`DecodeResponse` are not extendable

Due to use of default instance it is not currently possible to make user defined instances of DecodeResponse. The default instance has been made to not force users writing their own instances for undecodable responses. Since users are unable to define their own instances the feature seems as not worth it.

However, there is an alternative way of achieving the same goal. Using the solution an instances for undecidable responses will not be for free, but instead would require user to write three lines of simple boilerplate including library import.

Solution

Payload.Client.DecodeResponse.purs:

newtype FailResponseDecoding a = FailResponseDecoding a
derive instance Newtype (FailResponseDecoding a) _

instance Warn ... => DecodeResponse (FailResponseDecoding a) where
  ...

-- This function is meant to be exported
-- | Implementation for an instance that fails decoding
failResponseDecoding :: forall a. String -> Maybe a
failResponseDecoding = map (unwrap :: FailResponseDecoding a -> a) <<< decodeResponse

If user has some type Foo that will fail decoding as a response:

instance DecodeResponse (FailResponseDecoding Foo) where
  decodeResponse = failResponseDecoding

Here user gets a warning. Please note that user gets the warning exactly in place where it can be fixed, not in the code that uses the cursed instance.

hoodunit / purescript-payload

`DecodeResponse` are not extendable #30

Solution