Conditional values in Farmer builders, or composing builders?

[Thorium]

I didn't find documentation about this:
What is the best practice to work with conditional values in Farmer custom builders?

For example, the VM IP ARM (PublicIpId) is an Option type:

 let deployment = arm {
        location Location.UKSouth
        // more resources here
        add_resource myVm
        output "vmIP" myVm.PublicIpId.Value.ArmExpression
    }

The code fails if PublicIpId is None.
When the deployments and resources are compex, I wouldn't want to create multiple copies of the same builders.

I'd like to do something like:

    let deployment = arm {
        location Location.UKSouth
        // more resources here
        add_resource myVm
        match myVm.PublicIpId with
        | Some ip -> output "vmIP" ip.ArmExpression
        | None -> ()
    }
// or
    let deployment = arm {
        location Location.UKSouth
        // more resources here
        add_resource myVm
        if myVm.PublicIpId.IsSome then
            output "vmIP" myVm.PublicIpId.Value.ArmExpression
    }

This control construct may only be used if the computation expression builder defines a 'For' method.

Or could I use composition and still take the advantage of the builder?
Currently I've only find a solution to jump out of the builder and start to build manually the state, losing the convenience of builder with proper type-conversions in overloaded methods:

    let composition =
        match myVm.PublicIpId with
        | None -> deployment
        | Some ip -> { deployment with Outputs = deployment.Outputs.Add("vmIP", ip.ArmExpression.Eval()) }

[Thorium]

    let deployment = arm {
        location Location.UKSouth
        add_resource myVm
        output "vmIP" (myVm.PublicIpId |> Option.map(fun ip -> ip.ArmExpression))
    }

Now, only thing more to add, what if there are a list of VMs...

[davidglassborow]

I've got exactly the same issue, trying to work out how to configure different instances (staging/acc/prod) from the same set of builders

[dburriss]

@isaacabraham I have run into this a few times and the workarounds are decidedly inelegant.

I attached a PR to this with a proposal.

I initially checked to see if I could get if working in the webapp CE but it has been a few years but I am not even sure it is possible with a non-sequence data structure? The benefit of this would be that it could be added to each CE and unblock this issue without requiring a change to each "flag" operator.

Anyway, my proposal is far simpler... we put an optional argument in each flag method. This keeps backward compatibility, is easy to implement, and allows for more sophisticated logic but keeps the resource definition straightforward.

This is a big blocker for me for something I am currently working on so I am happy to put in some work to enable this. Let me know what you think.

[dburriss]

Sorry I really did not read this issue well :( I see this is specifically for outputs, or maybe deployments. Mine is related to the actual CE resource builders. I can open a separate issue?

[mattgallagher92]

Thanks @dburriss! My reading of this issue is that it's about the general difficulty in using something that varies according to some condition within Farmer computation expressions. I believe that @Thorium was just using the arm CE as an example. As such, I think that your contribution belongs in this issue and I don't think that there's a need to open a new one 🙂

Let's discuss the actual proposal on the PR.

[mattgallagher92]

As an aside, the approach that I have used in the past is the one mentioned in the description - using copy and update expressions on the underlying record.

[mattgallagher92]

Note: @dburriss's suggestion works for on/off values, but doesn't cover the general case where we want to pass in data of different types in different cases. For example, in the description, we either pass in an ARM expression or nothing. More thought is needed for cases beyond on/off.

[ninjarobot]

I'm curious what Copilot can come up with here...