feat:prefix envs (#201)

* feat: allow to prefix envs

* chore: fumpt

README.md

@@ -296,6 +296,56 @@ func main() {
 }
 ```
 
+### Prefixes
+
+You can prefix sub-structs env tags, as well as a whole `env.Parse` call.
+
+Here's an example flexing it a bit:
+
+```go
+package main
+
+import (
+	"fmt"
+	"log"
+
+	"github.com/caarlos0/env/v6"
+)
+
+type Config struct {
+	Home string `env:"HOME"`
+}
+
+type ComplexConfig struct {
+	Foo   Config `envPrefix:"FOO_"`
+	Clean Config
+	Bar   Config `envPrefix:"BAR_"`
+	Blah  string `env:"BLAH"`
+}
+
+func main() {
+	cfg := ComplexConfig{}
+	if 	err := Parse(&cfg, Options{
+		Prefix: "T_",
+		Environment: map[string]string{
+			"T_FOO_HOME": "/foo",
+			"T_BAR_HOME": "/bar",
+			"T_BLAH":     "blahhh",
+			"T_HOME":     "/clean",
+		},
+	}); err != nil {
+		log.Fatal(err)
+	}
+
+	// Load env vars.
+	if err := env.Parse(cfg, opts); err != nil {
+		log.Fatal(err)
+	}
+
+	// Print the loaded data.
+	fmt.Printf("%+v\n", cfg.envData)
+}
+```
 
 ### On set hooks
 
@@ -372,7 +422,7 @@ func main() {
 
 ## Defaults from code
 
-You may define default value also in code, by initialising the config data before it's filled by `env.Parse`.  
+You may define default value also in code, by initialising the config data before it's filled by `env.Parse`.
 Default values defined as struct tags will overwrite existing values during Parse.
 
 ```go

env.go

@@ -111,6 +111,9 @@ type Options struct {
 	// OnSet allows to run a function when a value is set
 	OnSet OnSetFn
 
+	// Prefix define a prefix for each key
+	Prefix string
+
 	// Sets to true if we have already configured once.
 	configured bool
 }
@@ -142,6 +145,9 @@ func configure(opts []Options) []Options {
 		if item.OnSet != nil {
 			opt.OnSet = item.OnSet
 		}
+		if item.Prefix != "" {
+			opt.Prefix = item.Prefix
+		}
 		opt.RequiredIfNoDef = item.RequiredIfNoDef
 	}
 
@@ -218,7 +224,12 @@ func doParse(ref reflect.Value, funcMap map[reflect.Type]ParserFunc, opts []Opti
 		}
 		if value == "" {
 			if reflect.Struct == refField.Kind() {
-				if err := doParse(refField, funcMap, opts); err != nil {
+				subOpts := make([]Options, len(opts))
+				copy(subOpts, opts)
+				if prefix := refType.Field(i).Tag.Get("envPrefix"); prefix != "" {
+					subOpts[0].Prefix += prefix
+				}
+				if err := doParse(refField, funcMap, subOpts); err != nil {
 					return err
 				}
 			}
@@ -239,7 +250,9 @@ func get(field reflect.StructField, opts []Options) (val string, err error) {
 	var notEmpty bool
 
 	required := opts[0].RequiredIfNoDef
+	prefix := opts[0].Prefix
 	key, tags := parseKeyForOption(field.Tag.Get(getTagName(opts)))
+	key = prefix + key
 	for _, tag := range tags {
 		switch tag {
 		case "":
@@ -256,7 +269,6 @@ func get(field reflect.StructField, opts []Options) (val string, err error) {
 			return "", fmt.Errorf("env: tag option %q not supported", tag)
 		}
 	}
-
 	expand := strings.EqualFold(field.Tag.Get("envExpand"), "true")
 	defaultValue, defExists := field.Tag.Lookup("envDefault")
 	val, exists, isDefault = getOr(key, defaultValue, defExists, getEnvironment(opts))

env_test.go

@@ -1422,6 +1422,52 @@ func TestRequiredIfNoDefOption(t *testing.T) {
 	})
 }
 
+func TestPrefix(t *testing.T) {
+	is := is.New(t)
+	type Config struct {
+		Home string `env:"HOME"`
+	}
+	type ComplexConfig struct {
+		Foo   Config `envPrefix:"FOO_"`
+		Bar   Config `envPrefix:"BAR_"`
+		Clean Config
+	}
+	cfg := ComplexConfig{}
+	err := Parse(&cfg, Options{Environment: map[string]string{"FOO_HOME": "/foo", "BAR_HOME": "/bar", "HOME": "/clean"}})
+	is.NoErr(err)
+	is.Equal("/foo", cfg.Foo.Home)
+	is.Equal("/bar", cfg.Bar.Home)
+	is.Equal("/clean", cfg.Clean.Home)
+}
+
+func TestComplePrefix(t *testing.T) {
+	is := is.New(t)
+	type Config struct {
+		Home string `env:"HOME"`
+	}
+	type ComplexConfig struct {
+		Foo   Config `envPrefix:"FOO_"`
+		Clean Config
+		Bar   Config `envPrefix:"BAR_"`
+		Blah  string `env:"BLAH"`
+	}
+	cfg := ComplexConfig{}
+	err := Parse(&cfg, Options{
+		Prefix: "T_",
+		Environment: map[string]string{
+			"T_FOO_HOME": "/foo",
+			"T_BAR_HOME": "/bar",
+			"T_BLAH":     "blahhh",
+			"T_HOME":     "/clean",
+		},
+	})
+	is.NoErr(err)
+	is.Equal("/foo", cfg.Foo.Home)
+	is.Equal("/bar", cfg.Bar.Home)
+	is.Equal("/clean", cfg.Clean.Home)
+	is.Equal("blahhh", cfg.Blah)
+}
+
 func isErrorWithMessage(tb testing.TB, err error, msg string) {
 	tb.Helper()