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The IO type is used to store a function that describes some computation with side effects, such as reading some data from a file, printing logging information to the console, or mutating the elements in the DOM. Describing actions in this way allows for IO instances to be composed and passed around while keeping functions pure and maintaining referential transparency.


The IO type consists of a single constructor which accepts a nullary function that describes the action to be performed, resulting in an IO of the value eventually returned from the action IO :: (() -> a) -> IO a.

//:: IO [String]
argsIO = IO(() => R.tail(process.argv));

//:: String -> IO String
readFile = filename => IO(() => fs.readFileSync(filename, 'utf8'));

//:: String -> IO ()
stdoutWrite = data => IO(() => process.stdout.write(data));

It is important to note that upon construction of an IO instance, the action will not be executed until the runIO method is called.


An IO instance implements the monad specification, allowing the results of the actions to be transformed or chained together. The runIO method can then be called to finally execute the action. Execution of an IO instance is recommended to be delegated to the outer edges of an application, to the point where an application will consist of a single IO instance at the entry point.

// Read each file provided in the arguments of the running application, echoing
// their contents to stdout after transforming the text to uppercase characters
//:: String -> IO ()
loudCat = argsIO.chain(R.traverse(IO.of, readFile))





:: (() -> a) -> IO a

Constructs an IO instance that represents some action that may possibly have side effects.

Static methods


:: IO a -> a

Executes the action described by the given IO instance. This is also available as an instance method.


:: a -> IO a

Produces an IO instance that results in the given value.

Instance methods


:: IO a ~> () -> a

Executes the action described by this IO instance. This is also available as a static method.

:: IO a ~> (a -> b) -> IO b

Transforms the result of this IO instance with the provided function.


:: IO (a -> b) ~> IO a -> IO b

Produces a new IO instance that when executed, applies the function resulting from the action in this IO instance to the value resulting from the action in the given IO instance.


:: IO a ~> (a -> IO b) -> IO b

Produces an IO instance that when executed, will apply the given function to the result of the action in this IO instance and then execute the resulting IO action.