add conditionals to module resource

[BrianHicks]

module resource should have conditional evaluation. Something like this:

module "x.hcl" "x" {
  when = [
    "eq {{system.os}} \"darwin\""
  ]
}

If the whens combine (with and) to false, the subtree rooted at the module is dropped from evaluation.

[BrianHicks]

This still makes sense a couple of days later… mostly. The string syntax for the conditionals is too different from the rest of our configuration. A better syntax could look llike:

module "x.hcl" "x" {
  when equal ["{{system.os}}", "darwin"]
}

To generalize: a when block will have a string for the operator, then the operator's arguments. These arguments could take pretty much any shape, depending on the operator.

@sehqlr this syntax might be useful for your param validation work as well?

[sehqlr]

Yes, this would be useful for parameters. I like the basic idea, but I
think the syntax will be tricky.

On Sep 21, 2016 8:36 AM, "Steven Borrelli" [email protected] wrote:

when "{{ system.os == "darwin" }}" ?

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[BrianHicks]

Right, whiteboard discussion in the office over. We're going to go forward with something like a switch construct that will be expanded like a macro (that is, it will not be evaluated at runtime, but before and pruned.)

select { // or `switch` or `select!` or `switch!`... You get the idea.
  case {
    conditional = "some syntax goes here"
    value = {
      module "x.hcl" "x" {}
    }
  }
}

If the conditional syntax ends up being within a string, we can move that to the path selector of case and get rid of value.

Preferred syntax among the group was something like "platform.os == 'darwin'" (Or so I remember; @stevendborrelli and @rebeccaskinner can you confirm)? At any rate, we want to not use all of text/template here because are "bob" or similar strings true or false?

[rebeccaskinner]

PR #362