Panic with `${env}` in help for positional arguments

[dbohdan]

I would like to use ${env} in the help message of a positional argument. It would make it consistent with my options. When I try to, help in Kong 1.12.1 panics saying "undefined variable".

Here is an demo.

Option with ${env}

module test

go 1.25.0

require github.com/alecthomas/kong v1.12.1

package main

import (
	"github.com/alecthomas/kong"
)

var cli struct {
	Foo string `help:"Foo (${env})" env:"FOO"`
}

func main() {
	ctx := kong.Parse(&cli)
	err := ctx.Run()
	ctx.FatalIfErrorf(err)
}

Let's build and run:

> go build && ./test -h
Usage: test [flags]

Flags:
  -h, --help          Show context-sensitive help.
      --foo=STRING    Foo (FOO)

Argument with ${env}

Let's use arg:"" to turn Foo into a positional argument, leaving everything else unchanged:

package main

import (
	"github.com/alecthomas/kong"
)

var cli struct {
	Foo string `arg:"" help:"Foo (${env})" env:"FOO"`
}

func main() {
	ctx := kong.Parse(&cli)
	err := ctx.Run()
	ctx.FatalIfErrorf(err)
}

The program panics:

> go build && ./test -h
panic: help for <foo>: undefined variable ${env}

goroutine 1 [running]:
github.com/alecthomas/kong.Parse({0x598660?, 0x757830?}, {0x0?, 0xc000110068?, 0x0?})
        /home/dbohdan/go/pkg/mod/github.com/alecthomas/[email protected]/global.go:11 +0xa5
main.main()
        /tmp/demo/main.go:12 +0x28

Argument without ${env}

Now let's remove ${env} from the description:

package main

import (
	"github.com/alecthomas/kong"
)

var cli struct {
	Foo string `arg:"" help:"Foo" env:"FOO"`
}

func main() {
	ctx := kong.Parse(&cli)
	err := ctx.Run()
	ctx.FatalIfErrorf(err)
}

It works again:

> go build && ./test -h
Usage: test <foo>

Arguments:
  <foo>    Foo ($FOO)

Flags:
  -h, --help    Show context-sensitive help.

[alecthomas]

env isn't supported with positional arguments IIRC

[dbohdan]

env isn't supported with positional arguments IIRC

It seems to be. env with positional arguments works. It has tests. Here is an earlier issue about positional arguments, env, and required: #153.