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Showing content from http://hackage.haskell.org/packages/archive/mtl/2.0.1.0/doc/html/Control-Monad-Writer-Class.html below:

Control.Monad.Writer.Class

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

Methods

tell :: w -> m ()Source

tell w is an action that produces the output w.

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

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 aSource

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.

censor :: MonadWriter w m => (w -> w) -> m a -> m aSource

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)

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