[Haskell-cafe] Use of errorWithoutStackTrace in the Prelude

Alexis King lexi.lambda at gmail.com
Wed Dec 7 08:00:49 UTC 2016

> On Dec 6, 2016, at 21:04, Eric Seidel <eric at seidel.io> wrote:
> The performance impact is part of the rationale, but a bigger issue is
> the burden on the type signatures. 
> Using the new `error` would not be enough to get useful locations for
> `head` and friends. The stack trace would only contain the call to error
> *inside* head, and not the call to head itself, which is pure noise. If
> we wanted to add the call to head, we would have to add a `HasCallStack`
> constraint to `head` itself (and all other partial functions).

Right, of course; my assumption was that it would be necessary to both
add HasCallStack and use `error`, though I figured you would also want
to use withFrozenCallStack to prevent the call stack from `error` from
bubbling through (since that really is just noise; nobody needs to know
the location of `error` inside of base). Upon further inspection,
though, I realized errorWithoutStackTrace effectively includes
withFrozenCallStack (rather than taking an alternate path that ignores
the call stack entirely), so adding the HasCallStack constraint seems
like it should be sufficient on its own.

> We discussed this back when I first implemented the implicit
> call-stacks, and decided to be conservative with their use in base for
> these reasons. Though it is worth noting that at this point we were
> still using an explicit implicit parameter instead of the `HasCallStack`
> alias, which made the constraint more unfamiliar especially to
> newcomers. Perhaps with the new alias the API-cluttering concerns are
> less of a problem.

Yeah, I could see the potential for confusion here, but IMO the
potential gains from offering source locations in error messages by
default is a bigger win for beginners than the slightly more complicated
type signatures are a loss. That’s obviously just my opinion though,
since I have no evidence whatsoever to back that up, but it’s definitely
something I’d like to see.

> In the meantime, I have a package
>  http://hackage.haskell.org/package/located-base
> that provides callstack-aware variants of many partial functions that I
> use, and would happily accept a PR for any others that you use!

This is a nice package; thank you for writing it! I’ll definitely keep
it in mind, though admittedly, it’s pretty unlikely I’d find it worth it
to import just to get the call stacks for a handful of functions I try
to avoid whenever possible, anyway (which is why I think these need to
be in base to be all that useful — my guess is that beginners are by far
the most likely to use partial functions in situations that may throw,
and they are unlikely to discover your package). That said, it’s good to
have, and I’ll relay its existence to my coworker.


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