Is the Tcl `file dirname` command usable for URL?

Question

From a Subversion HTTP URL, I would like to remove some extra files/folders level to get a branch root URL in a Tcl script.

E.g.:

http://svn.example.com/repos/trunk/file.tcl

-->

http://svn.example.com/repos/trunk/

---

I tried to use the file dirname command but I'm experiencing some "unexpected behavior" (at least from my point of view):

file dirname {http://svn.example.com/repos/trunk/file.tcl}

returns (Windows 7, Tcl 8.5 from an Altera Quartus distribution):

http:svn.example.com/repos/trunk

The '//' slashes of the URL prefix have been removed!?!

---

In other Tcl releases (Windows 7, Tcl 8.5 from Cygwin / Linux, Tcl 8.5) I get a different but still incorrect result:

http:/svn.example.com/repos/trunk

(One '/' of the two '//' slashes has been removed)

Why such a result?

Is there any alternative? (apart from using a custom function with string last, string range...)

---

Note: the file separator command returns different results depending from the release:

• "\" in the Windows "native" Tcl from Altera Quartus;
• "/" in the Tcl Cygwin / Linux releases.

Answer 1

The file command is designed for working with files and filenames, not URLs. This means that it does things (such as removing excess / characters) that are correct for filenames, but wrong for URLs, and this is entirely deliberate.

The uri package in Tcllib is what you're really looking for, since that lets you extract the path (which is filename-like) from a URL so you can manipulate it:

package require uri
set uri http://svn.example.com/repos/trunk/file.tcl

# Split a URL up into its components
set uricomponents [uri::split $uri]
# ==> fragment {} port {} path repos/trunk/file.tcl scheme http host svn.example.com query {} pwd {} user {}

# Manipulate the dictionary, in this case to adjust the path part
dict with uricomponents {
    set path [file dirname $path]
}

# Compose back into a normal URL
set newuri [uri::join {*}$uricomponents]
# ==> http://svn.example.com/repos/trunk

Yes, dict with is ideal for this sort of thing.