Add a naïve parser for .env files (#89)

* Add a naïve parser for .env files

* Remove dead code

* Cleanup

README.md

@@ -104,7 +104,7 @@ func main() {
 		port  = fs.Int("port", 8080, "listen port for server (also via PORT)")
 		debug = fs.Bool("debug", false, "log debug information (also via DEBUG)")
 	)
-	ff.Parse(fs, os.Args[1:], ff.WithEnvVarNoPrefix())
+	ff.Parse(fs, os.Args[1:], ff.WithEnvVars())
 
 	fmt.Printf("port %d, debug %v\n", *port, *debug)
 }

env_parser.go

@@ -0,0 +1,58 @@
+package ff
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"strconv"
+	"strings"
+)
+
+// EnvParser is a parser for .env files. Each line is tokenized on the first `=`
+// character. The first token is interpreted as the flag name, and the second
+// token is interpreted as the value. Both tokens are trimmed of leading and
+// trailing whitespace. If the value is "double quoted", control characters like
+// `\n` are expanded. Lines beginning with `#` are interpreted as comments.
+//
+// EnvParser respects WithEnvVarPrefix, e.g. an .env file containing `A_B=c`
+// will set a flag named "b" if Parse is called with WithEnvVarPrefix("A").
+func EnvParser(r io.Reader, set func(name, value string) error) error {
+	s := bufio.NewScanner(r)
+	for s.Scan() {
+		line := strings.TrimSpace(s.Text())
+		if line == "" {
+			continue // skip empties
+		}
+
+		if line[0] == '#' {
+			continue // skip comments
+		}
+
+		index := strings.IndexRune(line, '=')
+		if index < 0 {
+			return fmt.Errorf("invalid line: %s", line)
+		}
+
+		var (
+			name  = strings.TrimSpace(line[:index])
+			value = strings.TrimSpace(line[index+1:])
+		)
+
+		if len(name) <= 0 {
+			return fmt.Errorf("invalid line: %s", line)
+		}
+
+		if len(value) <= 0 {
+			return fmt.Errorf("invalid line: %s", line)
+		}
+
+		if unquoted, err := strconv.Unquote(value); err == nil {
+			value = unquoted
+		}
+
+		if err := set(name, value); err != nil {
+			return err
+		}
+	}
+	return nil
+}

env_parser_test.go

@@ -0,0 +1,66 @@
+package ff_test
+
+import (
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/peterbourgon/ff/v3"
+	"github.com/peterbourgon/ff/v3/fftest"
+)
+
+func TestEnvFileParser(t *testing.T) {
+	t.Parallel()
+
+	for _, testcase := range []struct {
+		file string
+		opts []ff.Option
+		want fftest.Vars
+	}{
+		{
+			file: "testdata/empty.env",
+			want: fftest.Vars{},
+		},
+		{
+			file: "testdata/basic.env",
+			want: fftest.Vars{S: "bar", I: 99, B: true, D: time.Hour},
+		},
+		{
+			file: "testdata/prefix.env",
+			opts: []ff.Option{ff.WithEnvVarPrefix("MYPROG")},
+			want: fftest.Vars{S: "bingo", I: 123},
+		},
+		{
+			file: "testdata/prefix-undef.env",
+			opts: []ff.Option{ff.WithEnvVarPrefix("MYPROG"), ff.WithIgnoreUndefined(true)},
+			want: fftest.Vars{S: "bango", I: 9},
+		},
+		{
+			file: "testdata/quotes.env",
+			want: fftest.Vars{S: "", I: 32, X: []string{"1", "2 2", "3 3 3"}},
+		},
+		{
+			file: "testdata/no-value.env",
+			want: fftest.Vars{WantParseErrorString: "invalid line: D="},
+		},
+		{
+			file: "testdata/spaces.env",
+			want: fftest.Vars{X: []string{"1", "2", "3", "4", "5", " 6", " 7 ", " 8 "}},
+		},
+		{
+			file: "testdata/newlines.env",
+			want: fftest.Vars{S: "one\ntwo\nthree\n\n", X: []string{`A\nB\n\n`}},
+		},
+		{
+			file: "testdata/capitalization.env",
+			want: fftest.Vars{S: "hello", I: 12345},
+		},
+	} {
+		t.Run(filepath.Base(testcase.file), func(t *testing.T) {
+			testcase.opts = append(testcase.opts, ff.WithConfigFile(testcase.file), ff.WithConfigFileParser(ff.EnvParser))
+			fs, vars := fftest.Pair()
+			vars.ParseError = ff.Parse(fs, []string{}, testcase.opts...)
+			fftest.Compare(t, &testcase.want, vars)
+		})
+	}
+}

ffcli/command_test.go

@@ -355,7 +355,7 @@ func TestIssue57(t *testing.T) {
 		},
 		{
 			args:        []string{"bar", "-undefined"},
-			parseErrStr: "error parsing commandline args: flag provided but not defined: -undefined",
+			parseErrStr: "error parsing commandline arguments: flag provided but not defined: -undefined",
 			runErrIs:    ffcli.ErrUnparsed,
 		},
 		{
@@ -368,7 +368,7 @@ func TestIssue57(t *testing.T) {
 		},
 		{
 			args:        []string{"bar", "baz", "-also.undefined"},
-			parseErrStr: "error parsing commandline args: flag provided but not defined: -also.undefined",
+			parseErrStr: "error parsing commandline arguments: flag provided but not defined: -also.undefined",
 			runErrIs:    ffcli.ErrUnparsed,
 		},
 	} {

options.go

@@ -0,0 +1 @@
+package ff