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

[Andreas Abel]

On 2020-05-12 02:51, Andrew Lelechenko wrote:
> 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.”

Here, it should relativize to:

    ... provided the hint is correct, i.e., is the correct length of the 
list.

If this half-sentence is added, then the description seems to say what 
you say in alternative 2.

> 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
> 
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