Documentation

A declare monad transformer parametrized by:

• d — the output to accumulate (declarations);
• m — the inner monad.

This monad transformer is similar to both state and writer monad transformers. Thus it can be seen as

• a restricted append-only version of a state monad transformer or
• a writer monad transformer with the extra ability to read all previous output.

Definitions of declare and look must satisfy the following laws:

monoid homomorphism (mempty)

declare mempty == return ()

monoid homomorphism (mappend)

declare x >> declare y == declare (x <> y) for every x, y

declare-look

declare x >> look == fmap (<> x) look <* declare x for every x

look as left identity

look >> m == m for every m

Retrieve a function of all the output so far.

Evaluate DeclareT d m a computation, ignoring new output d.

Execute DeclateT d m a computation, ignoring result and only producing new output d.

Evaluate DeclareT d m a computation, starting with empty output history.

A declare monad parametrized by d — the output to accumulate (declarations).

This monad is similar to both state and writer monads. Thus it can be seen as

• a restricted append-only version of a state monad or
• a writer monad with the extra ability to read all previous output.

Run Declare d a computation with output history d, producing result a and new output d.

Evaluate Declare d a computation, ignoring output d.

Execute Declate d a computation, ignoring result and only producing output d.

Evaluate DeclareT d m a computation, starting with empty output history.