I'd like to propose a GHC extension called (for now) `ArgumentBody`. `ArgumentBody` is a simple syntax extension, than, when enabled, permits the following code:

main = when True do
  putStrLn "Hello!"

main = forM values \value ->
  print value

main = forM values \case
    Just x -> print x
    Nothing -> print y

In this code we do not need `$` before `do`, lambda, or lambda-case (if -XLambdaCase is enabled). This change would not extend to `let`, `if`, `case`, or any other constructs.

Pros:

1. Code is simpler and it greatly reduces the need for "operator line noise" of $ everywhere.

2. We can avoid using the type-checker hack for $ for things such as runSt.

Cons:

1. Adds complexity to the compiler. (NB: The change is minimal and not invasive at all.)

2. Contributes to a proliferation of extensions that other tools must support.  (NB: This is just a parser change so should be easy for all tools to support.)

I'm very interested in hearing both favoring and dissenting opinions on this proposed change. If this change is approved of, names besides -XArgumentBody can be considered.

See more info on Trac.

-- Andrew Gibiansky

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