Default empty Arrays about purescript-simple-json

[kritzcreek]

Nothing and [] are kind of the same thing

😱

nononooooonoo. Especially in decoding you don't want your parser to make these implicit assumptions. One common situation I can think of is an API call querying a collection.

Now (1) null might mean the collection didn't exist, whereas (2) an empty Array means it's empty. That's a very different thing though. 1) means a programmer error, and 2) means the user should populate it with some data.

Don't surprise me with dynamic coercing behaviour. I can always throw in a fromMaybe [] if I feel like it.

[justinwoo]

Gist example for those reading this thread later: https://gist.github.com/justinwoo/9d0bb67a84c227f327da7171bb7105c2

[jacereda]

Why not simply use Array a and live with the serialised key=[]?

[justinwoo]

I think in this case, having readJSON read from an input and then applying a series of modify functions should work right? Since the key :: Maybe a -> key :: a transformations will provide the types there and your end result being some concrete type of a record should work?

e.g.

import Data.Nullable (toMaybe)

type MyThing =
  { a :: String
  , b :: Array String
  }

json = """
  { "a": "sedf", "b": null }
"""

parse :: String -> E MyThing
parse str = modify (SProxy :: SProxy "b") (fromMaybe [] <<< toMaybe) <$> readJSON str

or something like that?

[justinwoo]

Nothing and [] are kind of the same thing

Probably works if you only use NonEmpty Array everywhere, but not entirely sure if that's what you want in your codebase either

[megamaddu]

@jacereda Sorry, only referring to deserialization.

You're probably right.. unfortunately C# is really loose with nullability so I have a bunch of apis to work with which like to leave off arrays when they're empty, so I'll probably need to write custom ReadForeign implementations for those records.

[megamaddu]

Is the toMaybe necessary? Or does the Maybe instance distinguish between missing and null?

[justinwoo]

the Maybe instance is just a hack and shouldn't be relied on at all. As noted in https://github.com/justinwoo/purescript-simple-json#warning-maybe

[megamaddu]

Seems clear enough.. are you suggesting it shouldn't be used or might be removed?

[justinwoo]

Yeah, mostly just shouldn't be used unless you're willing to lock down versions of simple-json (which is the easy part if you use psc-package I guess) and read all the release notes to make sure the behavior wasn't changed again