On Sun, Feb 28, 2016 at 5:20 PM, Jonne Ransijn
Dear Haskell-Cafe mailing list people (?) I've been writing parenthesis around do blocks since forever now, but I don't get why they are necessary. I can't seem to come up with a program where they are necessary. Am I missing something or are parenthesis around do blocks nececairy for no reason? Since parsing 'do' blocks as if they have parenthesis around them doesn't seem to break any code, why not do so?
when (doBlocksNeedParenthesis) do putStrLn "This code is invalid."
when (doBlocksNeedParenthesis) $ do putStrLn "This code is valid."
when (doBlocksHaveInvisibleParenthesis) do putStrLn "These are equal v"
when (doBlocksHaveInvisibleParenthesis) (do putStrLn "These are equal ^")
This syntax can be ambiguous; consider:
(flip when) do putStrLn "Where do the parentheses go?" True
I admit this is contrived; we could choose to put parentheses around a do-block only if it is the terminal argument. There is a case that is completely unambiguous:
(flip when) do { putStrLn "Braces might as well be parentheses." } True
Certainly, I don't see why this shouldn't parse. Regards, Tom