Support for algebraic sum types

[liminalisht]

First, let me say great work. I've really enjoyed working with servant-elm so far.

The only trouble I've run into so far with servant-elm occurs when the api for which I'm generating elm code involves algebraic sum types in some way.

E.g. Let's say my api looks like this:

type NameCompletionAPI = "nameCompletion" :> ReqBody '[JSON] NameCompletionRequest :> Get '[JSON] [Name]

And suppose the NameCompletionRequest type looks like this:

data NameCompletionRequest = NameCompletionRequest { nameSubstring :: Text , fromNationality :: Nationality , position :: Position } deriving (Generic)

and Position is a simple algebraic sum type:

data Position = Beginning | Middle | End deriving (Eq, Generic)

(Suppose too Nationality is just a new type wrapper around Text, and that we've defined all the ToJSON, FromJSON, and ElmType or ToElmType instances for these data types.)

Then on the main branch, when I go to generate this code, I'll get something like:
*** Exception: toElmTypeWithSources: Sum (Constructor "Beginning" Unit) (Sum (Constructor "Middle" Unit) (Constructor "End" Unit))

And on the devel branch, I'll get something like:

*** Exception: src/Elm/Encoder.hs:(13,1)-(58,28): Non-exhaustive patterns in function render

I was wondering if support for algebraic sum types was in the cards any time soon.

I guess this may be fundamentally more of an issue with elm-export. Let me know if it's best brought up somewhere else.

[mattjbray]

Thanks!

Yes, servant-elm is really just a thin wrapper around elm-export, which does not yet support generating Elm encoders/decoders for sum types. As far as I know there's no technical reason for this, it just hasn't been done yet. So the proper place to raise this is over at krisajenkins/elm-export.

As an added complication, servant-elm currently uses a forked version of elm-export that adds a couple of features. I hope that they will be accepted upstream, but in the meantime let's leave this issue open to remind me to rebase the fork when sum type encoders/decoders land.

[liminalisht]

Ok great I'll raise the issue there then. Thanks.

[fosskers]

Is this still an issue?

[mattjbray]

elm-export still doesn't support sum types (but see krisajenkins/elm-export#34 for some promising progress).

On the other hand, servant-elm no longer uses a fork of elm-export, so perhaps we can close this.