writer package:monadology

Construct a writer computation from a (result, output) pair. (The inverse of runWriter.)

A writer monad parameterized by the type w of output to accumulate. The return function produces the output mempty, while >>= combines the outputs of the subcomputations using mappend.

A writer monad parameterized by:

• w - the output to accumulate.
• m - The inner monad.

The return function produces the output mempty, while >>= combines the outputs of the subcomputations using mappend.

Extract the output from a writer computation.

• execWriter m = snd (runWriter
  m)

Extract the output from a writer computation.

• execWriterT m = liftM snd
  (runWriterT m)

Map both the return value and output of a computation using the given function.

• runWriter (mapWriter f m) = f (runWriter
  m)

Map both the return value and output of a computation using the given function.

• runWriterT (mapWriterT f m) = f
  (runWriterT m)

Unwrap a writer computation as a (result, output) pair. (The inverse of writer.)