Control.Program.Writer

Types and functions for handling appendable output in the environment of a Program.

Writer Effect

newtype Writer w

A record of functions which represents the operations on an appendable output.

Constructors

Writer

Fields

• writeValue :: w -> IO ()

Program-based Writer

newWriter :: Monoid w => IO (Writer w, IO w)

Creates a new record of functions for appendable output, backed by an IORef.

Returns the record of functions and an action which reads the accumulated output, usually used after running a corresponding Program with the Writer in its environment.

tell :: e `Has` Writer w => w -> Program e ()

Produces the output w. In other words, w is appended to the accumulated output.