Aeson.Schema.TH.Unwrap
    (FunctorHandler(..), unwrapSchema, unwrapSchemaUsing)
import Data.Aeson.Schema.TH.Utils (reifySchemaName, schemaVToTypeQ)

-- | A helper that generates a 'Data.Aeson.Schema.TH.get' expression and a type alias for the result
-- of the expression.
--
-- > mkGetter "Node" "getNodes" ''MySchema ".nodes[]"
-- >
-- > {- is equivalent to -}
-- >
-- > -- | Node ~ { b: Maybe Bool }
-- > type Node = [unwrap| MySchema.nodes[] |]
-- >
-- > getNodes :: Object MySchema -> [Node]
-- > getNodes = [get| .nodes[] |]
--
-- 'mkGetter' takes four arguments:
--
--   [@unwrapName@] The name of the type synonym to store the unwrapped schema as
--
--   [@funcName@] The name of the getter function
--
--   [@startSchema@] The schema to extract/unwrap from
--
--   [@ops@] The operation to pass to the 'Data.Aeson.Schema.TH.get' and
--           'Data.Aeson.Schema.TH.unwrap' quasiquoters
--
-- There is one subtlety that occurs from the use of the same @ops@ string for both the
-- 'Data.Aeson.Schema.TH.unwrap' and 'Data.Aeson.Schema.TH.get' quasiquoters:
-- 'Data.Aeson.Schema.TH.unwrap' strips out intermediate functors, while 'Data.Aeson.Schema.TH.get'
-- applies within the functor. So in the above example, @".nodes[]"@ strips out the list when
-- saving the schema to @Node@, while in the below example, @".nodes"@ doesn't strip out the list
-- when saving the schema to @Nodes@.
--
-- > mkGetter "Nodes" "getNodes" ''MySchema ".nodes"
-- >
-- > {- is equivalent to -}
-- >
-- > -- | Nodes ~ List { b: Maybe Bool }
-- > type Nodes = [unwrap| MySchema.nodes |]
-- >
-- > getNodes :: Object MySchema -> Nodes
-- > getNodes = [get| .nodes |]
--
-- As another example,
--
-- > mkGetter "MyName" "getMyName" ''MySchema ".f?[].name"
-- >
-- > {- is equivalent to -}
-- >
-- > -- | MyName ~ Text
-- > type MyName = [unwrap| MySchema.f?[].name |]
-- >
-- > getMyBool :: Object MySchema -> Maybe [MyName]
-- > getMyBool = [get| .f?[].name |]
mkGetter :: String -> String -> Name -> String -> DecsQ
mkGetter unwrapName funcName startSchemaName ops = do
  getterExp@GetterExp{..} <- parseGetterExp ops
  unless (isNothing start) $
    fail $ "Getter expression should start with '.': " ++ ops

  startSchema <- reifySchemaName startSchemaName

  let unwrapResult = unwrapSchema getterOps startSchema
      funcResult = unwrapSchemaUsing ApplyFunctors getterOps startSchema
      getterFunc = generateGetterExp getterExp
      unwrapName' = mkName unwrapName
      funcName' = mkName funcName

  sequence
    [ tySynD unwrapName' [] unwrapResult
    , sigD funcName' [t| Object $(schemaVToTypeQ startSchema) -> $funcResult |]
    , funD funcName' [clause [] (normalB getterFunc) []]
    ]