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Re: port-filename and path canonicalization

From: Ludovic Courtès
Subject: Re: port-filename and path canonicalization
Date: Tue, 20 Apr 2010 18:57:07 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.1 (gnu/linux)


Andy Wingo <address@hidden> writes:

> On Tue 20 Apr 2010 01:12, address@hidden (Ludovic Courtès) writes:
>> Andy Wingo <address@hidden> writes:
>>> I recently added a global fluid, %file-port-name-canonicalization, which
>>> defaults to #f. But if it's 'absolute, the port name of a file port will
>>> be canonicalized to the absolute path; or, if it's 'relative, the port
>>> name is the canonical name of the file, relative to the %load-path, or
>>> the file name as given otherwise.
>>> The intention was to allow the user to control (port-filename P), so
>>> that the user could find e.g. the absolute path corresponding to that
>>> port at the time that it was made.
>> My feeling is that ports shouldn’t have to deal with paths because
>> that’s a separate concern.  The %file-port-name-canonicalization fluid
>> seems like an inelegant hack to me.
>> When applications have special requirements about paths, then it should
>> be up to the application logic to deal with that.

IOW, applications are free to do:

  (open-input-file (canonicalize-path filename))

instead of:

  (open-input-file filename)

And that’s all it takes at the application level.

> 2. I think a fluid is still necessary, because a file being
> compiled can do an `include' or `include-from-path', or even
> `open-input-file' in a macro, and all these cases you would want the
> same %file-port-name-canonicalization to take effect.

Indeed, this one is tricky.

I still think it’s application-specific, though.  How about calling the
fluid, say, %compiler-file-name-canonicalization instead?  :-)

> 3. The only correct time to do a path canonicalization is when the file
> is opened, because at another time, you might not be in the same current
> directory, so relative paths would resolve incorrectly.


> 4. The application-level code is nastier if it has to canonicalize,
> because a relative canonicalization

What do you mean by “relative canonicalization”?

(I have Glibc’s ‘canonicalize_file_name ()’ in mind, which returns an
absolute path, so I’m confused.)

> cannot in general be passed to open-input-file. For example
>   (open-input-file "../../module/ice-9/boot-9.scm")
> is not the same as
>   (open-input-file "ice-9/boot-9.scm")

Agreed.  :-)

> So you'd have to do a set-port-filename! on the port, mucking up your
> code -- and how would you decide what to set? In N places you'd have to
> duplicate fport_canonicalize_filename, and you'd probably have to make
> scm_i_relativize_path public.

I failed to get the transition at “So”.  :-)

What does scm_i_relativize_path do?  (It lacks a leading comment, hint
hint.  ;-))

Thanks for taking the time to explain!


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