mtl: Context of execWriterT
ross at soi.city.ac.uk
Thu Apr 14 15:20:19 CEST 2011
On Thu, Apr 14, 2011 at 03:02:18PM +0200, Daniel Fischer wrote:
> Alas, Functor is not a superclass of Monad, so a Monad constraint is not
> more restrictive than a Functor constraint.
Yes, that is the issue -- as those classes are now, making this change
would break programs.
Several functions are affected by the same issue:
ReaderT: ask asks
StateT: evalStateT execStateT get put gets liftListen liftPass
WriterT: execWriterT tell listen listens pass censor
It's particularly annoying for ReaderT and WriterT, now that they're
Applicative transformers as well as Monad transformers.
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