Maybe type classes

[dwhitney]

Hi I enjoy using this library. It's really reduced a lot of boilerplate for me, so thanks so much for putting it together!

One thing that I wish it had out of the box is type class instances for Maybe. Using NullOrUndefined feels a bit leaky to me, and wrapping my Maybes in newtypes just to get the functionality I wanted was becoming fairly tedious since I use Maybe so much. I also noticed there some similar sentiment on the latest PureScript Unscripted, so I decided to make this PR.

Things to note:

• when reading json, fields with Maybe types that have corresponding json values that are null or undefined will decode to a Nothing. Otherwise it will be Just value
• when writing fieldName :: Just value to json, the resulting json will be "fieldName":"value", and when writing fieldName :: Nothing, the resulting json will be "fieldName":null
• I've added a foreign import to allow null to be written to the resulting json, and because I did that I didn't want it leaking out of the module, so I've added all of the relevant functions as exports.
• I've added four additional tests to test the edge cases and standard cases.

Hope that you find this helpful. If you'd like any changes, I'm happy to make them!

[dwhitney]

done! travis should be doing its work any moment

On Thu, Sep 21, 2017 at 1:58 PM, Justin Woo notifications@github.com wrote:

@justinwoo commented on this pull request.

In src/Simple/JSON.purs https://github.com/justinwoo/purescript-simple-json/pull/16#discussion_r140315536 :

@@ -143,6 +166,12 @@ instance writeForeignArray :: WriteForeign a => WriteForeign (Array a) where instance writeForeignNullOrUndefined :: WriteForeign a => WriteForeign (NullOrUndefined a) where writeImpl (NullOrUndefined a) = maybe undefined writeImpl a

+instance writeForeignMaybe :: WriteForeign a => WriteForeign (Maybe a) where

• writeImpl (Just a) = writeImpl a
• writeImpl Nothing = null

• +foreign import null :: Foreign

Ah, null isn't exposed by Nullable but really, I'd even prefer toForeign $ toNullable Nothing

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[jacereda]

Merged, thanks!