mcfg: implement Sources

mcfg/source.go

@@ -51,3 +51,21 @@ func (cfg *Cfg) allParamValues() []ParamValue {
 type Source interface {
 	Parse(*Cfg) ([]ParamValue, error)
 }
+
+// Sources combines together multiple Source instances into one. It will call
+// Parse on each element individually. Later Sources take precedence over
+// previous ones in the slice.
+type Sources []Source
+
+// Parse implements the method for the Source interface.
+func (ss Sources) Parse(c *Cfg) ([]ParamValue, error) {
+	var pvs []ParamValue
+	for _, s := range ss {
+		innerPVs, err := s.Parse(c)
+		if err != nil {
+			return nil, err
+		}
+		pvs = append(pvs, innerPVs...)
+	}
+	return pvs, nil
+}

mcfg/source_test.go

@@ -3,6 +3,7 @@ package mcfg
 import (
 	"encoding/json"
 	"fmt"
+	. "testing"
 	"time"
 
 	"github.com/mediocregopher/mediocre-go-lib/mrand"
@@ -140,3 +141,21 @@ func (scs srcCommonState) assert(s Source) error {
 		massert.Subset(scs.expPVs, gotPVs),
 	).Assert()
 }
+
+func TestSources(t *T) {
+	cfg := New()
+	a := cfg.ParamRequiredInt("a", "")
+	b := cfg.ParamRequiredInt("b", "")
+	c := cfg.ParamRequiredInt("c", "")
+
+	err := cfg.populateParams(Sources{
+		SourceCLI{Args: []string{"--a=1", "--b=666"}},
+		SourceEnv{Env: []string{"B=2", "C=3"}},
+	})
+	massert.Fatal(t, massert.All(
+		massert.Nil(err),
+		massert.Equal(1, *a),
+		massert.Equal(2, *b),
+		massert.Equal(3, *c),
+	))
+}