Archive for March, 2013

Philosophical Issues with Rakudo’s .parse

March 6, 2013

For a little bit now I’ve had the ABC module working on both Niecza and Rakudo. I got a new Linux box yesterday; this morning I was playing around with my ABC tools there, trying to see if I could get them to work. I installed them with panda, so I was using ABC with Niecza by default on my MBP and with Rakudo by default on my new machine. I tried it with a file of my own compositions. Apparently I hadn’t tried this in a while, because it had several issues. And the two systems handled them completely differently.

On Niecza, I got this message:

Unhandled exception: Did not match ABC grammar: last tune understood:
 A Hat for the Whales
d|:e~A3 BGEF|G2BG DG (3ABd|eBdB ~A3G|Bdef gfed|
e~A3 BGEF|G2BG DG (3ABd|eBdB AAGA|[1Beed e3d:|

On Rakudo, there were no error messages, but “A Hat for the Whales” was the last tune processed, even though there were a dozen more tunes in the ABC file after it.

Without thinking about it too much I corrected this batch of errors. Now on Niecza the entire file parsed, but I got the error

Unhandled exception: Illegal key signature B minor

Under Rakudo, the same ABC file just appeared to work.

What’s going on here? Well, key signature processing looks something like this (simplified):

    token key-def { <basenote> <chord_accidental>? <mode>? }
    token mode { <minor> | <major> }
    token minor { "m" ["in" ["o" ["r"]?]?]? }
    token major { "maj" ["o" ["r"]?]? }

No space allowed between basenote and mode. So when you invoke parse with that rule on “B minor”, in Niecza it fails, because the entire string was not successfully parsed. In Rakudo it succeeds, because “B” is a valid key signature, and it doesn’t care that the entire string was not parsed. In practice, this means that three of the tunes in the collection got the wrong key signature: B major instead of the correct B minor.

To my mind, Niecza’s behavior is more intuitive. If you’re trying to parse a file, parsing half of it and then silently ignoring the rest is wildly unhelpful behavior. In every use I’ve had for parse so far, I’ve wanted it to parse everything in the string I sent to the function.

But hey, I understand that some people might have different needs. Can we at least add a flag that will give you a parsing failure if the entire string is not parsed? :all, perhaps?


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