Proposal: add HasCallStack for all partial functions in base

Edward Kmett ekmett at
Mon Jun 17 17:09:15 UTC 2019

Re: your code. This is passing the callStack to each instance and dropping it on the floor for the cases where you ignore the constraint.

I’m starting to warm to the idea of putting HasCallStack constraints on the obviously partial combinators in base if we can demonstrate that the performance impact isn't bad in practice, and even really, to some extent if there is a somewhat middling impact on the performance of code that leans on these hard to debug combinators, so long as the performance of their more total siblings remains unaffected. The impact on the perceived debuggability of Haskell seems _likely_ to significantly outweigh the performance concerns. 

Heck, off of the HEAD of cabal, which we’re encouraging folks to build to play with the ghc 8.8.1 alpha, just today we ran into an issue where the very build system we are using spat out an oh so informative “Prelude.foldr1: Empty list” when using a pkgconfig-depends stanza that didnt include any explicit bounds.

It seems worth implementing and measuring quite how bad the impact would be before pulling the trigger for good. The impact should be reasonable if the constraints only really live right at the outside of base though, then users can choose to propagate the constraint further outwards, just like they get to do with “error” today.

Slightly more controversially, MonadFail’s fail could be similarly modified with near zero user facing impact other than a bit of additional static info flowing to explicit callstacks to make it take a HasCallStack constraint. The code in existing instances would all still work. This one I think would need more benchmarking though, as recovering from a fail is a far “lighter” affair than catching an exception, but I’d be happy to raise it to the CLC as a question, especially if we can get benchmarks in hand.


> On Jun 17, 2019, at 4:06 PM, LuoChen <luochen1990 at> wrote:
> I have just confirmed that we can specify HasCallStack separately for different instance of same typeclass, and here is the test code.
> Oliver Charles <ollie at> 于2019年6月17日周一 上午3:02写道:
>> Surely then the call stack is only foldr1, and not foldr1's callsite?
>>> On Sun, 16 Jun 2019, 8:27 pm Henning Thielemann, <lemming at> wrote:
>>> On Sun, 16 Jun 2019, Simon Jakobi wrote:
>>> > >  I think only the partial implementations should get that constraint.
>>> > Oops, I had the misconception that one couldn't add a HasCallStack 
>>> > constraint to method implementations.
>>> I think you have to declare:
>>> instance Foldable [] where
>>>     foldl1 = foldl1List
>>> foldl1List :: HasCallStack => (a->a->a) -> [a] -> a
>>> foldl1List = ...
>>> Would this work?_______________________________________________
>>> Libraries mailing list
>>> Libraries at
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