Monad transformer for evaluating to a fixpoint The idea is that some transforms need to be run multiple times. Deciding whether to run a transform again can be somewhat tedious though, as you cannot necessarily just run some transform f on x until f x == x.

This might not be ideal for a few reasons: * x might not implement Eq; * x might implement Eq, but could contain floats of NaN, in which case NaN /= NaN; or * checking equality can be expensive.

Instead, this provides a function called progress, with the same type as return, that marks the current transform as having "made progress": that is, it should be re-run again. Then you can provide fixpoint with a function of type a -> FixT _ a, which will be re-run until no progress is made.