How do I conditionally declare variables in go templates?

Question

I am working on helmfile and stuck at the point where I want the conditional variable declaration. Roughly like this but in go standard templates.

ENV = "prod"
folder = ""
if ENV == "prod":
    folder = "production"
elif ENV == "qa"
    folder == "qa"
else
    folder = "dev"

How can I achieve something like that? Thanks

Answer 1

If you do have the Sprig functions available to you (in both standard Helm and in Helmfile) then you can create a dict, then look up a value in it (probably using the standard index function) with a default.

{{ $ENV := ... }}
{{ $folders := dict "prod" "production" "qa" "qa" }}
{{ $folder := index $folders $ENV | default "dev" }}

Helmfile also has its own version of get that includes defaulting functionality; so in Helmfile specifically (but not in core Go text/template or the Sprig extensions, so not standard Helm) you can say

{{/* get "key" "default" (dict ...) */}}
{{ $folder := get $ENV "dev" $folders }}

Both of these in Python would look like:

folders = { "prod": "production", "qa": "qa" }
folder = folders.get(ENV, "dev")

which should be equivalent to your logic.

Answer 2

Use the {{if}} action and the eq template function to compare the ENV variable to certain values:

{{$ENV := "prod"}}
{{$folder := ""}}

{{if eq $ENV "prod"}}
    {{$folder = "production"}}
{{else if eq $ENV "qa"}}
    {{$folder = "qa"}}
{{else}}
    {{$folder = "dev"}}
{{end}}

Note that this is not "conditional declaration". We declare $folder "unconditionally" and used {{if}} to change its value.

Here's a runnable demo to test the above template:

func main() {
    t := template.Must(template.New("").Parse(src))

    for _, env := range []string{"prod", "qa", "other"} {
        if err := t.Execute(os.Stdout, env); err != nil {
            panic(err)
        }
    }
}

const src = `{{$ENV := .}}
{{- $folder := "" -}}

{{- if eq $ENV "prod"}}
    {{$folder = "production"}}
{{else if eq $ENV "qa"}}
    {{- $folder = "qa" -}}
{{else}}
    {{- $folder = "dev" -}}
{{end}}
$ENV = {{$ENV}}; $folder = {{$folder}}`

Output (try it on the Go Playground):

$ENV = prod; $folder = production
$ENV = qa; $folder = qa
$ENV = other; $folder = dev