On Wed, Feb 4, 2015 at 4:39 PM, Eric Seidel
A constraint (?x :: CallStack) will always be solved for the source location that gave rise to it, when it comes from a function signature (as opposed to a use of the implicit param) you'll get the immediate call-site. Furthermore, if that call-site has a CallStack implicit param in scope, the stacks will be appended (this appending of call-stacks is the bit that I'm not sure will see much use). Does that make sense?
Sure. One place I can think of where the chaining would be useful is you can put it on a helper function that then calls the logging function, so the output can skip that intermediate function. This makes it better than python or C++ logging, which often provides no way to do that.
You're quite right, there are plenty of reasons to want this functionality beyond error reporting, though I'd personally like to use this for `error`, `undefined`, and `assert` as well!
True enough, though assert kind of already has the feature. In fact, you could remove the assert special case hack, though there are probably people relying on it to get source info. In any case it's up to the GHC people to say what they think. In case it's not already obvious, big +1 for "merge as soon as possible" from me.