JSON instances. Should they be in Types.hs ?

{-# OPTIONS_GHC -fno-warn-orphans #-}

--{-# LANGUAGE CPP                 #-}
--{-# LANGUAGE DataKinds           #-}
--{-# LANGUAGE DeriveAnyClass      #-}
{-# LANGUAGE DeriveGeneric       #-}
--{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances   #-}
--{-# LANGUAGE NamedFieldPuns #-}
--{-# LANGUAGE OverloadedStrings   #-}
{-# LANGUAGE OverloadedStrings #-}
--{-# LANGUAGE PolyKinds           #-}
--{-# LANGUAGE QuasiQuotes         #-}
--{-# LANGUAGE QuasiQuotes #-}
--{-# LANGUAGE Rank2Types #-}
--{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE RecordWildCards #-}
--{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE StandaloneDeriving #-}
--{-# LANGUAGE TemplateHaskell       #-}
--{-# LANGUAGE TypeFamilies        #-}
--{-# LANGUAGE TypeOperators       #-}

module Hledger.Data.Json (
  -- * Instances
  -- * Utilities
) where

import           Data.Aeson
--import           Data.Aeson.TH
import qualified Data.ByteString.Lazy as BL
import           Data.Decimal
import           Data.Maybe
import           GHC.Generics (Generic)
import           System.Time (ClockTime)

import           Hledger.Data.Types

-- To JSON

instance ToJSON Status
instance ToJSON GenericSourcePos

-- https://github.com/simonmichael/hledger/issues/1195

-- The default JSON output for Decimal can contain 255-digit integers
-- (for repeating decimals caused by implicit transaction prices).
-- JSON output is intended to be consumed by diverse apps and
-- programming languages, which can't handle numbers like that.
-- From #1195:
-- > - JavaScript uses 64-bit IEEE754 numbers which can only accurately
-- >   represent integers up to 9007199254740991 (i.e. a maximum of 15 digits).
-- > - Java’s largest integers are limited to 18 digits.
-- > - Python 3 integers are unbounded.
-- > - Python 2 integers are limited to 18 digits like Java.
-- > - C and C++ number limits depend on platform — most platforms should
-- >   be able to represent unsigned integers up to 64 bits, i.e. 19 digits.
-- What is the best compromise for both accuracy and practicality ?
-- For now, we provide both the maximum precision representation
-- (decimalPlaces & decimalMantissa), and a floating point representation
-- with up to 10 decimal places (and an unbounded number of integer digits).
-- We hope the mere presence of the large number in JSON won't break things,
-- and that the overall number of significant digits in the floating point
-- remains manageable in practice. (I'm not sure how to limit the number
-- of significant digits in a Decimal right now.)
instance ToJSON Decimal where
  toJSON d = object
    ["decimalPlaces"   .= toJSON decimalPlaces
    ,"decimalMantissa" .= toJSON decimalMantissa
    ,"floatingPoint"   .= toJSON (fromRational $ toRational d' :: Double)
    where d'@Decimal{..} = roundTo 10 d

instance ToJSON Amount
instance ToJSON AmountStyle
instance ToJSON Side
instance ToJSON DigitGroupStyle
instance ToJSON MixedAmount
instance ToJSON BalanceAssertion
instance ToJSON AmountPrice
instance ToJSON MarketPrice
instance ToJSON PostingType

instance ToJSON Posting where
  toJSON Posting{..} = object
    ["pdate"             .= pdate
    ,"pdate2"            .= pdate2
    ,"pstatus"           .= pstatus
    ,"paccount"          .= paccount
    ,"pamount"           .= pamount
    ,"pcomment"          .= pcomment
    ,"ptype"             .= ptype
    ,"ptags"             .= ptags
    ,"pbalanceassertion" .= pbalanceassertion
    -- To avoid a cycle, show just the parent transaction's index number
    -- in a dummy field. When re-parsed, there will be no parent.
    ,"ptransaction_"     .= maybe "" (show.tindex) ptransaction
    -- This is probably not wanted in json, we discard it.
    ,"poriginal"         .= (Nothing :: Maybe Posting)

instance ToJSON Transaction
instance ToJSON TransactionModifier
instance ToJSON PeriodicTransaction
instance ToJSON PriceDirective
instance ToJSON DateSpan
instance ToJSON Interval
instance ToJSON AccountAlias
instance ToJSON AccountType
instance ToJSONKey AccountType
instance ToJSON AccountDeclarationInfo
instance ToJSON Commodity
instance ToJSON TimeclockCode
instance ToJSON TimeclockEntry
instance ToJSON ClockTime
instance ToJSON Journal

instance ToJSON Account where
  toJSON a = object
    ["aname"        .= aname a
    ,"aebalance"    .= aebalance a
    ,"aibalance"    .= aibalance a
    ,"anumpostings" .= anumpostings a
    ,"aboring"      .= aboring a
    -- To avoid a cycle, show just the parent account's name
    -- in a dummy field. When re-parsed, there will be no parent.
    ,"aparent_"     .= maybe "" aname (aparent a)
    -- Just the names of subaccounts, as a dummy field, ignored when parsed.
    ,"asubs_"       .= map aname (asubs a)
    -- The actual subaccounts (and their subs..), making a (probably highly redundant) tree
    -- ,"asubs"        .= asubs a
    -- Omit the actual subaccounts
    ,"asubs"        .= ([]::[Account])

deriving instance Generic (Ledger)
instance ToJSON Ledger

-- From JSON

instance FromJSON Status
instance FromJSON GenericSourcePos
instance FromJSON Amount
instance FromJSON AmountStyle
instance FromJSON Side
instance FromJSON DigitGroupStyle
instance FromJSON MixedAmount
instance FromJSON BalanceAssertion
instance FromJSON AmountPrice
instance FromJSON MarketPrice
instance FromJSON PostingType
instance FromJSON Posting
instance FromJSON Transaction
instance FromJSON AccountDeclarationInfo
-- XXX The ToJSON instance replaces subaccounts with just names.
-- Here we should try to make use of those to reconstruct the
-- parent-child relationships.
instance FromJSON Account

-- Decimal, various attempts
-- https://stackoverflow.com/questions/40331851/haskell-data-decimal-as-aeson-type
----instance FromJSON Decimal where parseJSON =
----  A.withScientific "Decimal" (return . right . eitherFromRational . toRational)
-- https://github.com/bos/aeson/issues/474
-- http://hackage.haskell.org/package/aeson-
-- $(deriveFromJSON defaultOptions ''Decimal) -- doesn't work
-- $(deriveFromJSON defaultOptions ''DecimalRaw)  -- works; requires TH, but gives better parse error messages
-- https://github.com/PaulJohnson/Haskell-Decimal/issues/6
--deriving instance Generic Decimal
--instance FromJSON Decimal
deriving instance Generic (DecimalRaw a)
instance FromJSON (DecimalRaw Integer)
-- @simonmichael, I think the code in your first comment should work if it compiles—though “work” doesn’t mean you can parse a JSON number directly into a `Decimal` using the generic instance, as you’ve discovered.
--Error messages with these extensions are always rather cryptic, but I’d prefer them to Template Haskell. Typically you’ll want to start by getting a generic `ToJSON` instance working, then use that to figure out what the `FromJSON` instance expects to parse: for a correct instance, `encode` and `decode` should give you an isomorphism between your type and a subset of `Bytestring` (up to the `Maybe` wrapper that `decode` returns).
--I don’t have time to test it right now, but I think it will also work without `DeriveAnyClass`, just using `DeriveGeneric` and `StandAloneDeriving`. It should also work to use the [`genericParseJSON`](http://hackage.haskell.org/package/aeson/docs/Data-Aeson.html#v:genericParseJSON) function to implement the class explicitly, something like this:
--{-# LANGUAGE DeriveGeneric #-}
--{-# LANGUAGE StandAloneDeriving #-}
--import GHC.Generics
--import Data.Aeson
--deriving instance Generic Decimal
--instance FromJSON Decimal where
--  parseJSON = genericParseJSON defaultOptions
--And of course you can avoid `StandAloneDeriving` entirely if you’re willing to wrap `Decimal` in your own `newtype`.

-- Utilities

-- | Read a json from a file and decode/parse it as the target type, if we can.
-- Example: >>> readJsonFile "in.json" :: IO MixedAmount
readJsonFile :: FromJSON a => FilePath -> IO a
readJsonFile f = do
  bs <- BL.readFile f
  let v = fromMaybe (error "could not decode bytestring as json value") (decode bs :: Maybe Value)
  case fromJSON v :: FromJSON a => Result a of
    Error e   -> error e
    Success t -> return t

-- | Write some to-JSON-convertible haskell value to a json file, if we can.
-- Example: >>> writeJsonFile "out.json" nullmixedamt
writeJsonFile :: ToJSON a => FilePath -> a -> IO ()
writeJsonFile f v = BL.writeFile f (encode v)