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

[Andrew Lelechenko]

I find the existing description in GHC.Exts self-contradictory, or at least of unclear interpretation. It says:

"The fromListN function takes the input list's length as a hint. Its behaviour should be equivalent to fromList.”

This phrase sounds like fromListN should not reject shorter or truncate longer lists, which makes fromListN in vector (and other libraries) incorrect. But then the description goes on:

"The hint can be used to construct the structure l more efficiently compared to fromList. If the given hint does not equal to the input list's length the behaviour of fromListN is not specified” 

Which means that actually fromListN can reject shorter lists, truncate longer lists or launch missiles, and makes fromListN in vector perfectly valid. 

I suggest we fix this ambiguity. There are two ways to do it:

1. We require that the first argument of fromListN is treated only as a hint, and fromListN N is observably equivalent to fromList, even when N does not match the length of the list, removing the last sentence about undefined behaviour. Such interpretation means that we need to fix vector (and other) packages; in many cases there is no way to define such fromListN more efficient than fromList.

2. We require that the first argument of fromListN is not a hint, but must-must-must match the length of the list, otherwise undefined behaviour happens (UB allows to retain fromListN = const fromList as the default definition). Such interpretation matches existing implementations and, I think, matches the intuition of most package authors.

What do you think?

Best regards,
Andrew

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