On Sat, Feb 13, 2016, at 15:32, Christopher Allen wrote:
There's another problem I discovered with HasCallStack. The implicit parameter constraint breaks sharing behavior for examples that use bottom.
This is a necessary consequence of the implementation of callstack-aware functions as overloaded functions. It's really no different from using a type-class, and is in my opinion the correct behavior. It seems what you really want is a way to prevent GHC from inferring the HasCallStack constraint in the first place. That's doable with an explicit type signature (or even a combinator to wrap the expression), but this isn't a great solution for your book. What I'd suggest instead is to define your own undefined in a Prelude-replacement (or simplification if you will). Something like undefined :: a undefined = withFrozenCallStack emptyCallStack Prelude.undefined should work to remove all traces of CallStacks (including when it blows up). error can be similarly wrapped to avoid having to deal with CallStacks, and $ and the FTP-related functions can be specialized to less-polymorphic versions that are easier to explain to beginners (at least until they're ready to be exposed to the real versions). I'm sorry that this change has caused you trouble.