The problem is that some implementations will silently truncate data, and while the original reason for the interface was desugaring list notation to a different data structure, it’s now used for other things. 

Frankly, the main performance win was meant to be with respect to single round of data structure allocation/ associated fragmentation.  For a statically known list.  

On Sun, May 17, 2020 at 7:45 AM Andreas Abel <andreas.abel@ifi.lmu.de> wrote:
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@gmail.com
> <mailto:josephcsible@gmail.com>> wrote:
>
>     On Thu, May 14, 2020 at 10:41 AM Carter Schonwald
>     <carter.schonwald@gmail.com <mailto:carter.schonwald@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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