Give every table its path in one pass, name it in every message, read the layout probes through predicates, and leave the shell to the CLI

builtins.go

@@ -472,8 +472,9 @@ func iban(r rng, cc string) string {
 
 const dayLayout = "2006-01-02"
 
-// Alike in no field; layoutDay differs from layoutProbe in its date fields only,
-// layoutClock in its clock fields only.
+// The instants a layout is proved against: layoutProbe2 is alike in no field, while
+// layoutDay differs from layoutProbe in its date fields alone and layoutClock in its
+// clock fields alone, so formatting two of them tells which kind a layout names.
 var (
 	layoutProbe  = time.Date(2001, 2, 3, 4, 5, 6, 0, time.UTC)
 	layoutProbe2 = time.Date(2010, 11, 12, 13, 14, 15, 0, time.UTC)
@@ -481,6 +482,18 @@ var (
 	layoutClock  = time.Date(2001, 2, 3, 13, 14, 15, 0, time.UTC)
 )
 
+func namesAField(layout string) bool {
+	return layoutProbe.Format(layout) != layoutProbe2.Format(layout)
+}
+
+func namesADateField(layout string) bool {
+	return layoutProbe.Format(layout) != layoutDay.Format(layout)
+}
+
+func namesAClockField(layout string) bool {
+	return layoutProbe.Format(layout) != layoutClock.Format(layout)
+}
+
 // quotedLayout reports whether an arg carries the single quotes a layout is written in.
 func quotedLayout(a string) bool {
 	return len(a) >= 2 && a[0] == '\'' && a[len(a)-1] == '\''
@@ -493,10 +506,10 @@ func layoutArg(a string) (string, error) {
 		if strings.HasPrefix(a, `"`) || strings.HasSuffix(a, `"`) {
 			return "", fmt.Errorf("layout %s is double-quoted; write '%s'", a, bare)
 		}
-		return "", fmt.Errorf("layout %s is not quoted; write '%s', which a shell keeps only inside a double-quoted argument", a, bare)
+		return "", fmt.Errorf("layout %s is not quoted; write '%s'", a, bare)
 	}
 	layout := a[1 : len(a)-1]
-	if layoutProbe.Format(layout) == layoutProbe2.Format(layout) {
+	if !namesAField(layout) {
 		return "", fmt.Errorf("layout '%s' names no field, so it is the constant %q; write it as text", layout, layout)
 	}
 	return layout, nil
@@ -518,15 +531,15 @@ func layoutArity(name string, n int, a []string) error {
 		return nil
 	}
 	hint := ""
-	if len(a) > n && !split(a[n-1:]) {
+	if len(a) > n && !holdsQuotedLayout(a[n-1:]) {
 		hint = fmt.Sprintf("; a layout holding a comma is quoted: '%s'", strings.Join(a[n-1:], ", "))
 	}
 	return fmt.Errorf("%s takes %d argument%s, got %d%s", name, n, plural(n), len(a), hint)
 }
 
-// split reports whether the surplus args already hold a quoted layout, which no
-// comma split apart.
-func split(a []string) bool {
+// holdsQuotedLayout reports whether the surplus args already carry a quoted layout,
+// which no comma split apart.
+func holdsQuotedLayout(a []string) bool {
 	for _, arg := range a {
 		if quotedLayout(arg) {
 			return true
@@ -535,8 +548,6 @@ func split(a []string) bool {
 	return false
 }
 
-func plural(n int) string { return map[bool]string{true: "s"}[n != 1] }
-
 func dateArgs(_ map[string]node, a []string) error {
 	if err := layoutArity("date", 3, a); err != nil {
 		return err
@@ -556,10 +567,10 @@ func dateArgs(_ map[string]node, a []string) error {
 	if err != nil {
 		return err
 	}
-	if layoutProbe.Format(layout) == layoutDay.Format(layout) {
+	if !namesADateField(layout) {
 		return fmt.Errorf("date(from,to,layout): '%s' names no date field; write time('%s')", layout, layout)
 	}
-	if from.Equal(to) && layoutProbe.Format(layout) == layoutClock.Format(layout) {
+	if from.Equal(to) && !namesAClockField(layout) {
 		return fmt.Errorf("date(%s,%s,'%s') is the constant %q; write it as text", a[0], a[1], layout, from.Format(layout))
 	}
 	return nil
@@ -573,7 +584,7 @@ func timeArg(_ map[string]node, a []string) error {
 	if err != nil {
 		return err
 	}
-	if layoutProbe.Format(layout) != layoutDay.Format(layout) {
+	if namesADateField(layout) {
 		return fmt.Errorf("time(layout): '%s' names a date field; write date(from,to,layout)", layout)
 	}
 	return nil