Unicode in perl, mkdir exercise

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I'm currently learning the art of Unicode Programming, and applying it to a personal project. Soon I realized how it is really difficult to get it right, and even to understand if you did it correctly: if the tool is wrong, you can be mistaken in evaluating the results of your work.

My small goal in this exercise is to understand what I should pass to mkdir versus what is good for File::Path::make_path. In other words: what do they expect? Will they handle the encoding depending on the locale, or should I do it for them?

I wrote the following scripts, which takes arguments from @ARGV and for each of them creates the directory $_, using both functions and both encoded and decoded froms.

#!/usr/bin/perl

use warnings;
use strict;
use utf8;
use v5.16;

use Encode;
use Encode::Locale;

use File::Path qw/make_path/;
use File::Spec;

# Everything under the './tree' directory
mkdir 'tree';
mkdir File::Spec->catdir('tree', $_)
    for ('mkdir', 'mkdir_enc', 'make_path', 'make_path_enc');

foreach (map decode(locale => $_) => @ARGV) {
    mkdir File::Spec->catdir('tree', 'mkdir', $_);
    mkdir encode(locale_fs => File::Spec->catdir('tree', 'mkdir_enc', $_));

    make_path(File::Spec->catdir('tree', 'make_path', $_));
    make_path(encode(locale_fs => File::Spec->catdir('tree', 'make_path_enc', $_)));
}

I executed the script as follows:

./unicode_mkdir.pl a→b←c

What I would expect is:

  • Either tree/mkdir [x]or tree/mkdir_enc contain directories named gibberish;
  • Either tree/make_path [x]or tree/make_path_enc contain directories named gibberish;

With great surprise I found out that all version work properly. I verified it with find:

$ find tree
tree
tree/mkdir_enc
tree/mkdir_enc/a→b←c
tree/mkdir
tree/mkdir/a→b←c
tree/make_path_enc
tree/make_path_enc/a→b←c
tree/make_path
tree/make_path/a→b←c

I realized that the tree command makes it so wrong… (a quite common disease) but at least I could see that the results are all the same:

$ tree tree
tree
├── make_path
│   └── a\342\206\222b\342\206\220c
├── make_path_enc
│   └── a\342\206\222b\342\206\220c
├── mkdir
│   └── a\342\206\222b\342\206\220c
└── mkdir_enc
    └── a\342\206\222b\342\206\220c

8 directories, 0 files

A ls -R command seems to confirm it.

$ ls -R tree
tree:
make_path  make_path_enc  mkdir  mkdir_enc

tree/make_path:
a→b←c

tree/make_path/a→b←c:

tree/make_path_enc:
a→b←c

tree/make_path_enc/a→b←c:

tree/mkdir:
a→b←c

tree/mkdir/a→b←c:

tree/mkdir_enc:
a→b←c

tree/mkdir_enc/a→b←c:

So my questions are:

  1. Am I doing it right code-wise ('course not)?

  2. Am I doing it right filesystem-wise?

  3. How can mkdir and make_path figure out and fix the wrong one?

  4. Or maybe I was just "reverse-lucky" (the kind of lucky which doesn't allow you to realize your error, since in your case it? In that case, how I can test it out effectively?

Any hint?

filesystems unicode perl
1 Answers
  1. How can mkdir and make_path figure out and fix the wrong one?

Perl strings have a "UTF-8 flag" that indicates whether the "characters" they contain are Unicode characters, vs. octets (eight-bit bytes). You can use the utf8::is_utf8 function (see http://perldoc.perl.org/utf8.html) to check if the UTF-8 flag is set for a given string; or you can use Dump from the Devel::Peek module, which prints out all the guts of a scalar, including the list of flags that are set.

So mkdir and make_path don't need to do anything too crazy; they can handle Unicode strings by encoding them as octet strings, just as you're doing when you call encode.

(Unfortunately, the UTF-8 flag thing has a lot of quirks, and not all functions honor it; for example, encode doesn't care whether its argument has that flag set, it just trusts that you wouldn't be calling it on a string unless the string was supposed to be interpreted as a sequence of Unicode characters. But if you use modern, Unicode-aware libraries, and use utf8, and just do everything Unicode-ishly except when specifically interacting with byte-oriented external systems (which you use Encode::encode and Encode::decode for), you should be fine.)

  1. Am I doing it right code-wise ('course not)?
  2. Am I doing it right filesystem-wise?

Yes, except I think you should pay more attention to error cases. What if your input can't be represented in the locale character set? What if it can, but the result isn't a valid filename in your operating system or filesystem?

To address this, you should make two or three changes:

  • You should provide an explicit third argument to Encode::encode to specify how it should handle non-encodable characters. (The default behavior is to replace them with a replacement character, such as ? for US-ASCII; that's probably not what you want.)
  • You should examine the return-value of mkdir.
  • You may want to use the error option to make_path, and examine the resulting arrayref; or, alternatively, you may want to wrap make_path in an eval block.
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