custom 'group by' with functions

[GiancarloAparicio]

Feat:
Have special functions to be able to customize the grouping, similar to dataview functions

Motive:
When you have very nested folders 'group by' is not so nice, especially on mobile

[claremacrae]

@GiancarloAparicio Thanks for the suggestion. That dataview page is very long.

Please could you sketch out an example here to show the kind of thing you have in mind, in the context of Tasks.

[claremacrae]

Or if it's buried in the sceenshot, please paste the text in here of the queries, so that we can experiment with them in our own vaults, without having to re-typed your code. Thanks.

[claremacrae]

Ok I do understand what you are asking for now, but still, having the text would be useful please.

[GiancarloAparicio]

@claremacrae For example useful functions (according to my perspective) would be:

# Numeric operations
round(16.555555) = 7
product([3,3]) = 9
length(list()) = 0

# Operations with strings
regexreplace("Suite 1000", "\d+", "-") = "Suite -"
replace("what", "what", "h") = "hat"
lower("Test") = "test"
upper("Test") = "TEST"
split("hello world", " ") = list("hello", "world")
join(list(1, 2, 3)) = "1, 2, 3"

# utility functions
choice(x > 4, y, z) = y if x > 4, else z
link("note") = return link obsidian
filter([1, 2, 3], (x) => x >= 2) = [2, 3]

This could give users more flexibility to group their tasks, for example:

```tasks
# same to: 'group by filename' but with wikilink
group by split(path,'/')[length(split(path,'/'))]
group by link(split(path,'/')[length(split(path,'/'))-1]) # parent folder 
group by link(split(path,'/')[length(split(path,'/'))-2]) # sub-parent folder 

# same to: 'group by folder' 
group by regexreplace(path,'.*(\/.*?)$','')
group by link(regexreplace(path,'.*(\/.*?)$','')) # Like wikilink
.
.
.
```

My original use case and its queries are these:

TASK FROM "200 - 🌎 Areas/🏛️ Universidad/👨‍💻 Systems engineer" 
SORT file.tasks.text DESC
WHERE !completed
GROUP BY (link(split(file.path,"/")[4])) 

not done
path includes Weeks
(due before in three weeks) OR (no due date)
hide due date
hide scheduled date
sort by due
group by folder 

[claremacrae]

Super. Thank you very much indeed @GiancarloAparicio !

[Robin-des-Bois]

I would love to be able to group by e.g. due.week or week(due)

[claremacrae]

Some time soon-ish I'm planning to refactor the code for sorting and grouping so that it is easier to maintain and modify, and easier to add new sorting types.

At that point, it should become easier to add options after some of the current grouping instructions.

I'm thinking that one way to solve this, for date groups, might be to a custom format string at the end of the instruction - something like:

group by done YYYY

(Grouping by year has just been requested in #1396)

[claremacrae]

I released custom group by function in Tasks 4.0.0.
This is the starting point for the documentation:
https://publish.obsidian.md/tasks/Scripting/Custom+Grouping