check refuses placeholders that could never expand

1 files changed, 14 insertions, 0 deletions

diff --git a/internal/plan/index.go b/internal/plan/index.go
index abe073f..0409295 100644
--- a/internal/plan/index.go
+++ b/internal/plan/index.go
@@ -6,8 +6,22 @@ import (
 	"fmt"
 	"strconv"
 	"strings"
+	"time"
 )
 
+// CheckTemplate reports the first reason s's placeholders could never
+// expand - an unknown placeholder, {mtime} or {now} without a format, an
+// unknown format code, {0}, {10} and up, an unclosed "{" - or nil. Capture
+// counts are not checked here; that needs the rule's name tests.
+func CheckTemplate(s string) error {
+	if _, err := MaxIndex(s); err != nil {
+		return err
+	}
+	facts := Facts{Name: "a.b", Captures: make([]string, 10), ModTime: time.Unix(0, 0), Now: time.Unix(0, 0)}
+	_, err := Expand(s, facts)
+	return err
+}
+
 // MaxIndex returns the highest {N} used in s, 0 when none. It reports the
 // same errors Expand does for a malformed placeholder: an unclosed
 // placeholder, or {0} (capture groups are numbered from 1). It shares