lazy evaluation in ternary function?

[zswanson]

I wasn't able to determine from the documentation, but is there is way to setup a ternary function such that if the initial test condition is true, the false branch isn't actually evaluated unless that conditional executes?

I'm using gomplate to render config templates inside a docker container, where the ternary condition first checks to see if an env var was populated and if not it does a lookup to AWS ParameterStore datasource. I'm observing that even if I provide all the values over env var, the ternary is checking parameter store anyways. In some cases such as automated testing I don't want it to touch parameter store at all.

{{- $env := ( .Env.ENV ) }}
{{ defineDatasource "smp" (strings.ReplaceAll "ENV" $env "aws+smp:///ENV/myapp") }}
{{- $env_http_port := (env.Getenv "HTTP_LISTEN_PORT") }}
{{- $http_port := (ternary (ds "smp" "/http-listen-port").Value $env_http_port (eq $env_http_port "")) }}

[hairyhenderson]

The short answer to this is "no, not with ternary" 😉

Ultimately the problem is that in Go templates, all arguments are evaluated. In Go you could do something like if cond1 || cond2, and cond2 would only be evaluated if cond1 first evaluated to false. But in the text/template language, all arguments to a function are evaluated first before calling the function.

So in the case of ternary, or even the built-in or function, all args must first be evaluated to booleans before the function itself is called.

The general workaround for this is to just use if/else blocks. In your case I think the following would work:

{{- $http_port := "" }}
{{- if eq $env_http_port "" }}
  {{- $http_port = (ds "smp" "/http-listen-port").Value }}
{{- else }}
  {{- $http_port = $env_http_port }}
{{- end }}

It's far more verbose, though. An alternate approach is to just assign to $env_http_port first and then you get:

{{- $http_port := $env_http_port }}
{{- if eq $http_port "" }}
  {{- $http_port = (ds "smp" "/http-listen-port").Value }}
{{- end }}

You didn't ask about it, but the defineDatasource line could be a bit simpler if you just use env.ExpandEnv:

{{ defineDatasource "smp" (env.ExpandEnv "aws+smp:///${ENV}/myapp") }}

So finally, here's a simpler version of your template that might be useful:

{{ defineDatasource "smp" (env.ExpandEnv "aws+smp:///${ENV}/myapp") }}
{{- $http_port := env.Getenv "HTTP_LISTEN_PORT" }}
{{- if eq $http_port "" }}
  {{- $http_port = (ds "smp" "/http-listen-port").Value }}
{{- end }}

Hope that helps!

[zswanson]

Super helpful, thanks very much!