emacs2nix --help is unclear

[DamienCassou]

$ ./melpa-stable-packages.sh 
Preprocessing library emacs2nix-0.1.0.0...
In-place registering emacs2nix-0.1.0.0...
Preprocessing executable 'melpa2nix' for emacs2nix-0.1.0.0...
Running melpa2nix...
Missing: --melpa DIR (-o|--output FILE)

Usage: melpa2nix [-t|--threads N] --melpa DIR [--stable] --work DIR
                 (-o|--output FILE) [--index-only] [PACKAGE]
  Generate Nix expressions from MELPA recipes

• what do all these options mean?
• when I pass --stable, I get Invalid option --stable
• same for --work

[ttuegel]

You should be able to get the full help text by running cabal run melpa2nix -- --help. It looks like optparse-applicative only displays a short usage message when the wrong options are given, instead of showing the full help output.