diff --git a/aconfig.go b/aconfig.go
index 2570b2a..39f2e60 100644
--- a/aconfig.go
+++ b/aconfig.go
@@ -9,17 +9,6 @@ import (
 	"strings"
 )
 
-const (
-	defaultValueTag = "default"
-	usageTag        = "usage"
-	jsonNameTag     = "json"
-	yamlNameTag     = "yaml"
-	tomlNameTag     = "toml"
-	hclNameTag      = "hcl"
-	envNameTag      = "env"
-	flagNameTag     = "flag"
-)
-
 // Loader of user configuration.
 type Loader struct {
 	config  Config
@@ -85,6 +74,10 @@ type Config struct {
 	// Files from which config should be loaded.
 	Files []string
 
+	// Envs hold the environment variable from which envs will be parsed.
+	// By default is nil and then os.Environ() will be used.
+	Envs []string
+
 	// Args hold the command-line arguments from which flags will be parsed.
 	// By default is nil and then os.Args will be used.
 	// Unless loader.Flags() will be explicitly parsed by the user.
@@ -164,6 +157,9 @@ func (l *Loader) init() {
 		dec.Init(l.fsys)
 	}
 
+	if l.config.Envs == nil {
+		l.config.Envs = os.Environ()
+	}
 	if l.config.Args == nil {
 		l.config.Args = os.Args[1:]
 	}
@@ -174,7 +170,7 @@ func (l *Loader) init() {
 	if !l.config.SkipFlags {
 		names := make(map[string]bool, len(l.fields))
 		for _, field := range l.fields {
-			flagName := l.fullTag(l.config.FlagPrefix, field, flagNameTag)
+			flagName := l.fullTag(l.config.FlagPrefix, field, "flag")
 			if flagName == "" {
 				continue
 			}
@@ -183,7 +179,7 @@ func (l *Loader) init() {
 				return
 			}
 			names[flagName] = true
-			l.flagSet.String(flagName, field.Tag(defaultValueTag), field.Tag(usageTag))
+			l.flagSet.String(flagName, field.Tag("default"), field.Tag("usage"))
 		}
 	}
 	if l.config.FileFlag != "" {
@@ -278,7 +274,7 @@ func (l *Loader) checkRequired() error {
 
 func (l *Loader) loadDefaults() error {
 	for _, field := range l.fields {
-		defaultValue := field.Tag(defaultValueTag)
+		defaultValue := field.Tag("default")
 		if err := l.setFieldData(field, defaultValue); err != nil {
 			return err
 		}
@@ -376,11 +372,11 @@ func (l *Loader) loadFileFlag() error {
 }
 
 func (l *Loader) loadEnvironment() error {
-	actualEnvs := getEnv()
+	actualEnvs := getEnv(l.config.Envs)
 	dupls := make(map[string]struct{})
 
 	for _, field := range l.fields {
-		envName := l.fullTag(l.config.EnvPrefix, field, envNameTag)
+		envName := l.fullTag(l.config.EnvPrefix, field, "env")
 		if envName == "" {
 			continue
 		}
@@ -411,7 +407,7 @@ func (l *Loader) loadFlags() error {
 	dupls := make(map[string]struct{})
 
 	for _, field := range l.fields {
-		flagName := l.fullTag(l.config.FlagPrefix, field, flagNameTag)
+		flagName := l.fullTag(l.config.FlagPrefix, field, "flag")
 		if flagName == "" {
 			continue
 		}
@@ -430,7 +426,7 @@ func (l *Loader) postFlagCheck(values map[string]interface{}, dupls map[string]s
 		delete(values, name)
 	}
 	for flag, value := range values {
-		if strings.HasPrefix(flag, l.config.EnvPrefix) {
+		if strings.HasPrefix(flag, l.config.FlagPrefix) {
 			return fmt.Errorf("unknown flag %s=%v (see AllowUnknownFlags config param)", flag, value)
 		}
 	}
diff --git a/aconfig_test.go b/aconfig_test.go
index 2d130c3..622fa67 100644
--- a/aconfig_test.go
+++ b/aconfig_test.go
@@ -3,7 +3,7 @@ package aconfig
 import (
 	"embed"
 	"fmt"
-	"io/ioutil"
+	"io"
 	"net/url"
 	"os"
 	"reflect"
@@ -27,7 +27,6 @@ func (l *LogLevel) UnmarshalText(text []byte) error {
 	default:
 		return fmt.Errorf("unknown log level: %s", text)
 	}
-
 	return nil
 }
 
@@ -38,9 +37,7 @@ func TestDefaults(t *testing.T) {
 		SkipEnv:   true,
 		SkipFlags: true,
 	})
-	if err := loader.Load(); err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, loader.Load())
 
 	want := TestConfig{
 		Str:      "str-def",
@@ -63,10 +60,7 @@ func TestDefaults(t *testing.T) {
 			Em: "em-def",
 		},
 	}
-
-	if got := cfg; !reflect.DeepEqual(want, got) {
-		t.Fatalf("want %v, got %v", want, got)
-	}
+	mustEqual(t, cfg, want)
 }
 
 func TestDefaults_AllTypes(t *testing.T) {
@@ -101,9 +95,7 @@ func TestDefaults_AllTypes(t *testing.T) {
 		SkipEnv:   true,
 		SkipFlags: true,
 	})
-	if err := loader.Load(); err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, loader.Load())
 
 	want := AllTypesConfig{
 		Bool:    true,
@@ -121,14 +113,11 @@ func TestDefaults_AllTypes(t *testing.T) {
 		Float32: 1234.213,
 		Float64: 1234.234,
 		Dur:     time.Hour + 2*time.Minute + 3*time.Second,
-		// TODO
+		// TODO: support time
 		// Time :2000-04-05 10:20:30 +0000 UTC,
 		Level: LogLevel(1),
 	}
-
-	if got := cfg; got != want {
-		t.Fatalf("want %v, got %v", want, got)
-	}
+	mustEqual(t, cfg, want)
 }
 
 func TestDefaults_OtherNumberFormats(t *testing.T) {
@@ -150,9 +139,7 @@ func TestDefaults_OtherNumberFormats(t *testing.T) {
 		SkipEnv:   true,
 		SkipFlags: true,
 	})
-	if err := loader.Load(); err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, loader.Load())
 
 	want := OtherNumberFormats{
 		Int:    7,
@@ -165,10 +152,7 @@ func TestDefaults_OtherNumberFormats(t *testing.T) {
 		Uint16: 83,
 		Uint32: 291,
 	}
-
-	if got := cfg; got != want {
-		t.Fatalf("want %v, got %v", want, got)
-	}
+	mustEqual(t, cfg, want)
 }
 
 func TestJSON(t *testing.T) {
@@ -181,14 +165,10 @@ func TestJSON(t *testing.T) {
 		SkipFlags:    true,
 		Files:        []string{filepath},
 	})
-	if err := loader.Load(); err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, loader.Load())
 
 	want := wantConfig
-	if got := cfg; !reflect.DeepEqual(want, got) {
-		t.Fatalf("want %v, got %v", want, got)
-	}
+	mustEqual(t, cfg, want)
 }
 
 func TestJSONWithOmitempty(t *testing.T) {
@@ -204,9 +184,7 @@ func TestJSONWithOmitempty(t *testing.T) {
 		AllowUnknownFields: true,
 		Files:              []string{createTestFile(t)},
 	})
-	if err := loader.Load(); err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, loader.Load())
 }
 
 func TestCustomFile(t *testing.T) {
@@ -222,14 +200,10 @@ func TestCustomFile(t *testing.T) {
 			".config": &jsonDecoder{},
 		},
 	})
-	if err := loader.Load(); err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, loader.Load())
 
 	want := wantConfig
-	if got := cfg; !reflect.DeepEqual(want, got) {
-		t.Fatalf("want %v, got %v", want, got)
-	}
+	mustEqual(t, cfg, want)
 }
 
 func TestFile(t *testing.T) {
@@ -242,9 +216,7 @@ func TestFile(t *testing.T) {
 		SkipFlags:    true,
 		Files:        []string{filepath},
 	})
-	if err := loader.Load(); err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, loader.Load())
 
 	want := TestConfig{
 		Str:      "str-json",
@@ -260,10 +232,7 @@ func TestFile(t *testing.T) {
 			IsAnon: true,
 		},
 	}
-
-	if got := cfg; !reflect.DeepEqual(want, got) {
-		t.Fatalf("want %v, got %v", want, got)
-	}
+	mustEqual(t, cfg, want)
 }
 
 //go:embed testdata
@@ -280,9 +249,7 @@ func TestFileEmbed(t *testing.T) {
 		Files:        []string{filepath},
 		FileSystem:   configEmbed,
 	})
-	if err := loader.Load(); err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, loader.Load())
 
 	want := TestConfig{
 		Str:      "str-json",
@@ -298,10 +265,7 @@ func TestFileEmbed(t *testing.T) {
 			IsAnon: true,
 		},
 	}
-
-	if got := cfg; !reflect.DeepEqual(want, got) {
-		t.Fatalf("want %v, got %v", want, got)
-	}
+	mustEqual(t, cfg, want)
 }
 
 func TestFileMerging(t *testing.T) {
@@ -317,9 +281,7 @@ func TestFileMerging(t *testing.T) {
 		MergeFiles:   true,
 		Files:        []string{file1, file2, file3},
 	})
-	if err := loader.Load(); err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, loader.Load())
 
 	want := TestConfig{
 		Str:      "111",
@@ -328,10 +290,7 @@ func TestFileMerging(t *testing.T) {
 			Float: 333.333,
 		},
 	}
-
-	if got := cfg; !reflect.DeepEqual(want, got) {
-		t.Fatalf("want %v, got %v", want, got)
-	}
+	mustEqual(t, cfg, want)
 }
 
 func TestFileFlag(t *testing.T) {
@@ -350,18 +309,13 @@ func TestFileFlag(t *testing.T) {
 		Files:        []string{file1},
 		Args:         flags,
 	})
-	if err := loader.Load(); err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, loader.Load())
 
 	want := TestConfig{
 		Str:      "111",
 		HTTPPort: 222,
 	}
-
-	if got := cfg; !reflect.DeepEqual(want, got) {
-		t.Fatalf("want %v, got %v", want, got)
-	}
+	mustEqual(t, cfg, want)
 }
 
 func TestBadFileFlag(t *testing.T) {
@@ -376,9 +330,7 @@ func TestBadFileFlag(t *testing.T) {
 		FileFlag:     "file_flag",
 		Args:         flags,
 	})
-	if err := loader.Load(); err == nil {
-		t.Fatal("should be an error")
-	}
+	failIfOk(t, loader.Load())
 }
 
 func TestNoFileFlagValue(t *testing.T) {
@@ -392,18 +344,13 @@ func TestNoFileFlagValue(t *testing.T) {
 		Files:        []string{file1},
 		Args:         []string{}, // no file_flag
 	})
-	if err := loader.Load(); err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, loader.Load())
 
 	want := TestConfig{
 		Str:      "111",
 		HTTPPort: 111,
 	}
-
-	if got := cfg; !reflect.DeepEqual(want, got) {
-		t.Fatalf("want %v, got %v", want, got)
-	}
+	mustEqual(t, cfg, want)
 }
 
 func TestEnv(t *testing.T) {
@@ -423,9 +370,7 @@ func TestEnv(t *testing.T) {
 		SkipFlags:    true,
 		EnvPrefix:    "TST",
 	})
-	if err := loader.Load(); err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, loader.Load())
 
 	want := TestConfig{
 		Str:      "str-env",
@@ -445,9 +390,7 @@ func TestEnv(t *testing.T) {
 		},
 	}
 
-	if got := cfg; !reflect.DeepEqual(want, got) {
-		t.Fatalf("want %v, got %v", want, got)
-	}
+	mustEqual(t, cfg, want)
 }
 
 func TestFlag(t *testing.T) {
@@ -469,13 +412,9 @@ func TestFlag(t *testing.T) {
 		"-tst.em=em-flag",
 	}
 
-	if err := loader.Flags().Parse(flags); err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, loader.Flags().Parse(flags))
 
-	if err := loader.Load(); err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, loader.Load())
 
 	want := TestConfig{
 		Str:      "str-flag",
@@ -495,9 +434,7 @@ func TestFlag(t *testing.T) {
 		},
 	}
 
-	if got := cfg; !reflect.DeepEqual(want, got) {
-		t.Fatalf("want %v, got %v", want, got)
-	}
+	mustEqual(t, cfg, want)
 }
 
 func TestExactName(t *testing.T) {
@@ -521,9 +458,7 @@ func TestExactName(t *testing.T) {
 		AllowUnknownEnvs: true,
 		EnvPrefix:        "TST",
 	})
-	if err := loader.Load(); err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, loader.Load())
 
 	want := ExactConfig{
 		Foo: Foo{
@@ -532,9 +467,7 @@ func TestExactName(t *testing.T) {
 		Bar: "bar-env",
 	}
 
-	if got := cfg; !reflect.DeepEqual(want, got) {
-		t.Fatalf("want %v, got %v", want, got)
-	}
+	mustEqual(t, cfg, want)
 }
 
 func TestSkipName(t *testing.T) {
@@ -555,9 +488,7 @@ func TestSkipName(t *testing.T) {
 		SkipFiles: true,
 		SkipFlags: true,
 	})
-	if err := loader.Load(); err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, loader.Load())
 
 	want := ExactConfig{
 		Foo: Foo{
@@ -566,9 +497,7 @@ func TestSkipName(t *testing.T) {
 		Bar: "def",
 	}
 
-	if got := cfg; !reflect.DeepEqual(want, got) {
-		t.Fatalf("want %v, got %v", want, got)
-	}
+	mustEqual(t, cfg, want)
 }
 
 func TestDuplicatedName(t *testing.T) {
@@ -588,9 +517,7 @@ func TestDuplicatedName(t *testing.T) {
 		SkipFlags:       true,
 		AllowDuplicates: true,
 	})
-	if err := loader.Load(); err != nil {
-		t.Error(err)
-	}
+	failIfErr(t, loader.Load())
 
 	want := ExactConfig{
 		Foo: Foo{
@@ -599,9 +526,7 @@ func TestDuplicatedName(t *testing.T) {
 		FooBar: "str-env",
 	}
 
-	if got := cfg; !reflect.DeepEqual(want, got) {
-		t.Fatalf("want %v, got %v", want, got)
-	}
+	mustEqual(t, cfg, want)
 }
 
 func TestFailOnDuplicatedName(t *testing.T) {
@@ -619,6 +544,8 @@ func TestFailOnDuplicatedName(t *testing.T) {
 	})
 
 	err := loader.Load()
+	failIfOk(t, err)
+
 	if !strings.Contains(err.Error(), "is duplicated") {
 		t.Fatalf("got %s", err.Error())
 	}
@@ -631,11 +558,10 @@ func TestFailOnDuplicatedFlag(t *testing.T) {
 	}
 
 	err := LoaderFor(&Foo{}, Config{}).Load()
+	failIfOk(t, err)
 
 	want := `init loader: duplicate flag "yes"`
-	if got := err.Error(); got != want {
-		t.Fatalf("got %s want %s", got, want)
-	}
+	mustEqual(t, err.Error(), want)
 }
 
 func TestUsage(t *testing.T) {
@@ -651,9 +577,7 @@ func TestUsage(t *testing.T) {
     	use... em...field. (default "em-def")
 `
 
-	if got != want {
-		t.Fatalf("want %v, got %v", want, got)
-	}
+	mustEqual(t, got, want)
 }
 
 func TestBadDefauts(t *testing.T) {
@@ -665,9 +589,7 @@ func TestBadDefauts(t *testing.T) {
 			SkipEnv:   true,
 			SkipFlags: true,
 		})
-		if err := loader.Load(); err == nil {
-			t.Fatal(err)
-		}
+		failIfOk(t, loader.Load())
 	}
 
 	f(&struct {
@@ -757,27 +679,21 @@ func TestBadFiles(t *testing.T) {
 			FailOnFileNotFound: true,
 			Files:              []string{filepath},
 		})
-		if err := loader.Load(); err == nil {
-			t.Fatal(err)
-		}
+		failIfOk(t, loader.Load())
 	}
 
-	t.Run("no_such_file.json", func(t *testing.T) {
+	t.Run("no_such_file.json", func(*testing.T) {
 		f("no_such_file.json")
 	})
 
 	t.Run("bad_config.json", func(t *testing.T) {
 		filepath := t.TempDir() + "unknown.ext"
 		file, err := os.Create(filepath)
-		if err != nil {
-			t.Fatal(err)
-		}
+		failIfErr(t, err)
 		defer file.Close()
 
 		_, err = file.WriteString(`{almost": "json`)
-		if err != nil {
-			t.Fatal(err)
-		}
+		failIfErr(t, err)
 
 		f(filepath)
 	})
@@ -785,9 +701,7 @@ func TestBadFiles(t *testing.T) {
 	t.Run("unknown.ext", func(t *testing.T) {
 		filepath := t.TempDir() + "unknown.ext"
 		file, err := os.Create(filepath)
-		if err != nil {
-			t.Fatal(err)
-		}
+		failIfErr(t, err)
 		defer file.Close()
 
 		f(filepath)
@@ -806,9 +720,7 @@ func TestFailOnFileNotFound(t *testing.T) {
 			Files:              []string{filepath},
 		})
 
-		if err := loader.Load(); err != nil {
-			t.Fatal(err)
-		}
+		failIfErr(t, loader.Load())
 	}
 
 	f("testdata/config.json")
@@ -826,9 +738,7 @@ func TestBadEnvs(t *testing.T) {
 		EnvPrefix:    "TST",
 	})
 
-	if err := loader.Load(); err == nil {
-		t.Fatal(err)
-	}
+	failIfOk(t, loader.Load())
 }
 
 func TestBadFlags(t *testing.T) {
@@ -839,12 +749,10 @@ func TestBadFlags(t *testing.T) {
 		FlagPrefix:   "tst",
 	})
 
-	if err := loader.Flags().Parse([]string{"-tst.param=10a01"}); err != nil {
-		t.Fatal(err)
-	}
-	if err := loader.Load(); err == nil {
-		t.Fatal(err)
-	}
+	args := []string{"-tst.param=10a01"}
+
+	failIfErr(t, loader.Flags().Parse(args))
+	failIfOk(t, loader.Load())
 }
 
 func TestUnknownFields(t *testing.T) {
@@ -859,9 +767,8 @@ func TestUnknownFields(t *testing.T) {
 	})
 
 	err := loader.Load()
-	if err == nil {
-		t.Fatal("must not be nil")
-	}
+	failIfOk(t, err)
+
 	if !strings.Contains(err.Error(), "unknown field in file") {
 		t.Fatalf("got %s", err.Error())
 	}
@@ -882,9 +789,8 @@ func TestUnknownEnvs(t *testing.T) {
 	})
 
 	err := loader.Load()
-	if err == nil {
-		t.Fatal("must not be nil")
-	}
+	failIfOk(t, err)
+
 	if !strings.Contains(err.Error(), "unknown environment var") {
 		t.Fatalf("got %s", err.Error())
 	}
@@ -902,9 +808,7 @@ func TestUnknownEnvsWithEmptyPrefix(t *testing.T) {
 		SkipFlags:    true,
 	})
 
-	if err := loader.Load(); err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, loader.Load())
 }
 
 func TestUnknownFlags(t *testing.T) {
@@ -923,20 +827,17 @@ func TestUnknownFlags(t *testing.T) {
 
 	// just for tests
 	flagSet := loader.Flags()
-	flagSet.SetOutput(ioutil.Discard)
+	flagSet.SetOutput(io.Discard)
 
 	// define flag with a loader's prefix which is unknown
 	flagSet.Int("tst.unknown", 42, "")
 	flagSet.String("just_env", "just_def", "")
 
-	if err := flagSet.Parse(flags); err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, flagSet.Parse(flags))
 
 	err := loader.Load()
-	if err == nil {
-		t.Fatal("must not be nil")
-	}
+	failIfOk(t, err)
+
 	if !strings.Contains(err.Error(), "unknown flag") {
 		t.Fatalf("got %s", err.Error())
 	}
@@ -956,18 +857,13 @@ func TestUnknownFlagsWithEmptyPrefix(t *testing.T) {
 
 	// just for tests
 	flagSet := loader.Flags()
-	flagSet.SetOutput(ioutil.Discard)
+	flagSet.SetOutput(io.Discard)
 
 	// define flag with a loader's prefix which is unknown
 	flagSet.Int("unknown", 42, "")
 
-	if err := flagSet.Parse(flags); err != nil {
-		t.Fatal(err)
-	}
-
-	if err := loader.Load(); err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, flagSet.Parse(flags))
+	failIfErr(t, loader.Load())
 }
 
 // flag.FlagSet already fails on undefined flag
@@ -986,11 +882,26 @@ func TestUnknownFlagsStdlib(t *testing.T) {
 
 	// just for tests
 	flagSet := loader.Flags()
-	flagSet.SetOutput(ioutil.Discard)
+	flagSet.SetOutput(io.Discard)
+
+	failIfOk(t, flagSet.Parse(flags))
+}
+
+func TestCustomEnvsAndArgs(t *testing.T) {
+	var cfg TestConfig
+	loader := LoaderFor(&cfg, Config{
+		SkipDefaults: true,
+		Envs:         []string{"PARAM=2"},
+		Args:         []string{"-str=4"},
+	})
 
-	if err := flagSet.Parse(flags); err == nil {
-		t.Fatal("must not be nil")
+	failIfErr(t, loader.Load())
+
+	want := TestConfig{
+		Str:   "4",
+		Param: 2,
 	}
+	mustEqual(t, cfg, want)
 }
 
 func TestCustomNames(t *testing.T) {
@@ -1000,28 +911,20 @@ func TestCustomNames(t *testing.T) {
 		C int `default:"-1" env:"three" flag:"four"`
 	}
 
+	setEnv(t, "ONE", "1")
+	setEnv(t, "three", "3")
+	defer os.Clearenv()
+
 	var cfg TestConfig
 	loader := LoaderFor(&cfg, Config{
 		Args: []string{"-two=2", "-four=4"},
 	})
 
-	setEnv(t, "ONE", "1")
-	setEnv(t, "three", "3")
-	defer os.Clearenv()
+	failIfErr(t, loader.Load())
 
-	if err := loader.Load(); err != nil {
-		t.Fatal(err)
-	}
-
-	if want := 1; cfg.A != want {
-		t.Errorf("got %#v, want %#v", cfg.A, want)
-	}
-	if want := 2; cfg.B != want {
-		t.Errorf("got %#v, want %#v", cfg.B, want)
-	}
-	if want := 4; cfg.C != want {
-		t.Errorf("got %#v, want %#v", cfg.C, want)
-	}
+	mustEqual(t, cfg.A, 1)
+	mustEqual(t, cfg.B, 2)
+	mustEqual(t, cfg.C, 4)
 }
 
 func TestDontGenerateTags(t *testing.T) {
@@ -1051,7 +954,7 @@ func TestDontGenerateTags(t *testing.T) {
 		DontGenerateTags: true,
 	}
 	LoaderFor(&testConfig{}, cfg).WalkFields(func(f Field) bool {
-		for _, tag := range []string{"json", "yaml", "toml", "env", "flag"} {
+		for _, tag := range []string{"json", "yaml", "env", "flag"} {
 			k := f.Name() + "::" + tag
 			if v, ok := want[k]; ok && v != f.Tag(tag) {
 				t.Fatalf("%v: got %v, want %v", tag, f.Tag(tag), v)
@@ -1105,26 +1008,18 @@ func TestWalkFields(t *testing.T) {
 
 	LoaderFor(&TestConfig{}, Config{}).WalkFields(func(f Field) bool {
 		wantFields := fields[i]
-		if f.Name() != wantFields.Name {
-			t.Fatalf("got name %v, want %v", f.Name(), wantFields.Name)
-		}
-
-		if parent, ok := f.Parent(); ok && parent.Name() != wantFields.ParentName {
-			t.Fatalf("got name %v, want %v", parent.Name(), wantFields.ParentName)
-		}
-		if f.Tag("default") != wantFields.DefaultValue {
-			t.Fatalf("got default %#v, want %#v", f.Tag("default"), wantFields.DefaultValue)
-		}
-		if f.Tag("usage") != wantFields.Usage {
-			t.Fatalf("got usage %#v, want %#v", f.Tag("usage"), wantFields.Usage)
+		mustEqual(t, f.Name(), wantFields.Name)
+		mustEqual(t, f.Name(), wantFields.Name)
+		if parent, ok := f.Parent(); ok {
+			mustEqual(t, parent.Name(), wantFields.ParentName)
 		}
+		mustEqual(t, f.Tag("default"), wantFields.DefaultValue)
+		mustEqual(t, f.Tag("usage"), wantFields.Usage)
 		i++
 		return true
 	})
 
-	if want := 3; i != want {
-		t.Fatalf("got %v, want %v", i, want)
-	}
+	mustEqual(t, i, 3)
 
 	i = 0
 	LoaderFor(&TestConfig{}, Config{}).WalkFields(func(f Field) bool {
@@ -1147,9 +1042,7 @@ func TestDontFillFlagsIfDisabled(t *testing.T) {
 		SkipFlags: true,
 		Args:      []string{},
 	})
-	if err := loader.Load(); err != nil {
-		t.Error(err)
-	}
+	failIfErr(t, loader.Load())
 
 	if flags := loader.Flags().NFlag(); flags != 0 {
 		t.Errorf("want empty, got %v", flags)
@@ -1204,9 +1097,7 @@ func TestBadRequiredTag(t *testing.T) {
 }
 
 func setEnv(t *testing.T, key, value string) {
-	if err := os.Setenv(key, value); err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, os.Setenv(key, value))
 }
 
 func int32Ptr(a int32) *int32 {
@@ -1215,9 +1106,7 @@ func int32Ptr(a int32) *int32 {
 
 func createTestFile(t *testing.T, name ...string) string {
 	t.Helper()
-	if len(name) > 1 {
-		t.Fatal()
-	}
+	mustEqual(t, len(name) < 2, true)
 
 	dir := t.TempDir()
 	t.Cleanup(func() {
@@ -1230,14 +1119,10 @@ func createTestFile(t *testing.T, name ...string) string {
 	}
 
 	f, err := os.Create(filepath)
-	if err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, err)
 	defer f.Close()
 	_, err = f.WriteString(testfileContent)
-	if err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, err)
 	return filepath
 }
 
@@ -1428,9 +1313,7 @@ func TestSliceStructs(t *testing.T) {
 		Files:        []string{"testdata/complex.json"},
 	})
 
-	if err := loader.Load(); err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, loader.Load())
 
 	want := ConfigTest{
 		VCenter: ConfigVCenter{
@@ -1451,9 +1334,7 @@ func TestSliceStructs(t *testing.T) {
 			},
 		},
 	}
-	if got := cfg; !reflect.DeepEqual(want, got) {
-		t.Fatalf("want %v, got %v", want, got)
-	}
+	mustEqual(t, cfg, want)
 }
 
 func TestMapOfMap(t *testing.T) {
@@ -1469,20 +1350,16 @@ func TestMapOfMap(t *testing.T) {
 		Files:        []string{"testdata/toy.json"},
 	})
 
-	if err := loader.Load(); err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, loader.Load())
 
-	var want = TestConfig{
+	want := TestConfig{
 		Options: map[string]float64{
 			"foo": 0.4,
 			"bar": 0.25,
 		},
 	}
 
-	if got := cfg; !reflect.DeepEqual(want, got) {
-		t.Fatalf("want %v, got %v", want, got)
-	}
+	mustEqual(t, cfg, want)
 }
 
 func TestBad(t *testing.T) {
@@ -1496,21 +1373,17 @@ func TestBad(t *testing.T) {
 	loader := LoaderFor(&cfg, Config{
 		SkipFlags: true,
 	})
-	if err := loader.Load(); err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, loader.Load())
 
 	p, err := url.ParseQuery("foo=bar")
 	if err != nil {
 		t.Fatal(err)
 	}
-	var want = TestConfig{
+	want := TestConfig{
 		Params: p,
 	}
 
-	if got := cfg; !reflect.DeepEqual(want, got) {
-		t.Fatalf("want %v, got %v", want, got)
-	}
+	mustEqual(t, cfg, want)
 }
 
 func TestFileConfigFlagDelim(t *testing.T) {
@@ -1531,16 +1404,33 @@ func TestFileConfigFlagDelim(t *testing.T) {
 		Files: []string{"testdata/toy.json"},
 	})
 
-	if err := loader.Load(); err != nil {
-		t.Fatal(err)
-	}
+	failIfErr(t, loader.Load())
 
-	var want = TestConfig{Options: struct {
+	want := TestConfig{Options: struct {
 		Foo float64
 		Bar float64
 	}{0.4, 0.25}}
 
-	if got := cfg; !reflect.DeepEqual(want, got) {
-		t.Fatalf("want %v, got %v", want, got)
+	mustEqual(t, cfg, want)
+}
+
+func failIfOk(t testing.TB, err error) {
+	t.Helper()
+	if err == nil {
+		t.Fatal("must be non-nil")
+	}
+}
+
+func failIfErr(t testing.TB, err error) {
+	t.Helper()
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func mustEqual(t testing.TB, got, want interface{}) {
+	t.Helper()
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("\nhave %+v\nwant %+v", got, want)
 	}
 }
diff --git a/reflection.go b/reflection.go
index 7ea1090..21f72c6 100644
--- a/reflection.go
+++ b/reflection.go
@@ -60,25 +60,25 @@ func (l *Loader) tagsForField(field reflect.StructField) map[string]string {
 	words := splitNameByWords(field.Name)
 
 	tags := map[string]string{
-		defaultValueTag: field.Tag.Get(defaultValueTag),
-		usageTag:        field.Tag.Get(usageTag),
+		"default": field.Tag.Get("default"),
+		"usage":   field.Tag.Get("usage"),
 
-		envNameTag:  l.makeTagValue(field, envNameTag, words),
-		flagNameTag: l.makeTagValue(field, flagNameTag, words),
+		"env":  l.makeTagValue(field, "env", words),
+		"flag": l.makeTagValue(field, "flag", words),
 	}
 
-	for _, tag := range []string{jsonNameTag, yamlNameTag, tomlNameTag, hclNameTag} {
-		tags[tag] = l.makeTagValue(field, tag, words)
+	for _, dec := range l.config.FileDecoders {
+		tags[dec.Format()] = l.makeTagValue(field, dec.Format(), words)
 	}
 	return tags
 }
 
 func (l *Loader) fullTag(prefix string, f *fieldData, tag string) string {
 	sep := "."
-	if tag == flagNameTag {
+	if tag == "flag" {
 		sep = l.config.FlagDelimiter
 	}
-	if tag == envNameTag {
+	if tag == "env" {
 		sep = l.config.envDelimiter
 	}
 	res := f.Tag(tag)
diff --git a/utils.go b/utils.go
index b15f248..9608c90 100644
--- a/utils.go
+++ b/utils.go
@@ -27,8 +27,7 @@ func assertStruct(x interface{}) {
 	}
 }
 
-func getEnv() map[string]interface{} {
-	env := os.Environ()
+func getEnv(env []string) map[string]interface{} {
 	res := make(map[string]interface{}, len(env))
 
 	for _, s := range env {
@@ -73,15 +72,16 @@ func (l *Loader) makeTagValue(field reflect.StructField, tag string, words []str
 		return v
 	}
 
-	switch tag {
-	case jsonNameTag, yamlNameTag, tomlNameTag, hclNameTag:
-		if l.config.DontGenerateTags {
-			return field.Name
+	for _, dec := range l.config.FileDecoders {
+		if tag == dec.Format() {
+			if l.config.DontGenerateTags {
+				return field.Name
+			}
 		}
 	}
 
 	name := strings.Join(words, "_")
-	if tag == envNameTag {
+	if tag == "env" {
 		return strings.ToUpper(name)
 	}
 	return strings.ToLower(name)
@@ -90,7 +90,7 @@ func (l *Loader) makeTagValue(field reflect.StructField, tag string, words []str
 // based on https://github.com/fatih/camelcase
 func splitNameByWords(src string) []string {
 	var runes [][]rune
-	lastClass, class := 0, 0
+	var lastClass, class int
 
 	// split into fields based on class of unicode character
 	for _, r := range src {
@@ -132,7 +132,8 @@ func splitNameByWords(src string) []string {
 }
 
 // copy-paste until https://github.com/golang/go/issues/46336 is fixed
-func cut(s, sep string) (before, after string, found bool) {
+// returns: before, after, isFound
+func cut(s, sep string) (_, _ string, _ bool) {
 	if i := strings.Index(s, sep); i >= 0 {
 		return s[:i], s[i+len(sep):], true
 	}
@@ -143,11 +144,11 @@ var _ fs.FS = &fsOrOS{}
 
 type fsOrOS struct{ fs.FS }
 
-func (fs *fsOrOS) Open(name string) (fs.File, error) {
-	if fs.FS == nil {
+func (f *fsOrOS) Open(name string) (fs.File, error) {
+	if f.FS == nil {
 		return os.Open(name)
 	}
-	return fs.FS.Open(name)
+	return f.FS.Open(name)
 }
 
 type jsonDecoder struct {