From the top of my head, I would expect that fromListN rejects both shorter and longer lists. If a user is under a wrong impression that some list has length N, it is better to fail early, before other, more dangerous implications of this impression are made (for example, using `unsafeIndex`).

However, my expectations disagree with the documentation, which states that the observable behaviour of fromListN should be equivalent to fromList: http://hackage.haskell.org/package/base-4.12.0.0/docs/GHC-Exts.html#v:fromListN I am not sure what was the motivation behind such choice.

Best regards,
Andrew

On 28 Feb 2020, at 02:53, Carter Schonwald <carter.schonwald@gmail.com> wrote:

Hey everyone: 
When you see fromListN as a function in a library, do you assume / presume it’s expecting an exactly N element list ? Or do you expect/tolerate other behavior ? 

Should it reject shorter lists?

Should it truncate or reject longer lists? 

A corner case of this came up in some bug discussion I was having regarding vector,  and I shall claim and or presume that most folks assume exact size with prompt rejection of too long or too short. 

Thoughts please ?

-Carter