[Haskell-cafe] dynamic code compilation and loading

Evan Laforge qdunkan at gmail.com
Tue Jan 17 08:33:53 UTC 2017

On Mon, Jan 16, 2017 at 11:16 PM, Dennis Raddle <dennis.raddle at gmail.com> wrote:
> I'm not clear on everything you've saying.
> My program, at present, opens some MIDI ports and leaves them open while it
> is running. It presents me with a command line prompt from which I can
> configure the program and initiate MIDI playback. I can also halt playback
> in progress. Every time I change the score (in the Sibelius typesetter
> program), I initiate a new playback action.
> What you are saying sounds like running the program once for each playback.
> I guess the program wouldn't leave the MIDI ports open, as it has to close
> them before it exits. But that should work fine. I would also need a
> different way of configuring the program -- the current settings would have
> to be stored in a file so they won't be lost when the program exits.

I was thinking you'd have ghci running persistently.  Start it up, run
the initialize function which will configure MIDI.  Then every time
you want to process a score, just rerun the "process" function.

> I wonder about two things. First, I would like the majority of the program
> to be compiled. It needs speed. Is there a way to compile everything but the
> module that "varies" a Score?

Yes, you can compile modules and ghci will load the compiled version.
When you change one and do a :r, it will recompile the changed module
and its dependents.  Normally it uses bytecode for this, but you can
also have it compile to binary.  See the ghci section of the ghc

> Second, the Vary.hs module would need to be located on the disk in the same
> directory as the music score, to keep things organized. But on my disk
> (MacBook Pro with SS drive) there is one tree for Haskell source,
> /Users/Dennis/haskell. I use the option "-i/Users/Dennis/haskell" which
> compiling or running ghci. The musical scores are all stored in a different
> tree, "/Users/Dennis/Dropbox/music/comp".
> So I don't know how to "import" a file that is not in my Haskell source
> tree, and more important, is not determined until run time.

You can set flags from ghci, so you can add a -i at runtime.  :l also
understands plain paths, so you could give it the complete path, or
generate it via :def.

> Oh yeah, I don't specify the specific score when I run my program, but
> rather let it search the /Users/Dennis/Dropbox/music/comp tree for the most
> recently modified score. This is convenient, because over the course of an
> hour or two that I compose, I would switch several times between scores, and
> as soon as I modify and save a particular score, that one becomes the source
> for playback.
> So the program would locate a recently modified Sibelius score, say
> $MUSIC/piano/tocatta1.sib. (Where $MUSIC is
> /Users/Dennis/Dropbox/music/comp). Then the program would assume there is
> some Haskell source in the same directory, called "toccata1.hs". This source
> would contain functions that vary the expression in ways I like for that
> particular composition.
> Does this sound doable with ghci? Or the GHC API?

ghci has some limited but surprisingly flexible scripting ability.
For instance, you could do :def L (\_ -> loadNewest), and then define
a loadNewest function that looks for the relevant .sib file and emits
the path to a parallel .hs file.  I actually use a :L macro to load
the currently edited file in vim, so I'd probably make a vim macro to
load (or create) an .hs for the "current" .sib file, and then use :L
to load it.  For interactive work, my :l is overridden to :m +Utils so
I can get a bunch of interactive utils without having to import them
into every module.

Also, you are not limited to just rerunning a single "vary" function.
You can use the full power of haskell to configure the realization.  I
use the REPL for analysis and debugging as well.

> About what I've learned about expression score realization, I could perhaps
> share some things at some point. Unfortunately I have bad work habits... I
> often don't take care to finish projects in a way that is presentable, so I
> have many years of half-finished compositions that can't even be played back
> with the current version of my program. I'm 48 years old and finally
> realizing just how important it is to organize and finish things. And to
> collaborate. I found another musician to collaborate with -- I will make
> computer "interpretations" of his compositions.

I keep each score saved with a "known good" version of the MIDI
output, along with a commit ID for the time it was generated.  So it
not only archives a known good realization, it also acts as a
regression test against code changes (checking all saved scores is
part of the commit validation, in addition to the more usual unit
tests), and if all else fails, I can get back to the original
performance by rewinding the repo.  mp3 is good for archiving of
course, but you'll never be able to change it again.

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