Question

I’m trying to parse a YAML chunk and create a Request instance from it.
So basically I have the following (implementations of FromJSON URI, FromJSON RequestMethod and FromJSON Header are omitted for brevity):

...
instance FromJSON (Request r) where
  parseJSON (Object v) = Request <$>
    v .: "uri" <*>
    v .: "method" <*>
    v .: "headers" <*>
    v .: "body"

Trying to compile (using GHC) this code produces:

    No instance for (FromJSON r)
      arising from a use of `.:'
    In the second argument of `(<*>)', namely `v .: "body"'
    In the expression:
      Request <$> v .: "uri" <*> v .: "method" <*> v .: "headers"
      <*> v .: "body"
    In an equation for `parseJSON':
        parseJSON (Object v)
          = Request <$> v .: "uri" <*> v .: "method" <*> v .: "headers"
            <*> v .: "body"

I feel it’s a newbie question (and that’s who I am), but apparently GHC cannot infer the type for the request body (which appears to be polymorphic) and I’m lacking the skills for handling this. Although I managed to “pacify” the compiler by replacing the instance declaration with:

instance FromJSON (Request Text) where
...

and adding {-# LANGUAGE FlexibleInstances #-} option, but I’m somehow sure it’s wrong.
So, how do I properly parse this (and similar cases)?

Thank you!

Answer 1

Thanks to @hammar, the answer boils down to the 2 following points:

1) To be able to join the computation, “r” (which refers to the request body) in the instance declaration has to be restricted to “FromJSON” class.

instance FromJSON r => FromJSON (Request r) where
  parseJSON (Object v) = Request <$>
    v .: "uri" <*>
    v .: "method" <*>
    v .: "headers" <*>
    v .: "body"

2) and, when the parser is used, it must be explicitly notified of the body type it is to handle:

testJSON :: Maybe (Request Text)
testJSON = decode "{uri: 'http://example.com/', method: GET, headers: [], body: 'hello'}"

I’m not sure how obvious that was, but GHC could’ve inferred the class restriction on its own, I guess. Not yet, perhaps.