G'day all.
Quoting Brian Smith
Even if "undecidable instances" was standardized, would we want it turned on by default? I am trying to write "real" programs in Haskell and I have never even comtemplated using undecidable instances.
There's only one situation where I've found it necessary, and I think it's due to a missing feature in Haskell. Sometimes when I'm doing generic programming, I need a function to have some group of typeclass constraints: foo :: (MonadIO m, Ord k, Foo k v) => k -> m v And sometimes these constraints appear in more than one function. For brevity, I want a "typeclass synonym". The only way I've found to do this is: class (Monad m, Ord k, Foo k v) => MyCtx m k v instance (Monad m, Ord k, Foo k v) => MyCtx m k v which now requires undecidable instances. Cheers, Andrew Bromage