Add completions for fish shell (#363)

As per title, adds shell completions for [fish
shell](https://github.com/fish-shell/fish-shell)

Closes #259

Author: kilyanni

README.md

@@ -37,7 +37,7 @@
   - [Default Values](#default-values)
   - [Place-holders in Help](#place-holders-in-help)
   - [Consuming all remaining arguments](#consuming-all-remaining-arguments)
-  - [Bash/ZSH Shell Completion](#bashzsh-shell-completion)
+  - [Bash/ZSH/Fish Shell Completion](#bashzshfish-shell-completion)
   - [Supporting -h for help](#supporting--h-for-help)
   - [Custom help](#custom-help)
 
@@ -531,7 +531,7 @@ And use it like so:
 ips := IPList(kingpin.Arg("ips", "IP addresses to ping."))
 ```
 
-### Bash/ZSH Shell Completion
+### Bash/ZSH/Fish Shell Completion
 
 By default, all flags and commands/subcommands generate completions
 internally.
@@ -548,8 +548,9 @@ for your target platform/shell). An alternative is to instruct your end
 user to source a script from their `bash_profile` (or equivalent).
 
 Fortunately Kingpin makes it easy to generate or source a script for use
-with end users shells. `./yourtool --completion-script-bash` and
-`./yourtool --completion-script-zsh` will generate these scripts for you.
+with end users shells. `./yourtool --completion-script-bash`,
+`./yourtool --completion-script-zsh`, and `./yourtool --completion-script-fish`
+will generate these scripts for you.
 
 **Installation by Package**
 
@@ -574,6 +575,18 @@ Or for ZSH
 eval "$(your-cli-tool --completion-script-zsh)"
 ```
 
+Or for fish
+
+```
+your-cli-tool --completion-script-fish | source
+```
+
+Or, to install the fish completions permanently:
+
+```
+your-cli-tool --completion-script-fish > ~/.config/fish/completions/your-cli-tool.fish
+```
+
 #### Additional API
 To provide more flexibility, a completion option API has been
 exposed for flags to allow user defined completion options, to extend

app.go

@@ -68,6 +68,7 @@ func New(name, help string) *Application {
 	a.Flag("completion-bash", "Output possible completions for the given args.").Hidden().BoolVar(&a.completion)
 	a.Flag("completion-script-bash", "Generate completion script for bash.").Hidden().PreAction(a.generateBashCompletionScript).Bool()
 	a.Flag("completion-script-zsh", "Generate completion script for ZSH.").Hidden().PreAction(a.generateZSHCompletionScript).Bool()
+	a.Flag("completion-script-fish", "Generate completion script for fish.").Hidden().PreAction(a.generateFishCompletionScript).Bool()
 
 	return a
 }
@@ -108,6 +109,15 @@ func (a *Application) generateZSHCompletionScript(c *ParseContext) error {
 	return nil
 }
 
+func (a *Application) generateFishCompletionScript(c *ParseContext) error {
+	a.Writer(os.Stdout)
+	if err := a.UsageForContextWithTemplate(c, 2, FishCompletionTemplate); err != nil {
+		return err
+	}
+	a.terminate(0)
+	return nil
+}
+
 // DefaultEnvars configures all flags (that do not already have an associated
 // envar) to use a default environment variable in the form "<app>_<flag>".
 //

model.go

@@ -16,6 +16,7 @@ var (
 		"completion-bash":        true,
 		"completion-script-bash": true,
 		"completion-script-zsh":  true,
+		"completion-script-fish": true,
 	}
 )
 

templates.go

@@ -260,3 +260,15 @@ if [[ "$(basename -- ${(%):-%x})" != "_{{.App.Name}}" ]]; then
     compdef _{{.App.Name}} {{.App.Name}}
 fi
 `
+
+var FishCompletionTemplate = `complete -c {{.App.Name}} -f -a '(
+    set -l c (commandline -opc)
+    set -e c[1]
+    set -l r ({{.App.Name}} --completion-bash $c)
+    if test -n "$r"
+        echo $r
+    else
+        __fish_complete_path (commandline -ct)
+    end
+)'
+`