Read type class

#1

I just realized that the prelude doesn’t have a Read type class.

Does anyone know any history behind this? I assume this was a conscious decision.

I tried searching the issues in the purescript and purescript-prelude repos on GitHub but there were a lot of results and none seemed relevant.

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#2

Perhaps purescript-in-purescript had this at some point. I’ll try looking there.

#3

I’m starting to think this probably comes down to having to maintain a PureScript parser in PureScript, since it seems like it should work in theory at runtime.

#4

AFAIK, this was intentionally left out. Haskell from First Principles recommends never using this type clsas. I think we only have Show so that one can print things from the REPL. However, even that type class is discouraged despite everyone using it.

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#5

Yeah, it’s definitely not something I miss. It’s rarely a good idea to use, I think; proper serialization tools are preferable. (Show kind of sucks for serialization anyway.) I don’t think it comes down to having to maintain a PureScript parser in PureScript; if you were going to go that route you would have to implement a PureScript evaluator too and that would be massively overkill, because the following approach suffices:

instance readMaybe :: Read a => Read (Maybe a) where
  read str = parseJust <|> parseNothing
    where
    parseJust = do
      inner <- str # stripPrefix "(Just " >=> stripSuffix ")"
      read inner

    parseNothing = do
      guard (str == "Nothing")
      pure Nothing

I think GHC generates code along these lines when you ask it to derive Read. For data types where you do need a conversion from string, those functions should be provided as monomorphic functions, in my view. For example: Data.Int.fromString :: String -> Maybe Int.

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#6

Yup, it was all this. If we’d defined it we would have wanted to make it support failure without crashing also, making it even more parser-like, so why not just use a real serialization format, etc.

I mean, in addition to that, we’re still not all that enthused about Show either really. :smile:

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