Proposal: Improve error messages for (!!) (include index and length of list)

[Thomas Schilling]

Yes, you'd have to calculate the length on the fly. i.e., something
like this (untested):

xs !! n | n < 0 = error "... negative index ..."
xs !! (I# n) = go xs n 0#
  where
    go [] idx len = error $ "... Index " ++ show (I# (idx +# len) ++ "
too large for list of length "
                                  ++ show (I# len)
    go (x:_) 0# _ = x
    go (_:xs) idx len = go xs (idx -# 0#) (len +# 1#)

On modern processors the extra addition and the extra parameter
shouldn't hurt, though we'd need a benchmark to make sure, of course.
You could also make the error message a bit less helpful and just
return how far the index pointed past the end of the list.


On 16 October 2014 13:46, Herbert Valerio Riedel <hvr at gnu.org> wrote:
> On 2014-10-16 at 08:20:55 +0200, Simon Hengel wrote:
>
> [...]
>
>> I propose to change the error messages for the non-report version to
>> include index and list length, something that is functionally equivalent
>> to:
>
> While I'm very sympathetic to better error messages; doesn't the
> implementation you gave defer garbage-collecting the start of the list,
> by keeping the head of the list alive until either the desired index has
> been reached or end-of-list is detected?
>
> e.g. consider something (silly) like ([1..] !! 10000000)
>
> Cheers,
>   hvr
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