RFC: Add HasCallStack constraint to partial Data.List functions.

Tom Ellis tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk
Tue Jun 1 19:09:44 UTC 2021

On Tue, Jun 01, 2021 at 08:50:43PM +0200, Andreas Abel wrote:
> On 6/1/21 2:57 AM, Henrik Nilsson wrote:
> >> But every now and
> >> then the reason, say, a list is non-empty is just a little bit too
> >> subtle to capture in a reasonable way ..., and the use of functions
> >> like foldl1 is entirely appropriate, along with a comment in the code
> >> explaining why it is safe.
> A bit off topic (my apologies for the hijacking), but if you my fellow
> Haskellers think you understand when the use of such a partial function is
> safe, I challenge you to look at:
>   https://gitlab.haskell.org/ghc/ghc/-/issues/19917
> I thought that e.g. `last (x : xs)` can never throw the error `Prelude.last:
> empty list` but I had to learn otherwise...
> (So my conviction is that `List.head`, `List.last` and the like should be
> eradicated entirely and any step that brings us closer to this goal gets my
> approval.)

Wow, this is *very* interesting.  So when someone grumbles

> I got the very unhelpful error message "Prelude.last: empty list"
> and I can't even see how an empty list can have entered my program

they may be completely correct: the error can be printed when there is
no empty list at all!


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