Question

Playing around with the ReadArgs package, it seems that it does not support single-argument situations.

{-# LANGUAGE ScopedTypeVariables #-}

import ReadArgs (readArgs)

main = do
  (foo :: Int) <- readArgs
  print foo

The error is (when using version 1.0):

No instance for (ReadArgs.ArgumentTuple Int)
  arising from a use of `readArgs'

My question is twofold:

1. How does readArgs work?
2. How can that library be adjusted to allow it to work with a single argument as well?

N.B. version 1.1 of ReadArgs eliminates this “error”; see comments.

Answer 1

I don’t quite understand all of the extensions I needed to enable, but you could define an instance of ReadArgs.ArgumentTuple a (even though it’s not really a semantically correct name) like this:

{-# LANGUAGE FlexibleInstances, UndecidableInstances,
             OverlappingInstances, ScopedTypeVariables #-}

import ReadArgs

instance (Argument a) => ArgumentTuple a where
  parseArgsFrom ss = do
    a :& () <- parseArgsFrom ss
    return a
  usageFor a = usageFor (a :& ())

main = do
    (foo :: Int) <- readArgs
    print foo

Also, I’m not really sure if there are any problems with this instance, though it works for the example you presented. I would assume there’s a reason it’s missing from the library, but I may be wrong.

Update

After looking trying a few things out, to be sure they still work (like the following example), I’m fairly convinced this doesn’t cause any problems, so maybe it’s (or something similar) existance was just an oversight.

main = do
    (foo :: Int, bar :: Int) <- readArgs
    print foo
    print bar