csound-expression-0.3.1: Csound combinator library




Library csound-expression allows csound code construction in functional way.

It has two parts CsoundExpr.Base and CsoundExpr.Opcodes. First part Csound.Base contains essential functionality of the package. Second part Csound.Opcodes contains opcode definitions.

This module provides overview of the library. For examples see sources, folder examples



Csound-expression is csound code generator. Program produces value of CsoundFile type. CsoundFile is Show. So that is the way to get csound code. Function csd can be invoked to make value of CsoundFile type.

csd :: Flags -> Header -> EventList Dur SignalOut -> CsoundFile
  • Flags is String. It's pasted in place of csounds flags.
  • Header is csound header declaration. See module CsoundExpr.Base.Header for more details.
  • EventList represents csound orchestra and score sections. This type comes from external library 'temporal-media' [1]. EventList contains values with time marks. Value begins at some time and lasts for some time. Very much like csound notes, but there is one difference no need for p-field parameters, translator derives them from note structure encoded in values of type SignalOut. EventList can be constructed directly with functions of 'temporal-media' library, but better way is to use some front-end. Package 'temporal-music-notation' [2] provides higher level musical functionality for EventList construction.

[1] http://hackage.haskell.org/package/temporal-media

[2] http://hackage.haskell.org/package/temporal-music-notation


Instruments are functions from some signal representation to signal. Score (from 'temporal-music-notation' library) or EventList (from 'temporal-media' library) is a Functor, so to play on instrument means to apply instrument. to container of its notes. Instrument can be made with opcodes. Translator derives p-fields from instrument structure. There are only two explicit p-fields itime and idur (p2 and p3 in csound).

Signals / Types

Signals are represented with trees. Tree contains information about how signal was build.

There are five types for signals (CsoundExpr.Base.Types).

Arate is audio rate signal

Krate is control rate signal

Irate is init value

SignalOut is no output at all (it's produced by opcodes like out, outs, xtratim)

BoolRate is comparision of two control or init rate signals (CsoundExpr.Base.Boolean)

There are two classes to allow csound's polymorphism : X and K

X = Arate | Krate | Irate

K = Krate | Irate

Csound's S - signal is represented with String. Ftable is represented with Irate. If you want to get Krate ftable you can construct it with function ifB.

There are two special types MultiOut (for opcodes that may produce several outputs, see CsoundExpr.Base.MultiOut) and SideEffect (for opcodes that rely on number of appearances in csound code, like unirand, see CsoundExpr.Base.SideEffect)


Naming conventions : Opcodes are named after csound's counterparts usually. Some opcodes in csound can produce signals of different rates by request (oscil, linseg). Those opcodes are labelled with suffix. Suffix defines output rate of signal (oscilA, oscilK). Some opcodes in csound have unfixed number of inputs due to setup parameters, almost all of them. Those opcodes have first argument that is list of setup parameters.


oscilA :: (X a, X b) => [Irate] -> a -> b -> Irate -> Arate
oscilK :: (K a, K b) => [Irate] -> a -> b -> Irate -> Krate        

Imperative style csound code

Most of csound opcodes can be used in functional way. You can plug them in one another, and make expressions, but some of them behave like procedures and rely on order of execution in instrument. Module CsoundExpr.Base.Imperative provides functions to write imperative csound code.

outList - to sequence procedures

'(<=>)' - Assignment

ar, kr, ir, gar, gkr, gir - named values, to produce signal with specified name and rate.

Functional style :

exmpInstr :: Irate -> SignalOut
exmpInstr pch = out $ oscilA [] (num 1000) (cpspch pch) $ 
       gen10 4096 [1]

Imperative style :

exmpImper :: Irate -> SignalOut 
exmpImper pch = outList [        
        ir "amp" <=> num 1000,
        ir "cps" <=> cpspch pch,
        ir "ft"  <=> gen10 4096 [1],
        ar "sig" <=> oscilA [] (ir "amp") (ir "cps") (ir "ft"),
        out (ar "sig")]


You can use polymorphic operations to do some arihmetic on signals from CsoundExpr.Base.Arithmetic. And Signal is Num. Eq is undefined though.


To see what will come out of an expression you can print it. Arate, Krate and Irate are Show.

User Defined opcodes

You can add your own opcodes to library, see CsoundExpr.Base.UserDefined


There are two ways to tell csd to include instrument in csound file. Instrument can be a part of Score or it can be midi instrument, then it should be mentioned in massign or pgmassign function. If you want to play midi-instr for some time t, you can tell it to csd function by invoking csd with none t ('temporal-media') or (toList $ rest t) ('temporal-music-notation') in place of EventList value.

flags = "-odac -iadc  -+rtmidi=virtual -M0"

header = [massign [] 1 instrMidi]

instrMidi :: SignalOut
instrMidi = out $ oscilA [] (num 1000) cpsmidi $ gen10 4096 [1]

-- play instrMidi for 2 minutes
exmpMidi = print $ csd flags header (none 120)


What can not be expressed


The major benefit and major problem of csound-expression is abscense of ids for p-fields, ftables, notes and instruments.

no ids for ... means ... no

p-fields/notes - opcodes that rely on p-fields or invoke instruments

ftables - k-rate ftables

instruments - convenient way to specify order of instruments

imperative program flow control

There is no program flow control opcodes (like if, then, goto). But you can use functional if/then from module CsoundExpr.Base.Boolean


I've decided to represent csound's S-rate with String. Signal is represented with tree and it means i can't include opcodes that produce Srate

Hack-way around (what somehow can be expressed)

instrument order

Orchestra section is generated from EventList. Different instruments have different tree structure and one instrument's tree can't be transformed into another one by replacing leaf-values only.

You can point to instrument by its structure. There is opcode in CsoundExpr.Base.Header that specifies order of instruments by list of notes. instrOrder takes in list of notes, if instrument's tree is equivalent to note it is placed in relation to list of notes.

There are ways to make mistake. Sometimes it's unpredictable.

In example below

q1 =/= q2

sco contains two instruments (one with x, and another one with cpspch x)

osc x = oscilA [] (num 1000) x $ gen10 4096 [1]
env   = lineK 1 idur 0

q1 x = osc x <*> env
q2 x = env   <*> osc x

sco1 = note 1 440
sco2 = note 1 $ cpspch 8.00

sco = fmap q1 $ sco1 +:+ sco2

I think maybe it's worthwhile to introduce some way of instrument id assignment.