Brainstorming Custom Functions

[springcomp]

This discussion is a blank canvas to brainstorm whether custom functions – as found in existing implementations – are worthy of consideration for inclusion into the set of builtin functions. This post will be edited along my though process.

I will focus on Custom Filters as implemented by Kyverno – one of the most complete set of custom functions I’m aware of.

Arithmetic operations

Function	Description	Alternative	Comment
add	Arithmetic addition	+ syntax	See Units¹
subtract	Arithmetic subtraction	− syntax	See Units¹
multiply	Arithmetic multiplication	× syntax	See Units¹
divide	Arithmetic division	÷ syntax	See Units¹
modulo	Arithmetic modulo	% syntax	See Units¹
…

These are currently implemented as arithmetic-expressions in JMESPath Community.
¹ Unis: an interesting use case is that all Kyverno’s arithmetic functions are unit-aware and perform the necessary conversions.

Object manipulation functions

Function	Description	Alternative	Comment
items	Extract object properties and values	items()	See Extended usage¹
object_from_list	Create object from list of keys and values	from_items()
…

¹ Extended usage: Kyverno’s items function behaves like JMESPath Community's. However, it also supports assigning arbitrary key names to the intermediate array, thus further transforming the initial input. It is a powerful filter that might only be replicated in JMESPath Community with a complex expression – if even possible.

String manipulation functions

Function	Description	Alternative	Comment
compare	String comparison	See limitations¹
equal_fold	Cases insensitive string comparison	lower(a) == lower(b)
replace	Replace in string	replace()
replace_all	Replace in string	replace()
split	Split string	split()
to_lower	Convert to lower case	lower()
to_upper	Convert to upper case	upper()
trim	Removes leading or trailing substrings	trim()
truncate	Truncate	[0:n] string slice syntax

¹ Limitations: JMESPath currently supports comparisons with numbers only. A common request is to extend comparison to other types.

Scratch pad

Function	Description	Alternative	Comment
…

¹ Note
² Note

TODO: study Gorilla Stack fork as well.

History

2023-03-04	Initial draft
2023-03-29	String functions