Style check/remove `do:` blocks lifted to lambdas

[metagn]

Abstract

There is a behavior in these lines in the compiler, that allow do: blocks without parameter lists (parsed as raw statement lists), to be lifted to lambdas with no parameters (the same as do ():). The proposal is to do one of:

• Turn this into a style check hint, that is, when --styleCheck is turned on, hint to the programmer that do (): should be used (my preferred solution)
• Deprecate/remove this behavior, and turn it into a special warning/error

Motivation

No response

Description

This is a weird special case that implies do: and do (): are meant to be the same construct, despite both commonly being used for different purposes. There is already discussion about this "inconsistency", see #264 and nim-lang/Nim#12117.

There is no reason for do: to be ambiguous here, it is not hard to accept that do: and do ...: parse as different things since the visual rule is as simple as "check if there is anything between do and :". Other keywords in the language like of, except, object/enum also have different meanings with different contexts.

nim-lang/Nim#12117 attempted to remove the distinction between do: and do (): by making them all parse as lambdas. I propose embracing the distinction and making do: not ambiguous, thereby making code more clear about what it does.

But, this doesn't have to be forced; the compiler can just complain about it being bad style instead of erroring. This is what I prefer, I consider it more practical. The special case is only a few lines, it doesn't really harm anyone beyond optics.

Post-implementation edit: nim-lang/Nim#13900 and nim-lang/Nim#5963 are bugs with this feature

Code Examples

proc twiceLambda(x: proc()) =
  x()
  x()

twiceLambda do (): echo "a" # fine
twiceLambda do: echo "a" # hint/warning/error: add parentheses to make a lambda

template twiceStmt(x: untyped) =
  x
  x

twiceStmt do: echo "a" # fine
twiceStmt do (): echo "a" # error

Backwards Compatibility

When tested against CI, only one package was affected by this in one line, argparse. The solution is literally just to change do: to do (): in that line.

If this is only a style check, there are obviously no compatibility problems.

[Araq]

That makes lots of sense.