I'm very much in the "Damn the torpedoes; full speed ahead" camp most of the time, but not here. Transformers is a tiny step up from base. Why would you make it depend on template haskell rather than the other way around?
Template Haskell currently has runIO :: IO a -> Q a , which allows you
to run IO actions with a guarantee of IO effects' ordering in a single
splice (but not necessarily the order in which all the Q splices are
run). If runIO is indeed a monad morphism, then there is a law-abiding
MonadIO Q instance that could be added to transformers:
instance MonadIO Q where
liftIO = runIO
This would be useful for dealing with monad transformer stacks with Q
as the base monad (for an example, see genifunctors
(https://github.com/danr/genifunctors/blob/4677bb57423b1b380ce9b50cc3d765a5c49a957a/Data/Generics/Genifunctors.hs#L47).
The only catch is that transformers would have to depend on
template-haskell. I think this would be okay since no quasiquotation
is involved, but I wanted to hear others' opinions.
Ryan
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