[Outreachy] Use of impure functional programming and use of vlists

[Magali Lemes <magalilemes00@gmail.com>]

Hello, Guix.

As my Outreachy internship approaches an end, I'd like to know which of 
the following options is better for walking and displaying the Git 
commit history:
1) having a list and then using for-each it to display the commit 
information;
2) display the list while building it.

When it comes to option number 2, the main advantage is that it's faster 
to perform operations such as 'guix git log --oneline | head -n5'. The 
downside is that it's not a pure functional programming approach.
Another question is, could vlists be used? Since it would be fast to 
have the commits in a hash table implemented with vlists, we could use 
'vlist-for-each' to display the commits. I haven't seen vlist-for-each 
being used anywhere in Guix, so I wondered if there's a special reason 
for it not being used, or if it hasn't really been necessary at all thus 
far.

Regards,
Magali