What do you assume when you see fromListN in a library?

[Andreas Abel]

A question would be how to validate the given length without performance 
penalty.  The existence of fromListN in addition to fromList is **only** 
justified by performance consideration, namely that the length of the 
list is known and does not have to be computed.

In general, it seems that the given length has to be trusted, however, 
there might be implementations that can validate the length parameter 
as-you-go without additional cost (in the good case that the length is 
correctly given).

On 2020-05-15 21:37, Carter Schonwald wrote:
> validating would *prevent inconsistent data*.
> 
> it is precisely the issue that current semantics are *not* consistent 
> across that needs to be addressed!
> 
> On Thu, May 14, 2020 at 6:38 PM Joseph C. Sible <josephcsible at gmail.com 
> <mailto:josephcsible at gmail.com>> wrote:
> 
>     On Thu, May 14, 2020 at 10:41 AM Carter Schonwald
>     <carter.schonwald at gmail.com <mailto:carter.schonwald at gmail.com>> wrote:
>      > My inclination is we change the semantics of fromListN to be
>     strictly validating with an error when the length is wrong. This is
>     the most consistent and humane of options.
> 
>     I disagree that validating would be consistent. Look how common the
>     phrases "the precondition is not checked" and "violation of this
>     condition is not detected" are in the containers library and so many
>     others on Hackage.
> 
>     Joseph C. Sible
> 
> 
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