Haskell Language Common monads as free monads Free (Writer w) ~~ Writer [w]
Example
Given
data Writer w a = Writer w a
we have
data Free (Writer w) a
= Pure a
| Free (Writer w (Free (Writer w) a))
which is isomorphic to
data ProgLog w a
= Done a
| After w (ProgLog w a)
or, equivalently, (if you promise to evaluate the log first), Writer [w] a.
