Add YAML linter.

cmd/yamll/README

@@ -0,0 +1,3 @@
+yamll: yaml linter
+
+Run this over YAML files to determine if they are well-formed or not.

cmd/yamll/main.go

@@ -0,0 +1,63 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"os"
+
+	"gopkg.in/yaml.v2"
+)
+
+type empty struct{}
+
+func errorf(format string, args ...interface{}) {
+	format += "\n"
+	fmt.Fprintf(os.Stderr, format, args...)
+}
+
+func usage(w io.Writer) {
+	fmt.Fprintf(w, `Usage: yamll [-hq] files...
+
+	For each file, yamll will make sure it is a well-formatted YAML
+	file.  Unless the -q option is passed, yamll will print the names
+	of each file and whether it was well-formed. With the -q option,
+	only malformed files are printed.
+`)
+}
+
+func init() {
+	flag.Usage = func() { usage(os.Stderr); os.Exit(1) }
+}
+
+func main() {
+	help := flag.Bool("h", false, "Print program usage.")
+	quiet := flag.Bool("q", false,
+		"Quiet mode - don't note well-formed files, only malformed ones.")
+	flag.Parse()
+
+	if *help {
+		usage(os.Stdout)
+		os.Exit(0)
+	}
+
+	for _, path := range flag.Args() {
+		in, err := ioutil.ReadFile(path)
+		if err != nil {
+			errorf("%s FAILED: %s", path, err)
+			continue
+		}
+
+		var e empty
+		err = yaml.Unmarshal(in, &e)
+		if err != nil {
+			errorf("%s FAILED: %s", path, err)
+			continue
+		}
+
+		if !*quiet {
+			fmt.Printf("%s: OK\n", path)
+		}
+	}
+}