Add support for Conditionals and Comparisons 'if-then-else'

[kenjones-cisco]

Add support for Conditionals and Comparisons 'if-then-else'

if A then B else C end will act the same as B if A produces a value other than false or null, but act the same as C otherwise.

Checking for false or null is a simpler notion of “truthiness” than is found in Javascript or Python, but it means that you’ll sometimes have to be more explicit about the condition you want: you can’t test whether, e.g. a string is empty using if .name then A else B end, you’ll need something more like if (.name | length) > 0 then A else B end instead.

If the condition A produces multiple results, then B is evaluated once for each result that is not false or null, and C is evaluated once for each false or null.

More cases can be added to an if using elif A then B syntax.

[rLitto]

Has it been implemented?

[CyberShadow]

Has it been implemented?

No:

yq e --null-input 'if true then 2 else 3 end'
Error: Parsing expression: Lexer error: could not match text starting at 1:1 failing at 1:3.
	unmatched text: "if"

@mikefarah I think you closed this by accident.

[76creates]

really odd that we don't have conditionals, i see all related issues are closed without any comments
#94
#96
#97

@mikefarah can you please comment on these? is it going to be implemented, if not can we get any rationale?

[mikefarah]

Oops - sorry I closed a bunch of issues back then that didn't seem to have anyone interested in them (then didn't see the comments). Re opening.

[sorvis]

There is a work around detailed here in the docs:

https://mikefarah.gitbook.io/yq/usage/tips-and-tricks#logic-without-if-elif-else

[sarimarton]

There is a work around detailed here in the docs:

https://mikefarah.gitbook.io/yq/usage/tips-and-tricks#logic-without-if-elif-else

How can I use it as an expression in a .map?

[planeiii-te]

It sure would be nice to have more than one demo. Maybe something more complex perhaps.

[robross0606]

The workaround does leave a lot to be desired. For example, I have been unable to figure out how to adapt that example to changing a single YAML string based on the value of an environment variable. Or any other (seemingly simpler) conditional related to a single string. Or to straight-up omit() a key if another value is set to a specific known value.

As far as I can tell, the second you select() something, it is no longer available to re-iterate for the "else" scenario. The only reason the workaround works in the example is because it is iterating once per value in the map. But what if you need to have an "else" for a single value?

I've tried using eval() with select() but haven't been able to get that to work either.

[robross0606]

Probably the biggest thing not clearly documented is, what happens with select() when nothing matches? For example, I've tried this:

 myvar=foo yq -n 'select(strenv(myvar) == "fred") | "hello"'

This prints "hello". But why if myvar clearly doesn't equal "fred"?

[git-developer]

Looks like the result of the select step (some kind of "missing context") is given to the "hello" step; the literal "hello" doesn't depend on this context, so it is not skipped but evaluated.

This workaround references the context . so that the "if-case" is skipped on missing context:

$ myvar=fred yq -n '(select(strenv(myvar) == "fred") | . = "is-fred") // "is-not-fred"'
is-fred

$ myvar=foo yq -n '(select(strenv(myvar) == "fred") | . = "is-fred") // "is-not-fred"'
is-not-fred