jxxcarlson / DrumLanguage

Elm-Euterpia project

This project has three aims. (1) Implement the fun but totally useless Techno Drum App (see ./app). (2) Implement some of the basic functionality of Euterpia, the music composition system created by Paul Hudak (see The Haskell School of Music). As the name indicates, the system is written in Haskell, and one uses it to write music directly. (3) implement a player interface for elm-euterpia (as we might call it) to a javascript music player system. It seems that Tone.js is a good candidate. MIDI should also be added (in and out).

The drum app is a very weak application of items (2) and (3). It is first test case for the viability of this project.

Notes on the Drum App

My approach was to map each letter a–z to a phonetic class, then map phonetic classes to musical pitches. The phonetic class o f a, e, i, o, u is VOWEL. Consonants are divided into various classes, e.g., m, n of type NASAL. Each class is assigned a pitch, and so any string of characters is mapped to a sequence of pitch names. For example, Hello is assigned the sequence G3 G2 C3 C3 G2. Here the numerical part of a pitch name refers to its octave. Thus G3 is one octave higher than G2. The pitches used to represent the pitch classes are G2 C3, E3, F3, G3, Bb3, D — a dominant ninth chord. This choice of mapping makes the drumming sound relatively harmonious.

Notes

• I've added another test app: ./app/EuterpiaTest.elm. Right now it is pretty useless and the sound playback is unreliable.

• The article on African drum languages gives some context for the drum app.

Current state

• The basic type system in the modulesPrimitive, Pitch, Duration, and Music.
• A Player module which is part of the interface to Tone.js
• A helper module Rational to implement the system of rational numbers used in Duration. Thus a quarter note will have duration 1/4, an eighth note 1/8, and a quarter followed by an eighth note will have duration 3/8.

An issue

I've implemented the Music a type as follows

type Music a
    = Prim (Primitive a)
    | Sequence (List (Music a))
    | Stack (List (Music a))
    | Nodify Control (Music a)

Compare this type definition to Hudack's, as found at the bottom of page 30 of The Haskell School of Music:

data Music a =
    Prim (Primitive a)
    | Music a:+:Music a
    | Music a:=:Music a
    | Modify Control (Music a)

The code Music a:+:Music a is for sequential composition, e.g., if p and q are phrases, then p :+: q is the longer phrase obtained by laying the two phrases end-to-end. In the Elm version, we say Sequence [p, q]. The duration of this piece of music is the sum of the durations of its components.

The code Music a :=: Music a is for parallel composition, e.g., if p and q are voices, say the treble and base in one of Bach's two-part inventions, then p :=: q is the music with those two parts. The duration of p :=: q is the maximum of the individual durations.