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Usually “patch” will mean a backend-specific (likely MIDI) definition, while “instrument” means a specific allocation of a patch in a particular score. I’m not totally consistent with that terminology, though I try.

Patches are defined in Local.Instrument, which will collect other the various instrument definitions together into an Instrument.Inst.Db. This is then used by the Instrument.Browser to search and display patches, or by the sequencer to look them up by name.

There are utilities to assist creating patches in Cmd.Instrument.MidiInst. Patch definitions can be very involved, but it’s mostly optional. The minimum you can get away with is a synth name and a pitchbend range. E.g. a minimal synth definition is

    synth :: MidiInst.Synth
    synth =
        MidiInst.synth "synth-name" "Synth description." $
            MidiInst.synth_controls controls patches
        controls = []
        patches = [MidiInst.default_patch (-12, 12) []]

After this you can create an instrument bound to the Instrument.InstTypes.Qualified name synth-name/ and it will have a pitch-bend range of (-12, 12), but no other configuration. The rest of the configuration is opt-in, so the more things you set in there, the more fancy things the instrument can do. Both the synth and patch names should conform to Ui.Id.valid. That’s not a technical requirement, but the restriction is good for consistency and keeps my options open.

As documented in Local.Instrument, some patches may require expensive things like parsing sysex dumps. Instrument.MakeDb, which is built as build/opt/make_db, will run them and write caches. Instrument.Sysex has support for parsing (and generating) sysexes.

Instruments have parts that affect the Cmd, Derive, and Perform systems.


Each instrument can bring Cmds into scope when the insert selection is focused on a track. Typically this is used for drums, which will want to bind specific strokes to keys instead of using the default pitch-oriented note entry.

Utilities for generating per-instrument cmds are in Cmd.Instrument.CUtil.

LInst config

The Cmd.Repl.LInst module has utilities to configure per-score instrument configuration, which lives in Ui.UiConfig.config_allocations.

The most useful functions are Cmd.Repl.LInst.list, to show an abbreviated version of the current configuration, Cmd.Repl.LInst.add to allocate a new instrument, and Cmd.Repl.LInst.remove to remove one. Other functions will set or clear various configuration fields.


At the derive level, an instrument is mostly just a string Derive.ScoreT.Instrument. However, a few other bits of instrument config apply, as defined by Derive.Deriver.Monad.Instrument. Specifically, a note track with a title >xyz will wind up calling Derive.Deriver.Lib.with_instrument on >xyz. A note track with an empty instrument name, like >, marks the track as a note track but won’t alter anything.

Analogous to CUtil, Derive.Instrument.DUtil has utilities for defining instrument-specific calls.


In theory, all the Perform level instrument configuration is specific to a backend, and a different backend may expose an entirely different set of fields. However, currently the only backend with non-trivial instrument needs is MIDI (lilypond config is in Ui.UiConfig.config_lilypond, but it just cares about the name to put it on the staff).

Performance specific details are in