writer package:dunai

Construct a writer computation from a (result, output) pair.

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

MSFs with a Writer monadic layer. This module contains functions to work with MSFs that include a Writer monadic layer. This includes functions to create new MSFs that include an additional layer, and functions to flatten that layer out of the MSF's transformer stack. It is based on the _strict_ writer monad Strict, so when combining it with other modules such as mtl's, the strict version has to be included, i.e. Strict instead of Writer or Lazy.

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.

Build an MSF in the Writer monad from one that produces the log as an extra output. This is the opposite of runWriterS.

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.)

Build an MSF that produces the log as an extra output from one on the Writer monad. This is the opposite of writerS.