Add JSONResponse type

[MaxGabriel]

Closes #1481

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[snoyberg]

I think it would be better without the datatype context ToJSON a => here. I don't remember all of the arguments, but it's typically considered a bad extension.

[MaxGabriel]

Hm, this gives some reasons https://wiki.haskell.org/Data_declaration_with_constraint

Haven’t tried this, but the wiki recommends replacing with a GADT

[snoyberg]

I don't think a GADT is necessary here. I'd replace with just:

newtype JSONResponse a = JSONResponse a

Is anything lost in such a change?

[MaxGabriel]

It's not that bad, but the type error is worse if you move the ToJSON constraint to ToContent and ToTypedContent:

/Users/maximiliantagher/Documents/Mercury/hs/mercury-web-backend/src/Application.hs:67:1: error:
    • No instance for (ToJSON Foo) arising from a use of ‘yesodRunner’
    • In the expression:
        yesodRunner getHomeR env6761_a2uNs (Just HomeR) req6761_a2uNt
      In a case alternative:
          "GET"
            -> yesodRunner getHomeR env6761_a2uNs (Just HomeR) req6761_a2uNt
      In the expression:
        case Network.Wai.Internal.requestMethod req6761_a2uNt of {
          "GET"
            -> yesodRunner getHomeR env6761_a2uNs (Just HomeR) req6761_a2uNt
          _ -> yesodRunner
                 (void badMethod) env6761_a2uNs (Just HomeR) req6761_a2uNt }

vs if the constraint is on JSONResponse itself:

/Users/maximiliantagher/Documents/Mercury/hs/mercury-web-backend/src/Handler/Home.hs:16:21: error:
    • No instance for (ToJSON Foo) arising from a use of ‘JSONResponse’
    • In the second argument of ‘($)’, namely ‘JSONResponse [Foo]’
      In the expression: return $ JSONResponse [Foo]
      In an equation for ‘getHomeR’:
          getHomeR = return $ JSONResponse [Foo]

[snoyberg]

Does the GADT approach solve that?

[StevenXL]

The version with GADTs works pretty well. The image below shows the declaration of the JSONResponse type constructor using GADTs, the type signature of the JSONResponse data constructor, and the error when you try apply JSONResponse to a value that is not an instance of ToJSON. I think it works out very well.

@MaxGabriel Given that it has been a long time since your PR, I'll make another PR with GADTs and reference this as well as #1481. (I can always back off if you say so. Thank you).

[StevenXL]

Closed by #1592