Question: Do block precedence

Jonne Ransijn

28 Feb 2016 28 Feb '16

6:20 p.m.

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 ^")

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Matt

28 Feb 28 Feb

6:33 p.m.

This has come up before as 'ArgumentDo': mailing list thread: Proposal: ArgumentDo https://mail.haskell.org/pipermail/ghc-devs/2015-September/009821.html Relevant Phabricator ticket https://phabricator.haskell.org/D1219 Reddit discussion https://www.reddit.com/r/haskell/comments/447bnw/does_argument_do_have_a_fut... Some people like it, though enough don't that the extension was abandoned. Matt Parsons On Sun, Feb 28, 2016 at 6:20 PM, Jonne Ransijn wrote:

...

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 ^")

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Tom Ellis

6:37 p.m.

On Sun, Feb 28, 2016 at 06:33:53PM -0500, Matt wrote:

...

This has come up before as 'ArgumentDo':

mailing list thread: Proposal: ArgumentDo https://mail.haskell.org/pipermail/ghc-devs/2015-September/009821.html Relevant Phabricator ticket https://phabricator.haskell.org/D1219 Reddit discussion https://www.reddit.com/r/haskell/comments/447bnw/does_argument_do_have_a_fut...

Some people like it, though enough don't that the extension was abandoned.

I don't particularly like it, and I don't think I would use it, but what's the argument against it being admitted as an extension? There are far more hairy extensions in GHC already. Tom

Thomas Tuegel

7:22 p.m.

On Sun, Feb 28, 2016 at 5:20 PM, Jonne Ransijn wrote:

...

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

Jonne Ransijn

7:27 p.m.

I would imagine

...

(flip when) do <N> putStrLn "Where do the parentheses go?" <N> True

should be parsed as

...

(flip when) (do { putStrLn "Where do the parentheses go?"; True; })

Jonne Ransijn

7:50 p.m.

Possible future suggestion: Add a `InlineStatements` language pragma which puts implicit parenthesis around `if-then-else`, `case-of` and `do` blocks, See: http://pastebin.com/T7L6GyBu (Permanent pastebin) for a example program. On Mon, Feb 29, 2016 at 1:22 AM, Thomas Tuegel wrote:

...

On Sun, Feb 28, 2016 at 5:20 PM, Jonne Ransijn wrote:

...
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

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Jonne Ransijn
Matt
Thomas Tuegel
Tom Ellis