{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE UndecidableInstances #-}

{-|
  This module provides a way to specify the shape of your JSON data at
  the type level.

  = Example

  > data User = User
  >   { name :: Text
  >   , lastLogin :: UTCTime
  >   }
  >   deriving stock (Show, Eq)
  >   deriving (ToJSON, FromJSON) via (SpecJSON User)
  > instance HasJsonEncodingSpec User where
  >   type EncodingSpec User =
  >     JsonObject '[
  >       Required "name" JsonString,
  >       Required "last-login" JsonDateTime
  >     ]
  >   toJSONStructure user =
  >     (Field @"name" (name user),
  >     (Field @"last-login" (lastLogin user),
  >     ()))
  > instance HasJsonDecodingSpec User where
  >   type DecodingSpec User = EncodingSpec User
  >   fromJSONStructure
  >       (Field @"name" name,
  >       (Field @"last-login" lastLogin,
  >       ()))
  >     =
  >       pure User { name , lastLogin }

  = Motivation

  The primary motivation is to allow you to avoid Aeson Generic instances
  while still getting the possibility of auto-generated (and therefore
  /correct/) documentation and code in your servant APIs.

  Historically, the trade-off has been:

  1. Use Generic instances, and therefore your API is brittle. Changes
     to Deeply nested object might unexpectedly change (and break) your
     API. You must structure your Haskell types exactly as they are
     rendered into JSON, which may not always be "natural" and easy to
     work with. In exchange, you get the ability to auto-derive matching
     ToSchema instances along with various code generation tools that
     all understand Aeson Generic instances.

  2. Hand-write your ToJSON and FromJSON instances, which means you
     get to structure your Haskell types in the way that works best
     for Haskell, while structuring your JSON in the way that works
     best for your API. It also means you can more easily support "old"
     decoding versions and more easily maintain backwards compatibility,
     etc. In exchange, you have to to hand-write your ToSchema instances,
     and code generation is basically out.

  The goal of this library is to provide a way to hand-write the encoding
  and decoding of your JSON using type-level 'Specification's, while
  still allowing the use of tools that can interpret the specification
  and auto-generate ToSchema instances and code.

  The tooling ecosystem that knows how to interpret 'Specification's
  is still pretty new, but it at least includes OpenApi compatibility
  (i.e. ToSchema instances) and Elm code generation.

-}
module Data.JsonSpec (
  Specification(..),
  HasJsonEncodingSpec(..),
  HasJsonDecodingSpec(..),
  SpecJSON(..),
  Tag(..),
  Field(..),
  JSONStructure,
  Rec(..),
  eitherDecode,
  StructureFromJSON,
  FieldSpec(..)
) where


import Data.Aeson (FromJSON(parseJSON), ToJSON(toJSON))
import Data.JsonSpec.Decode (HasJsonDecodingSpec(DecodingSpec,
  fromJSONStructure), StructureFromJSON(reprParseJSON), eitherDecode)
import Data.JsonSpec.Encode (HasJsonEncodingSpec(EncodingSpec,
  toJSONStructure), StructureToJSON(reprToJSON))
import Data.JsonSpec.Spec (Field(Field, unField), FieldSpec(Optional,
  Required), Rec(Rec, unRec), Specification(JsonArray, JsonBool,
  JsonDateTime, JsonEither, JsonInt, JsonLet, JsonNullable, JsonNum,
  JsonObject, JsonRef, JsonString, JsonTag), Tag(Tag), JSONStructure)
import Prelude ((.), (<$>), (=<<))


{- |
  Helper for defining 'ToJSON' and 'FromJSON' instances based on
  'HasEncodingJsonSpec'.

  Use with -XDerivingVia like:

  > data MyObj = MyObj
  >   { foo :: Int
  >   , bar :: Text
  >   }
  >   deriving (ToJSON, FromJSON) via (SpecJSON MyObj)
  > instance HasEncodingSpec MyObj where ...
  > instance HasDecodingSpec MyObj where ...
-}
newtype SpecJSON a = SpecJSON {unSpecJson :: a}
instance (StructureToJSON (JSONStructure (EncodingSpec a)), HasJsonEncodingSpec a) => ToJSON (SpecJSON a) where
  toJSON = reprToJSON . toJSONStructure . unSpecJson
instance (StructureFromJSON (JSONStructure (DecodingSpec a)), HasJsonDecodingSpec a) => FromJSON (SpecJSON a) where
  parseJSON v =
    SpecJSON <$>
      (fromJSONStructure =<< reprParseJSON v)