How dynamically assign nested mutable value?

[jweckman]

Question

The latest version supports creating cell paths using variables and mutating using them with eg:

$env.config.max_history_size = 20000

I think a common real-world use-case is to dynamically mutate certain nested keys inside a JSON/YAML if they exist. I got very close to succeeding but seem to fail right before the finish line:

def add_build_args_to_compose [
    path,
    args
] {
    let compose = open $path
    mut res = $compose
    let build_services = do {
        $compose.services
            | transpose service vals
            | each {|x| if $x.vals.build? != null { $x.service } }
    }
    for bs in $build_services {
        mut to_get = [services $bs build] | into cell-path
        $res | get $to_get | $in.args = $args # "$in needs to be a mutable variable"
        # $res | get $to_get | upsert args $args # does not seems to mutate
    }
    $res

I'm trying to fix a design flaw in docker-compose where they don't allow reading ARG values from a file. I want to mutate only the services that have a build key such that they get the passed args and then write to a new docker-compose.yml file.

Trying to to mutate $in fails because its not mutable (...what?)
and upserting/updating does not seem to mutate $res at all.

Is there something i'm missing? I would ideally like to do:

$res.services.($bs).build.args = $args

OR

$res.($to_get).args = $args

Is there some syntax i'm missing or is it impossible to mutate using variables inside a cell-path?

Alternatively, the update command could be extended to be used like what Clojure seemingly has:
https://clojuredocs.org/clojure.core/update-in

I personally have this need at work fairly often where i get some JSON/YAML and it needs to be dynamically mutated somewhere deep inside. Immutability is great and all but in this case it seems way too convoluted. If it stays like this i will have to make a python script and call that to mutate JSON/YAML and i really don't want to do that.

Additional context and details

No response

[devyn]

I think the only way to do it right now is to use update nested, but you could always write a custom command that recurses through a path you provide as an argument.

[jweckman]

I'm basically a beginner with functional so it took me quite a while to get that this was what i was supposed to do (can probably still be made a bit more elegant but works for me):

def add_build_args_to_compose [
    compose_file,
    args
] {
    let compose = open $compose_file
    let build_services = do {
        $compose.services
            | transpose service vals
            | each {|x| if $x.vals.build? != null { $x.service } }
    }
    $build_services
        | reduce --fold $compose {|it, acc| $acc | insert ([services $it build args] | into cell-path) $args }

That said, i think both of these should be supported by the language before 1.0:

$data.($some_cell_path) = 'new_value' # mutable way, not supported

$data | update $some_cell_path 'new_value' # immutable way, looks like it works now!

There are at least three other issues directly connected to this. The core team could maybe come up with a plan for merging/closing some of them:
#5966
#4834
#9082

[WindSoilder]

Closing the issue because the question is answered.