writer package:automaton

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

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

An Automaton with a WriterT layer outputs an extra monoid value on every step. 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.

Convert an extra log output into a WriterT effect. 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.)

Convert a WriterT effect into an extra log output. This is the opposite of writerS.