[darcs-devel] patch list concatenation

[Ganesh Sittampalam]

On 24/05/2021 07:36, Ben Franksen wrote:

>>> * use +<<+ instead of  +<+ reverseFL
>>
>> Fine though I'm not actually sure this is an improvement, it doesn't 
>> have a complexity advantage and is another operator to remember.
> 
> It is true that it does not have better asymptotic complexity but it
> fuses the two traversals.

Right. My feeling was that it doesn't really matter. If this was the
only consideration, we could also use a RULES pragma to rewrite it
inside the module.

> Regarding the cognitive burden: We have four concatenation operators:
> +<+ +>+ +<<+ +>>+. They all have the same meaning, but differ in the
> argument and result types. When reading code, it doesn't matter which
> one is used: you can mentally replace them all with "concatenate these
> two lists" without spending any thought on the concrete types.

I suspect my initial reaction was because I didn't remember what it did,
as it's used quite rarely. I might remember next time :-)

Ganesh