Fix providing multiple values via environment variables

README.md

@@ -94,6 +94,22 @@ $ NUM_WORKERS=4 ./example
 Workers: 4
 ```
 
+You should use a JSON array of strings (value will be converted if
+necessary) in the case of multiple values:
+
+```go
+var args struct {
+    Workers []int `arg:"env"`
+}
+arg.MustParse(&args)
+fmt.Println("Workers:", args.Workers)
+```
+
+```
+$ WORKERS='["1", "99"]' ./example
+Workers: [1 99]
+```
+
 ### Usage strings
 ```go
 var args struct {

parse.go

@@ -2,6 +2,7 @@ package arg
 
 import (
 	"encoding"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"os"
@@ -275,7 +276,23 @@ func process(specs []*spec, args []string) error {
 		}
 		if spec.env != "" {
 			if value, found := os.LookupEnv(spec.env); found {
-				err := scalar.ParseValue(spec.dest, value)
+				var err error
+				if spec.multiple {
+					// expect a JSON array of strings in an environment
+					// variable in the case of multiple values
+					var values []string
+					err = json.Unmarshal([]byte(value), &values)
+					if err != nil {
+						return fmt.Errorf(
+							"error processing environment variable %s (it should be a JSON array of strings):\n%v",
+							spec.env,
+							err,
+						)
+					}
+					err = setSlice(spec.dest, values, !spec.separate)
+				} else {
+					err = scalar.ParseValue(spec.dest, value)
+				}
 				if err != nil {
 					return fmt.Errorf("error processing environment variable %s: %v", spec.env, err)
 				}

parse_test.go

@@ -580,6 +580,42 @@ func TestEnvironmentVariableRequired(t *testing.T) {
 	assert.Equal(t, "bar", args.Foo)
 }
 
+func TestEnvironmentVariableSliceArgumentString(t *testing.T)  {
+	var args struct {
+		Foo []string `arg:"env"`
+	}
+	setenv(t, "FOO", "[\"bar\", \"baz\"]")
+	MustParse(&args)
+	assert.Equal(t, []string{"bar", "baz"}, args.Foo)
+}
+
+func TestEnvironmentVariableSliceArgumentInteger(t *testing.T)  {
+	var args struct {
+		Foo []int `arg:"env"`
+	}
+	setenv(t, "FOO", "[\"1\", \"99\"]")
+	MustParse(&args)
+	assert.Equal(t, []int{1, 99}, args.Foo)
+}
+
+func TestEnvironmentVariableSliceArgumentFloat(t *testing.T)  {
+	var args struct {
+		Foo []float32 `arg:"env"`
+	}
+	setenv(t, "FOO", "[\"1.1\", \"99.9\"]")
+	MustParse(&args)
+	assert.Equal(t, []float32{1.1, 99.9}, args.Foo)
+}
+
+func TestEnvironmentVariableSliceArgumentBool(t *testing.T)  {
+	var args struct {
+		Foo []bool `arg:"env"`
+	}
+	setenv(t, "FOO", "[\"true\", \"false\", \"0\", \"1\"]")
+	MustParse(&args)
+	assert.Equal(t, []bool{true, false, false, true}, args.Foo)
+}
+
 type textUnmarshaler struct {
 	val int
 }