On Thu, Jan 21, 2016, at 04:07, Simon Peyton Jones wrote:
| It’d probably need a built-in function | | setCallStack :: CallStack -> (AppendsCallStack => a) -> a
Correct. This is easy to write in Core but not in Haskell.
Ugh, I just realized that we can't write setCallStack (with implicit parameters) in Haskell either. Well, we can, but it adds an entry to the stack.. Why? Let's look at the implementation setCallStack :: CallStack -> (HasCallStack => a) -> a setCallStack stk do_this = let ?callStack = stk in do_this Rebinding ?callStack works just fine, but the occurrence of do_this causes GHC to push an entry onto the stack, which is less than ideal. What does this look like in practice? If we evaluate setCallStack foo (error "die") the resulting stack will be error *do_this* foo The rebinding trick works for withFrozenCallStack precisely because we freeze the CallStack, so the push from do_this is ignored. So, long story short, I'm not convinced of the utility of setCallStack. I think perhaps we should not provide it, and just do the rebinding trick inside withFrozenCallStack (which was the only use-case for setCallStack to begin with).