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The Effect monad, for handling native side effects.


bower install purescript-effect


Values in PureScript do not have side-effects by default. This package provides the standard type PureScript uses to handle "native" effects, i.e. effects which are provided by the runtime system, and which cannot be emulated by pure functions. Some examples of native effects are:

  • Console IO
  • Random number generation
  • Exceptions
  • Reading/writing mutable state

And in the browser:

  • DOM manipulation
  • XMLHttpRequest / AJAX calls
  • Interacting with a websocket
  • Writing/reading to/from local storage

All of these things may be represented in PureScript by the Effect type. A value of the type Effect a represents a computation which may perform some native effects, and which produces a value of the type a once it finishes. For example, the module Effect.Random from purescript-random exports a value random, whose type is Effect Number. When run, this effect produces a random number between 0 and 1. To give another example, the module Effect.Console exports a function log, whose type is String -> Effect Unit. This function takes a string and produces an effect which, when run, prints the provided string to the console.

module RandomExample where
import Prelude
import Effect (Effect)
import Effect.Random (random)
import Effect.Console (log)

printRandom :: Effect Unit
printRandom = do
  n <- random
  log (show n)

In this example, printRandom is an effect which generates a random number between 0 and 1 and prints it to the console. You can test it out in the repl:

> import RandomExample
> printRandom

The unit is there because all effects must produce a value. When there is nothing useful to return, we usually use the type Unit, which has just one value: unit.

Effects and Purity

Note that using Effect does not compromise the purity of your program. Functions which make use of Effect are still pure, in the sense that all functions will return the same outputs given the same inputs.

This is enabled by having the Effect type denote values which can be run to produce native effects; an Effect a is a computation which has not yet necessarily been performed. In fact, there is no way of "running" an Effect manually; we cannot provide a (safe) function of the type forall a. Effect a -> a, because such a function would violate purity; it could produce different outputs given the same input. (Note that there is actually such a function in the module Effect.Unsafe, but it is best avoided outside of exceptional circumstances.)

Instead, the recommended way of "running" an effect is to include it as part of your program's main value.

A consequence of being able to represent native effects purely using Effect is that you can construct and pass Effect values around your programs freely. For example, we can write functions which can decide whether to run a given effect just once, many times, or not at all:

-- | Runs an effect three times, provided that the given condition is
-- | true.
thriceIf :: Boolean -> Effect Unit -> Effect Unit
thriceIf cond effect =
  if cond
    then do
      pure unit

Using Effects via the Foreign Function Interface

A computation of type Effect a is implemented in JavaScript as a zero-argument function whose body is expected to perform its side-effects before finally returning its result. For example, suppose we wanted a computation which would increment a global counter and return the new result each time it was run. We can implement this as follows:

-- Counter.purs
foreign import incrCounter :: Effect Int

and in the corresponding JavaScript module:

// Counter.js
exports.incrCounter = function() {
  if (!window.globalCounter) {
    window.globalCounter = 0;
  return ++window.globalCounter;

For more information about the FFI (Foreign Function Interface), see the documentation repository. This package also provides a module Effect.Uncurried to simplify the process of making uncurried effectful JavaScript functions available to PureScript via the FFI.

The Effect type is Magic

The PureScript compiler has special support for the Effect monad. Ordinarily, a chain of monadic binds might result in poor performance when executed. However, the compiler can generate code for the Effect monad without explicit calls to the monadic bind function >>=.

Take the random number generation example from above. When compiled, the compiler produces the following JavaScript:

var printRandom = function __do() {
    var $0 = Effect_Random.random();
    return Effect_Console.log($0))();

whereas a more naive compiler might, for instance, produce:

var printRandom = Control_Bind.bind(Effect.bindEffect)(Effect_Random.random)(function ($0) {
    return Effect_Console.log($0));

While this is a small improvement, the benefit is greater when using multiple nested calls to >>=.

API reference

Module documentation is published on Pursuit.