On Tue, Jul 24, 2018 at 5:52 AM,Ryan Reich wrote:

This seems like the canonical illustration of the dangers of UndecidableInstances.

No I don't buy that explanation. The worst UndecidableInstances can do is send the type checker into a loop. For mutually recursive functions to cause looping, you don't need classes at all: foo :: Bool -> () foo x = bar x bar :: () -> Bool bar x = foo x No typechecking will detect that loop. This `HasBool` example is just an obfuscated way to write that loop(?) There's some odd thigs in the OP: Why use OVERLAPPABLE? Neither of those instances need it. Or are there more instances in the imports? (There must at least be the definition for class `Has`.) If so, what are their constraints? And what are their implementations? Perhaps that's causing the looping? Will `runRIO` or `view` use one of those other instances? What does this mean: "I can compile an equivalent program; it loops on execution."? There doesn't need to be a `Bool` in the "environment": the `HasBool a` instance introduces `Bool` in its constraint. I suggest boiling this down to a stand-alone program not needing imports. Then tell the whole story. AntC On Mon, Jul 23, 2018 at 2:10 PM, Theodore Lief Gannon < tanuki at gmail.com http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe> wrote:

* λ. :set -XFlexibleContexts -XFlexibleInstances -XMultiParamTypeClasses *>* -XUndecidableInstances *>* λ. import RIO *>* λ. import Data.Has *>* λ. class HasBool a where boolL :: Lens' a Bool *>* λ. instance {-#OVERLAPPABLE#-} HasBool a => Has Bool a where hasLens = *>* boolL *>* λ. instance {-#OVERLAPPABLE#-} Has Bool a => HasBool a where boolL = *>* hasLens *>* λ. runRIO () $ view boolL *>* ^CInterrupted. *>>* The RIO environment () doesn't contain a Bool. I can see how the *>* typechecker might get lost in this, but I can compile an equivalent *>* program; it loops on execution. *>>* _______________________________________________ *>* Haskell-Cafe mailing list *>* To (un)subscribe, modify options or view archives go to: *>* http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe *>* Only members subscribed via the mailman list are allowed to post. *>