Add HasCallStack where error can occur

[dspies-leapyear]

When this fails due to an empty list there's no stack trace pointing to the bad "elements" call.

[sol]

This requires more work for backwards compatibility. However, I think the error message is clear enough even without accurate source locations, so for me personally closing this as won't fix would be ok too.

EDIT: To be clear, I'm not a maintainer. This is just a third party comment.

[sol]

This only works with recent versions of GHC.

My call-stack package provides a shim for backwards compatibility. @nick8325 are you fine with adding that dependency? If yes, we could also add source locations to e.g. ===.

If not, we would want to CPP the HasCallStack => of the type signature.

[dspies-leapyear]

👍

[sol]

@nick8325 even with this change this will still require GHC 7.4.*.

[dspies-leapyear]

@sol The error message is certainly clear, but it's not useful for debugging if elements is called in a lot of places. For debugging purposes, I'd like to know which call to elements is causing the failure without having to put trace calls everywhere or create my own elements wrapper (which is what I'm doing currently).