Monad transformer

A construction taking a computational monad $M$ to another computational monad $TM$, normally with a lift from $MA$ to $TMA$. The order of monad transformers can affect which information survives an exception or a control transfer.

A continuation monad whose final answer is computed in a base computational monad. The answer type $R$ is fixed throughout a computation.

An unlabelled exception can be represented by an algebraic data type with successful and failed alternatives, sequenced inside a base computational monad. Sequencing skips the next computation after failure; a handler runs an alternative computation. The base effects already performed need not be undone.

A state monad layered over a base computational monad. An unsuccessful base computation may return no final program state; this matters when combining it with an exception monad transformer.