on Mon Mar 19 2007, Vladimir Prus <ghost-AT-cs.msu.su>
wrote:
> Looking at python.jam, I see this:
>
> includes ?= [ path-to-native
$(:E=python$(version):R=$(:E=include:R=$(prefix))) ] ;
>
> and I have not the slightest clue what this is supposed
to do. I suspect it
> concatenates, $(prefix), "include" and
"python$(version),
Yes, bingo! Maybe it was clearer than you thought?
> but the meaning of $(:E) syntax is not obvious for me,
$(:E=xxx...)
is the same as
local tmp = xxx ;
$(tmp...)
> and use of nested expansions does not seem clear.
It seemed clearer to me than writing it in multiple steps.
> Would it be better to stay away from arcane features of
bjam, rather
> than use them like that?
Perhaps. However, I understand what they do, and I know
they will
work with what the user writes in user-config.jam whether he
uses
native path syntax or just sticks with forward slashes.
I've never been able to get comfortable with the use of
"normalized"
paths as in path.jam; I'm always afraid of losing track of
whether I'm
working with native or "normalized" syntax, and I
end up with a
dilemma: if I write functions that only accept
"normalized" paths, I
can't expose them to end-users without explaining how to
form such
paths on windows. If I write functions that accept native
paths and
translate them into normalized paths, I can't call them from
other
internal functions that use normalized paths without first
translating
the paths back to native. So I've never found a way to use
path.jam
that's both consistent and forgiving.
In general I think $(X:Y=...) is an arcane syntax, but once
you learn
what it does, it's not too hard to read. The same applies
to
$(:E=...), and it's actually a pretty natural way to view
what happens
with bjam variable expansion. After all, wouldn't you
expect
local x ;
$($(x):E=foo)
to yield "foo"? Since x is empty, it seems
obvious to me that
$(:E=foo)
is equivalent.
--
Dave Abrahams
Boost Consulting
www.boost-consulting.com
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