Re: [Boost-build] Overuse of jam features

From: David Abrahams (dave_at_[hidden])
Date: 2007-03-19 18:25:11

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