An idiom for :include {once}

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.

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.

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


-- 
pub  1024D/FEA2A3FE 2002-06-18 Adriaan de Groot <[email protected]>
     Key fingerprint = 934E 31AA 80A7 723F 54F9  50ED 76AC EE01 FEA2 A3FE




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