[jamming] Path specification (was: Majam - keeping jam up-to-date)

[Kai Backman]

 Here is a two part solution:

1. Define a portable path format for use in jam, jambase and jamfile's
on all platforms

2. Add two modifiers, similar to what Alen has adopted, one for
converting native into portable ':~' and one to convert from portable
to native ':/'


 Based on my quick reading of the OpenVMS links provided by John and
Tim a portable path could be defined as follows:

Absolute path:
"drive:/subpath1/subpath2/file.ext;v"
 where drive: and ;v optional, leading / mandatory

Relative path:
"../subpath/file.ext;v"
 where ;v optional, leading non-/ mandatory



--------------------------- Examples

 To illustrate I'll temporarily adopt the fictional expansion
construct $("":MODIFIER) and '->' means evaluates to:


Win32:
$("C:\Program Files\Microsoft\":~)
 -> "C:/Program Files/Microsoft/"

OBJS = "../../obj/foo.obj" "../../obj/bar.obj" ;
$(OBJS:/)
 -> "..\..\obj\foo.obj" "..\..\obj\foo.obj"
 notice how ':/' does -not- join the list


Unix:
$("/usr/bin/:/usr/local/bin":~)
 -> "/usr/bin/" "usr/local/bin"
 notice how ':~' splits the paths into a list

$("C:/var/www/foo.html;10":/)
 -> "/var/www/foo.html"
 with perhaps a warning about the discarded components?


VMS:
$("SYS$SYSDEVICE:[SYS0.SYSCOMMON.]LOGIN.COM;5":~)
 -> "SYS$SYSDEVICE:/SYS0/SYSCOMMON/LOGIN.COM;5"

$("../data/output.bin":/)
 -> "SYS$SYSDEVICE:[SYS0.DATA]OUTPUT.BIN"
 assuming the default device and directory from above



--------------------------- Discussion

 Benefits:
- Lets writers of Jam, Jambase and Jamfile's use a single portable
path specification.

- Explicit way to mangle between OS specific and portable
paths. Leaves open the option of using local paths as input for people
building on a single platform.


 Open questions:
- Is the handling of ".." correct for VMS? I got the impression that
VMS doesn't have a concept of parent directory natively so the
resolution needs to happen explicitly?

- How should we handle mangling of large numbers of environment
variables like CC, INCLUDE and similar? Is it sufficient to explicitly
specify them in Jambase/Jamrules "CC=$(CC:~) ;" or do we need more
convenience?

- Would we like to include the $("") construct permanently to easy
input of local paths?

- What did I overlook?

 I think the problem of building the default HDRS from native include
paths is tangential but separate from this issue and I can't see any
significant complications there.

 Kai



-- 
Kai Backman, Software Engineer, kaib at google.com