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JSON return type for Handlers? #1481

MaxGabriel opened this Issue Feb 2, 2018 · 3 comments


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MaxGabriel commented Feb 2, 2018

So I was thinking about this blog post recently, that uses UndecidableInstances to let you specify the return type of your Handlers that return JSON as Handler <RecordName> rather than Handler Value

Why not just have something like this?

data JSONResponse a = ToJSON a => JSONResponse a

instance ToContent (JSONResponse a) where
  toContent (JSONResponse a) = toContent $ toEncoding a

instance ToTypedContent (JSONResponse a) where
  toTypedContent = TypedContent typeJson . toContent

Then handlers are typed as:

postSignupR :: Handler (JSONResponse CreateUserResponse)

and the function returns:

return $ JSONResponse $ CreateUserResponse userId

Slightly more overhead than the blog post version. I don’t know if UndecidableInstances is actually worth avoiding though. Thoughts? I think endorsing one or the other of these options would be good for Yesod to do—it seems much better than returning Value.


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MaxGabriel commented Feb 2, 2018

cc @gregwebs since you authored that blog post


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gregwebs commented Feb 9, 2018

Looks good

MaxGabriel added a commit that referenced this issue Mar 12, 2018

@MaxGabriel MaxGabriel referenced this issue Mar 12, 2018


Add JSONResponse type #1494

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@StevenXL StevenXL referenced this issue Apr 12, 2019


Introduce JSONResponse. #1592

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StevenXL commented Apr 13, 2019

Closed by #1592

@StevenXL StevenXL closed this Apr 13, 2019

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