Does 'MonadState s m => MonadState s (ReaderT r m)', found in
Control.Monad.Reader violate the coverage condition as I believe it
does?
Yes, it does.
Can one write a function using this library to force the type
inference engine to loop indefinitely? If not, what mitigating
conditions prevent it? Can I write a similar declaration in my
library and compile it with -fallow-undecidable-instances and without
worry?
The coverage condition is a sufficient condition for termination of the
type checker.
Another sufficient condition is the Terminating Weak Coverage condition
(Definition 14). This condition is met by that instance, as well as yours.
If you want to get non-terminating, you must write a funny instance like:
MonadState s m => MonadState (MymonadT s) (MymonadT m)
and then set up a constraint
MonadState x (MymonadT x)
I did not try it, but GHC should run out of stack on this one or give up.
I'm currently working with Martin Sulzmann on a relaxed set of
conditions for FDs and type functions.
Cheers,
Tom
--
Tom Schrijvers
Department of Computer Science
K.U. Leuven
Celestijnenlaan 200A
B-3001 Heverlee
Belgium
tel: +32 16 327544
e-mail: [EMAIL PROTECTED]
url: http://www.cs.kuleuven.be/~toms/
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