Here's a sketch of an idea as a solution to my dilemma, which unfortunately
requires associated types. Any suggestions how it might be translatable
into functional dependencies? (I should say, I've not got a HEAD ghc, and
am just going by memory on my indexed types syntax.)
class Witness w where
type M w a b
(>>=) :: M w a b x -> (x -> M w b c y) -> M w a c y
(>>) :: M w a b x -> M w b c y -> M w a c y
f >> g = f >>= const g
return :: x -> M w a a x
fail :: String -> M w a b x
instance Monad m => Witness m where
M m a b = m
(>>=) = Prelude.(>>=)
(>>) = Prelude.(>>)
return = Prelude.return
fail = Prelude.fail
with these definitions it seems that any existing monad will continue to
work as it always had, but I can now add new special sorts of monadish
objects, such as
data RepositoryMonad a b x = RM ...
instance Witness RepositoryMonad where
M RepositoryMonad x y = RepositoryMonad x y
...
which would allow me to create a monad in which the actions are limited
according to witness types, such as
applyPatchToWorking :: Patch a b -> RepositoryMonad (rec,a) (rec,b) ()
--
David Roundy
http://www.darcs.net
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