Adding separate tag option

As outlined in #49, there is a need to mimic the behavior of other
applications by interweaving positional and non-positional parameters.

This change adds the 'separate' option that will force a arg of type
[]string to only read the next supplied value.

For example, when dealing with the following arg type:

var MyArgs struct {
    Pos []string `arg:"positional"`
    Separate []string `arg:"-s,separate"`
}

This commit will parse the following command line:

./app pos1 pos2 -s=separate1 -s=separate2 pos3 -s=separate3 pos4

Such that MyArgs.Pos will be [pos1 pos2 pos3 pos4] and MyArgs.Separate
will be [separate1 separate2 separate3].

Unit tests for the above have also been written and are included in this
commit, as well as the addition of a section to README.md and an example
func in example_test.go.

Fixes #49

4 files changed, 96 insertions, 5 deletions

diff --git a/README.md b/README.md
index 8b6f3d8..21a8789 100644
--- a/README.md
+++ b/README.md
@@ -108,7 +108,7 @@ arg.MustParse(&args)
 
 ```shell
 $ ./example -h
-usage: [--verbose] [--dataset DATASET] [--optimize OPTIMIZE] [--help] INPUT [OUTPUT [OUTPUT ...]] 
+usage: [--verbose] [--dataset DATASET] [--optimize OPTIMIZE] [--help] INPUT [OUTPUT [OUTPUT ...]]
 
 positional arguments:
   input
@@ -148,6 +148,22 @@ fmt.Printf("Fetching the following IDs from %s: %q", args.Database, args.IDs)
 Fetching the following IDs from foo: [1 2 3]
 ```
 
+### Arguments that can be specified multiple times, mixed with positionals
+```go
+var args struct {
+    Commands  []string `arg:"-c,separate"`
+    Files     []string `arg:"-f,separate"`
+    Databases []string `arg:"positional"`
+}
+```
+
+```shell
+./example -c cmd1 db1 -f file1 db2 -c cmd2 -f file2 -f file3 db3 -c cmd3
+Commands: [cmd1 cmd2 cmd3]
+Files [file1 file2 file3]
+Databases [db1 db2 db3]
+```
+
 ### Custom validation
 ```go
 var args struct {
diff --git a/example_test.go b/example_test.go
index 6fb5197..c34effa 100644
--- a/example_test.go
+++ b/example_test.go
@@ -71,6 +71,21 @@ func Example_multipleValues() {
 	fmt.Printf("Fetching the following IDs from %s: %q", args.Database, args.IDs)
 }
 
+// This eample demonstrates multiple value arguments that can be mixed with
+// other arguments.
+func Example_multipleMixed() {
+	os.Args = []string{"./example", "-c", "cmd1", "db1", "-f", "file1", "db2", "-c", "cmd2", "-f", "file2", "-f", "file3", "db3", "-c", "cmd3"}
+	var args struct {
+		Commands  []string `arg:"-c,separate"`
+		Files     []string `arg:"-f,separate"`
+		Databases []string `arg:"positional"`
+	}
+	MustParse(&args)
+	fmt.Println("Commands:", args.Commands)
+	fmt.Println("Files", args.Files)
+	fmt.Println("Databases", args.Databases)
+}
+
 // This example shows the usage string generated by go-arg
 func Example_usageString() {
 	// These are the args you would pass in on the command line
diff --git a/parse.go b/parse.go
index 60f35ee..4f62c60 100644
--- a/parse.go
+++ b/parse.go
@@ -20,6 +20,7 @@ type spec struct {
 	multiple   bool
 	required   bool
 	positional bool
+	separate   bool
 	help       string
 	env        string
 	wasPresent bool
@@ -189,6 +190,8 @@ func NewParser(config Config, dests ...interface{}) (*Parser, error) {
 						spec.required = true
 					case key == "positional":
 						spec.positional = true
+					case key == "separate":
+						spec.separate = true
 					case key == "help":
 						spec.help = value
 					case key == "env":
@@ -314,11 +317,14 @@ func process(specs []*spec, args []string) error {
 				for i+1 < len(args) && !isFlag(args[i+1]) {
 					values = append(values, args[i+1])
 					i++
+					if spec.separate {
+						break
+					}
 				}
 			} else {
 				values = append(values, value)
 			}
-			err := setSlice(spec.dest, values)
+			err := setSlice(spec.dest, values, !spec.separate)
 			if err != nil {
 				return fmt.Errorf("error processing %s: %v", arg, err)
 			}
@@ -350,7 +356,7 @@ func process(specs []*spec, args []string) error {
 	for _, spec := range specs {
 		if spec.positional {
 			if spec.multiple {
-				err := setSlice(spec.dest, positionals)
+				err := setSlice(spec.dest, positionals, true)
 				if err != nil {
 					return fmt.Errorf("error processing %s: %v", spec.long, err)
 				}
@@ -388,7 +394,7 @@ func validate(spec []*spec) error {
 }
 
 // parse a value as the appropriate type and store it in the struct
-func setSlice(dest reflect.Value, values []string) error {
+func setSlice(dest reflect.Value, values []string, trunc bool) error {
 	if !dest.CanSet() {
 		return fmt.Errorf("field is not writable")
 	}
@@ -401,7 +407,7 @@ func setSlice(dest reflect.Value, values []string) error {
 	}
 
 	// Truncate the dest slice in case default values exist
-	if !dest.IsNil() {
+	if trunc && !dest.IsNil() {
 		dest.SetLen(0)
 	}
 
diff --git a/parse_test.go b/parse_test.go
index 91e17bb..267e57c 100644
--- a/parse_test.go
+++ b/parse_test.go
@@ -739,3 +739,57 @@ func TestHyphenInMultiPositional(t *testing.T) {
 	require.NoError(t, err)
 	assert.Equal(t, []string{"---", "x", "-", "y"}, args.Foo)
 }
+
+func TestSeparate(t *testing.T) {
+	for _, val := range []string{"-f one", "-f=one", "--foo one", "--foo=one"} {
+		var args struct {
+			Foo []string `arg:"--foo,-f,separate"`
+		}
+
+		err := parse(val, &args)
+		require.NoError(t, err)
+		assert.Equal(t, []string{"one"}, args.Foo)
+	}
+}
+
+func TestSeparateWithDefault(t *testing.T) {
+	args := struct {
+		Foo []string `arg:"--foo,-f,separate"`
+	}{
+		Foo: []string{"default"},
+	}
+
+	err := parse("-f one -f=two", &args)
+	require.NoError(t, err)
+	assert.Equal(t, []string{"default", "one", "two"}, args.Foo)
+}
+
+func TestSeparateWithPositional(t *testing.T) {
+	var args struct {
+		Foo []string `arg:"--foo,-f,separate"`
+		Bar string   `arg:"positional"`
+		Moo string   `arg:"positional"`
+	}
+
+	err := parse("zzz --foo one -f=two --foo=three -f four aaa", &args)
+	require.NoError(t, err)
+	assert.Equal(t, []string{"one", "two", "three", "four"}, args.Foo)
+	assert.Equal(t, "zzz", args.Bar)
+	assert.Equal(t, "aaa", args.Moo)
+}
+
+func TestSeparatePositionalInterweaved(t *testing.T) {
+	var args struct {
+		Foo  []string `arg:"--foo,-f,separate"`
+		Bar  []string `arg:"--bar,-b,separate"`
+		Pre  string   `arg:"positional"`
+		Post []string `arg:"positional"`
+	}
+
+	err := parse("zzz -f foo1 -b=bar1 --foo=foo2 -b bar2 post1 -b bar3 post2 post3", &args)
+	require.NoError(t, err)
+	assert.Equal(t, []string{"foo1", "foo2"}, args.Foo)
+	assert.Equal(t, []string{"bar1", "bar2", "bar3"}, args.Bar)
+	assert.Equal(t, "zzz", args.Pre)
+	assert.Equal(t, []string{"post1", "post2", "post3"}, args.Post)
+}