Re: An idiom for :include {once}

Adriaan de Groot wrote:

> When you use :include, you can specify {once} as an attribute, to
> prevent AAP from including the file more than once. This is useful
> when the included file contains :route commands, which cannot be
> redefined.

I already explained the intend of using {once} in another message.

> Unfortunately, this places the burden of deciding what to include once
> _at the point of inclusion_, not in the included file. And it's the
> included file, after all, that knows whether it can be included more
> than once or not.  Hence, {once} isn't a very useful mechanism for
> including things once because the recipe doing the :including pretty
> much needs to specify {once} for _every_ include, just to be safe.
> 
> Once workaround is to make {once} default for include, and add a
> {noonce} to allow multiple inclusion explicitly.

That would not be backwards compatible.  I don't like it anyway.
":include recipe" should include the recipe without surprises.

> Or, here's an idiom comparable to C's #ifndef #define idiom to prevent 
> multiple inclusion in the file that is being included.
> 
> filename_aap_seen?=1
> @if _no.filename_aap_seen == "1":
>     # Normal content of file
> filename_aap_seen=2
> 
> This assumes - just like the C idiom - that noone else messes with the 
> ..._seen variables. Because the filename_aap_seen variable is given a value, 
> the first assignment is done exactly once, and the bottom assignment prevents 
> the @if statement from being executed a second time.
> 
> Another form of the same idiom is
> 
> @if not globals().get("_no.filename_aap_seen"):
>     # Normal contents
> filename_aap_seen=1

Yes, this is a good mechanism to avoid defining something twice when you
include a recipe in several places.  A bit simpler:

	@if not _top.get("seen_this_recipe"):
		_top.seen_this_recipe = 1

		:route something	# define route only once
		# more commands A
	
	MyVar = something		# do this in every scope
	# more commands B


Hmm, the indenting doesn't look that nice if the block of "commands A"
once gets longer.  Perhaps we should add a ":finish" command, that stops
processing the recipe?  People using Vim will recognize it.

	MyVar = something		# do this in every scope
	# more commands B

	@if _top.get("seen_this_recipe"):
		:finish

	_top.seen_this_recipe = 1
	:route something		# define route only once
	# more commands A
	

-- 
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 /// Bram Moolenaar -- [email protected] -- http://www.Moolenaar.net   \\\
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