Implement sub-command completion for fish shell

completions/pyenv.fish

@@ -1,5 +1,22 @@
-function __fish_pyenv
-  pyenv commands
+function __fish_pyenv_needs_command
+  set cmd (commandline -opc)
+  if [ (count $cmd) -eq 1 -a $cmd[1] = 'pyenv' ]
+    return 0
+  end
+  return 1
 end
 
-complete -f -c pyenv -a '(__fish_pyenv)'
+function __fish_pyenv_using_command
+  set cmd (commandline -opc)
+  if [ (count $cmd) -gt 1 ]
+    if [ $argv[1] = $cmd[2] ]
+      return 0
+    end
+  end
+  return 1
+end
+
+complete -f -c pyenv -n '__fish_pyenv_needs_command' -a '(pyenv commands)'
+for cmd in (pyenv commands)
+  complete -f -c pyenv -n "__fish_pyenv_using_command $cmd" -a "(pyenv completions $cmd)"
+end