pipe : FileListSplit and better errors

pipe/pipe.go

@@ -2,6 +2,7 @@ package pipe
 
 import (
 	"errors"
+	"fmt"
 	"io"
 	"log"
 	"os"
@@ -27,3 +28,30 @@ func GetStdin() ([]string, error) {
 	s, _ := strings.CutSuffix(string(data), Newline)
 	return strings.Split(s, Newline), nil
 }
+
+// reads files from stdin if present, otherwise from the filename:
+func FileListSplitMaybeStdin(f string) ([]string, error) {
+	filelist, err := GetStdin()
+	if err != nil {
+		// BORK: don't see a neater way of wrapping these two when we depend on the non-nil in this block:
+		// if i just return FileListSplit(), we lose the first err
+		filelist, err2 := FileListSplit(f)
+		if err2 != nil {
+			return nil, fmt.Errorf("%w: %w", err, err2)
+		}
+		return filelist, nil
+	}
+	return filelist, nil
+}
+
+// opens the given filename and splits into lines:
+func FileListSplit(f string) ([]string, error) {
+	if f == "" {
+		return nil, errors.New("filename is empty")
+	}
+	dat, err := os.ReadFile(f)
+	if err != nil {
+		return nil, fmt.Errorf("error opening file: %w", err)
+	}
+	return strings.Split(string(dat), Newline), nil
+}