Fix ListT MMonad instance

[ivan-timokhin]

Current instance (e3fc775) is

instance MMonad ListT where
    embed f m = Select (enumerate (embed f m))

…which is just an infinite loop, unless I'm completely misunderstanding something.

For a simple demonstration,

runEffect $ for (enumerate $ pure False) (lift . print)

prints False, whereas

runEffect $ for (enumerate $ embed lift $ pure False) (lift . print)

loops indefinitely, even though embed lift should be equal to id by MMonad laws.

[Gabriella439]

I verified that both examples succeed and produced the same result after this fix. The fix is up on Hackage as pipes-4.3.2

[ivan-timokhin]

Thanks!

[Gabriella439]

You're welcome!