Archive for November, 2011

November 11, 2011

chromatic has come up with a lovely new metaphor to replace the idea of technical debt: “technical insurance”. I love this sentence: “Given the premiums for your team, you then have to decide whether or not to buy a technical insurance policy.”

It hits at something that has bugged me about testing enthusiasts — the notion that you should always have tests. Because there is a balance that needs to be found — the cost of the tests versus the cost of not having the tests.

I’ve talking about this with respect to the ABC module before. Properly testing the abc2ly script would require constructing an OCR system for reading sheet music. That would be very expensive technical insurance indeed.

And what would it be protecting against? abc2ly is a open source tool for creating sheet music. It makes no money for me. As far as I know, I am the only active user. The only real risk I can see is if I am printing a long musical document, and a change I’ve made breaks something subtle that only affects, say, one thing on page 14, so I don’t notice the problem in a timely fashion. So basically, there’s a potential for wasting some paper.

In other words, in this case technical insurance would be expensive, and have very limited benefits. I’d be a fool to “buy” it.

On the other hand, the Perl 6 spectests are a fantastic technical insurance policy. For the most part they are low cost (easy to write) AND they are shared across multiple implementations of Perl 6. On the flip side, they help protect against subtle flaws in the implementations, which have the potential of causing problems for every person using Perl 6. We’d have to be crazy to do without this policy.


Fixing Tags

November 9, 2011

So, in my last post I identified 3906 files in my MP3 collection with missing tags. This time I set out to fix some of them.

So, first I went through the list I generated with the last script and singled out all 2294 files which used a standard pattern of Artist / Album / Track Number - Track Name. Then I wrote this script:

my $for-real = Bool::True;

constant $TAGLIB  = "taglib-sharp,  Version=, Culture=neutral, PublicKeyToken=db62eba44689b5b0";
constant TagLib-File    = CLR::("TagLib.File,$TAGLIB");
constant String-Array   = CLR::("System.String[]");

for lines() -> $filename {
    my @path-parts = $filename.split('/').map(&Scrub);
    my $number-and-title = @path-parts.pop;
    next unless $number-and-title ~~ m/(\d+) \- (.*) .mp3/;
    my $track-number = ~$0;
    my $title = ~$1;
    my $album = @path-parts.pop;
    my $artist = @path-parts.pop;
    say "$artist: $album: $title (track $track-number)";

    if $for-real {
        my $file;
        try {
            $file = TagLib-File.Create($filename);
            CATCH { say "Error reading $filename" }

        $file.Tag.Track = $track-number.Int;
        $file.Tag.Album = $album;
        $file.Tag.Title = $title;
        $file.Tag.Performers = MakeStringArray($artist);
        try {
            CATCH { say "Error saving changes to $filename" }

sub Scrub($a) {
    $a.subst('_', ' ', :global);

sub MakeStringArray(Str $a) {
    my $sa =;
    $sa.Set(0, $a);

For the main loop, the first half uses standard Perl techniques to extract the artist, album, and track info from the path. The second half sets the tags. Opening the file is the same as last time, and then setting Track, Album, and Title is as simple as could be. The Performers tag is a bit tricky, because it’s a string array (the others are simple strings or integers) and Niecza doesn’t know how to do the coercion automatically. MakeStringArray gets the job done nicely.

So, if you’ve done this sort of thing in Perl 5 using the MP3 CPAN modules, there’s nothing at all revolutionary about this code. But it feels really good to be able to do it with Perl 6!

Examining MP3 Tags

November 6, 2011

I’ve been playing around with Niecza’s ability to handle CLR libraries. It’s actually kind of intoxicating; it’s the closest thing yet to having a CPAN for Perl 6. So I decided to see what I could do with the TagLib# library for dealing with media file tags.

Now, I’ve got an MP3 library with 23564 MP3 files in it, the majority of which were created by older ripping files that didn’t do anything with the ID tags. Most of those have been updated to include tags, but every now and then I add one of the old directories to iTunes and get a bunch of “Unknown Artist” / “Unknown Album” tracks.

So I thought a nice first project would be figuring out which of the tracks was correct. The first thing to do was to get TagLib# properly installed on my MacBook Pro. make install didn’t add the DLL to the main GAC; I ended up installing it there by hand, which was trivially easy once I knew what to do:

sudo gacutil -i taglib-sharp.dll
sudo gacutil -i policy.2.0.taglib-sharp.dll

Once I had that done, I experimented with it for a bit, and ended up with this script:

constant $TAGLIB  = "taglib-sharp,  Version=, Culture=neutral, PublicKeyToken=db62eba44689b5b0";
constant TagLib-File    = CLR::("TagLib.File,$TAGLIB");

for lines() -> $filename {
    try {
        my $file = TagLib-File.Create($filename);
        unless $file.Tag.JoinedPerformers ~~ m/\S/ && $file.Tag.Title ~~ m/\S/ {
            say $filename;
        CATCH { say "Error reading $filename" }

The first line specifies the exact assembly we want to use; you can get the details from gacutil -l. The next line effectively imports the TagLib::File class into Niecza. I get my filenames from stdin, as that allows me to use find to generate the list of MP3 files.

This was my first use of exception handling in Perl 6. I needed it because TagLib-File.Create throws an exception when it isn’t happy with the MP3 file. When it is happy with it, $file is an object of type CLR::TagLib::Mpeg::AudioFile. $file.Tag.JoinedPerformers gives the list of performers (AKA artists) as a single string; $file.Tag.Title gives the title as a string. Unless we find a valid non-space character in both of them, we flag the file by printing it out.

Really, the only way it could be significantly simpler than this would be if the constant TagLib-File line were unnecessary!

End result: I have a list of 3906 files it flagged, 77 of which were read errors.

My next step is to write some code which translates the filenames (which are mostly of the form /Volumes/colomon/Albums/Dervish/Live_in_Palma/04-Slow_Reels.mp3) into artist, album, and track name fields, and then set those tags. Based on my initial experiments, I think it’s is going to be incredibly easy…