2009/11/26 Michael Mossey
First of all, thanks for the ideas, everyone. I think I'm starting to get the usefulness of type classes.
With regard to your question, Eugene, you are probably right. In fact my rough draft of this code from four months ago used a Map with time as the key (of type Rational). I was trying to come up with a quick example and didn't think straight.
You may observe, however, that the concept "note" has a lot of uses and contexts. Sometimes it means simply a pitch, like middle C. Sometimes it means a black dot on a piece of staff paper, in which case it has a duration (as a number of beats). The context of that black dot gives it a dynamic level, an associated instrument (timbre), and of course, a time. In this larger context, I'm not sure if there is a deep difference between storing time inside the object or outside it, except for the practical matter of wanting to index notes by time (in which case it is useful to use time outside the object as the key). You tell me: do you think it makes a deep difference?
I argue that in the situation you provided, the pitch, duration, timbre and instrument are "essential" attributes of the dot, whereas time is the position of the dot on paper and should be separated from its essence. I think two concepts should be separated if one makes sense and is useful without the other. A note out of its time context is certainly useful, for example, it may probably be converted to a MIDI command or to a graphical glyph (which is further to be positioned by a layout engine).
Thanks, Mike
Eugene Kirpichov wrote:
Hi,
Are you sure you need to store the time *inside* your "objects" instead of using, say, pairs "(Time, YourObject)" (and lists of them instead of lists of your objects)? It would seem strange to me that a note HAS-A time even in an OO design: more likely, a note is associated with a time, and this is modeled by pairing them.
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