ternary-style expansion modifier

[rsgalloway]

would be great if we could do inline conditionals as part of the expansion modifiers:

ENV: ${ENV:=prod}
SUBFORK_DOMAIN: ${ENV==dev:?myapp-dev.fork.io:myapp.fork.io}

so that when resolving SUBFORK_DOMAIN, we get:

$ envstack -r SUBFORK_DOMAIN
SUBFORK_DOMAIN=myapp.fork.io
$ ENV=dev envstack -r SUBFORK_DOMAIN
SUBFORK_DOMAIN=myapp-dev.fork.io

[rsgalloway]

rough pseudo-code:

if match := re.match(r"\$\{(.+?)\s*([=!]=)\s*(.+?)\s*\?\s*(.+?)\s*:\s*(.+?)\}", expr):
    left, op, right, true_val, false_val = match.groups()
    result = true_val if compare(left, op, right) else false_val

support chained ternaries or fallback operators later, e.g.:

SUBFORK_DOMAIN: ${ENV == dev ? myapp-dev : ENV == staging ? myapp-stage : myapp}.fork.io