Add prefix optionv0.1.0

3 files changed, 38 insertions, 7 deletions

diff --git a/example.go b/example.go
index 5bffebd..a5c459f 100644
--- a/example.go
+++ b/example.go
@@ -15,3 +15,8 @@ func ExampleFlagSet() {
 	fs.Bool("verbose", false, "Be verbose")
 	ParseFlagSet(fs, os.Args[1:])
 }
+
+func ExamplePrefix() {
+	flag.String("db", "", "Database thing")
+	Parse(Prefix("APP_"))
+}
diff --git a/flag.go b/flag.go
index 9e12188..39a6183 100644
--- a/flag.go
+++ b/flag.go
@@ -19,10 +19,26 @@ type FlagConverterFunc func(flag string) string
 type UsageUpdaterFunc func(key, usage string) string
 
 type config struct {
+	prefix        string
 	flagConverter FlagConverterFunc
 	usageUpdater  UsageUpdaterFunc
 }
 
+// FlagConverter converts the flag name to an environment variable key.
+func FlagConverter(f FlagConverterFunc) Option {
+	return func(cfg *config) {
+		cfg.flagConverter = f
+	}
+}
+
+// Prefix sets the prefix for all envvars.
+// This is applied after the FlagConverter function.
+func Prefix(s string) Option {
+	return func(cfg *config) {
+		cfg.prefix = s
+	}
+}
+
 // UsageUpdater enables configurable flag usage updates.
 func UsageUpdater(f UsageUpdaterFunc) Option {
 	return func(cfg *config) {
@@ -44,13 +60,6 @@ func UsageSuffixer() Option {
 	}
 }
 
-// FlagConverter converts the flag name to an environment variable key.
-func FlagConverter(f FlagConverterFunc) Option {
-	return func(cfg *config) {
-		cfg.flagConverter = f
-	}
-}
-
 // ParseWithEnv parses the command-line flags from os.Args[1:]. Must be called
 // after all flags are defined and before flags are accessed by the program.
 func Parse(opts ...Option) error {
@@ -78,6 +87,7 @@ func parseFlagSetWithEnv(fs *flag.FlagSet, arguments []string, opts ...Option) e
 	var nerr error
 	fs.VisitAll(func(f *flag.Flag) {
 		envKey := cfg.flagConverter(f.Name)
+		envKey = cfg.prefix + envKey
 		if cfg.usageUpdater != nil {
 			f.Usage = cfg.usageUpdater(envKey, f.Usage)
 		}
diff --git a/flag_test.go b/flag_test.go
index bed8d8d..f09bd82 100644
--- a/flag_test.go
+++ b/flag_test.go
@@ -74,6 +74,22 @@ func TestParseFlagSet(t *testing.T) {
 			env:      map[string]string{"TES_T_ING_123": "fromenv"},
 			expected: "fromenv",
 		},
+		"prefixed-no-flag": {
+			// Flags take precedence
+			name:     "testing-123",
+			values:   []string{},
+			opts:     []Option{Prefix("APP_")},
+			env:      map[string]string{"APP_TESTING_123": "fromenv"},
+			expected: "fromenv",
+		},
+		"prefixed-with-flag": {
+			// Flags take precedence
+			name:     "testing-123",
+			values:   []string{"-testing-123", "passed"},
+			opts:     []Option{Prefix("APP_")},
+			env:      map[string]string{"APP_TESTING_123": "fromenv"},
+			expected: "passed",
+		},
 	}
 
 	for name, tt := range tests {