handle missing space in usage block

docopt.go

@@ -116,7 +116,13 @@ func parse(doc string, argv []string, help bool, version string, optionsFirst bo
 	usage := usageSections[0]
 
 	options := parseDefaults(doc)
-	pat, err := parsePattern(formalUsage(usage), &options)
+	formal, err := formalUsage(usage)
+	if err != nil {
+		output = handleError(err, usage)
+		return
+	}
+
+	pat, err := parsePattern(formal, &options)
 	if err != nil {
 		output = handleError(err, usage)
 		return
@@ -542,10 +548,14 @@ func tokenListFromPattern(source string) *tokenList {
 	return newTokenList(result, errorLanguage)
 }
 
-func formalUsage(section string) string {
+func formalUsage(section string) (string, error) {
 	_, _, section = stringPartition(section, ":") // drop "usage:"
 	pu := strings.Fields(section)
 
+	if len(pu) == 0 {
+		return "", newLanguageError("no fields found in usage (perhaps a spacing error).")
+	}
+
 	result := "( "
 	for _, s := range pu[1:] {
 		if s == pu[0] {
@@ -556,7 +566,7 @@ func formalUsage(section string) string {
 	}
 	result += ")"
 
-	return result
+	return result, nil
 }
 
 func extras(help bool, version string, options patternList, doc string) string {

docopt_test.go

@@ -151,9 +151,13 @@ func TestFormalUsage(t *testing.T) {
     prog is a program`
 	usage := parseSection("usage:", doc)[0]
 	if usage != "Usage: prog [-hv] ARG\n           prog N M" {
-		t.Error()
+		t.Fatal()
+	}
+	formal, err := formalUsage(usage)
+	if err != nil {
+		t.Fatal(err)
 	}
-	if formalUsage(usage) != "( [-hv] ARG ) | ( N M )" {
+	if formal != "( [-hv] ARG ) | ( N M )" {
 		t.Error()
 	}
 	return