fix(repl): print every line of a usage error at the prompt

changes/unreleased/repl-usage-error-lines.fixed.md

@@ -0,0 +1 @@
+- **The terminal prompt prints every line of a usage error.** A repository or notebook command given wrong arguments — `%load --id x`, `%load --id=a --id=b`, `%load --tags=a a.ipynb` — printed only the usage line at the prompt, leaving out the line saying what was wrong (`--id is not an option of %load`), which the Jupyter kernel's `UsageError` traceback showed. The prompt now prints the usage and the problem, as the kernel does.

internal/frontend/repl/loop.go

@@ -56,7 +56,7 @@ func Loop(r LineReader, out io.Writer, s *Session) error {
 			printLines(w, metaOut)
 			_ = w.Flush()
 			if merr != nil {
-				printLines(w, []string{"error: " + merr.Error()})
+				printLines(w, metaErrorLines(merr))
 				_ = w.Flush()
 			}
 			if quit {
@@ -124,6 +124,16 @@ func bareExpression(src string) (string, bool) {
 	return trimmed, true
 }
 
+// metaErrorLines prints a meta command's error: every line of a usage error,
+// the first under the error prefix as the kernel's value; other errors as before.
+func metaErrorLines(err error) []string {
+	var usage *UsageError
+	if errors.As(err, &usage) && len(usage.Lines) > 0 {
+		return append([]string{errPrefix + usage.Error()}, usage.Lines[1:]...)
+	}
+	return []string{errPrefix + err.Error()}
+}
+
 func printLines(w io.Writer, lines []string) {
 	for _, l := range lines {
 		_, _ = io.WriteString(w, l+"\n")

internal/frontend/repl/repository_test.go

@@ -61,6 +61,25 @@ func wantUsage(t *testing.T, err error, line string) {
 
 func joined(lines []string) string { return strings.Join(lines, "\n") }
 
+// TestLoopPrintsEveryLineOfAUsageError drives repository usage errors through the
+// terminal loop: the usage line under the error prefix, then what was wrong.
+func TestLoopPrintsEveryLineOfAUsageError(t *testing.T) {
+	repoSession(t)
+	var out strings.Builder
+	if err := Loop(&scriptReader{lines: []string{"%load --id x", "%load --id=a --id=b", "%projects now"}}, &out, NewSession()); err != nil {
+		t.Fatalf("Loop error: %v", err)
+	}
+	got := strings.Split(strings.TrimSuffix(out.String(), "\n"), "\n")
+	want := []string{
+		"error: " + usageLoadRepo, "--id is not an option of %load",
+		"error: " + usageLoadRepo, "--id was given twice",
+		"error: " + usageProjects,
+	}
+	if strings.Join(got, "\n") != strings.Join(want, "\n") {
+		t.Errorf("the loop printed:\n%s\nwant:\n%s", strings.Join(got, "\n"), strings.Join(want, "\n"))
+	}
+}
+
 func TestRepoShowsAndSetsTheBaseURL(t *testing.T) {
 	s, api := repoSession(t)
 	if got := joined(runMeta(t, s, "%repo")); got != "API base path: "+api.URL() {