mtl-2.2.1: Monad classes, using functional dependencies

Description

Inspired by the paper Functional Programming with Overloading and Higher-Order Polymorphism, Mark P Jones (http://web.cecs.pdx.edu/~mpj/pubs/springschool.html) Advanced School of Functional Programming, 1995.

Synopsis

# Documentation

class (Monoid w, Monad m) => MonadWriter w m | m -> w where #

Methods

writer :: (a, w) -> m a #

writer (a,w) embeds a simple writer action.

tell :: w -> m () #

tell w is an action that produces the output w.

listen :: m a -> m (a, w) #

listen m is an action that executes the action m and adds its output to the value of the computation.

pass :: m (a, w -> w) -> m a #

pass m is an action that executes the action m, which returns a value and a function, and returns the value, applying the function to the output.

Instances

listens f m is an action that executes the action m and adds the result of applying f to the output to the value of the computation.
• listens f m = liftM (id *** f) (listen m)
censor f m is an action that executes the action m and applies the function f to its output, leaving the return value unchanged.
• censor f m = pass (liftM (\x -> (x,f)) m)