Move how the template language is written into internal/grammar

internal/grammar/reference.go

@@ -0,0 +1,41 @@
+package grammar
+
+import (
+	"fmt"
+	"strings"
+)
+
+// A reference names a node by path rather than as a sibling field: {/a.b} from the
+// data root, {.a} from the folder this file sits in, {..a} from the folder above.
+func IsRef(name string) bool { return strings.HasPrefix(name, ".") || strings.HasPrefix(name, "/") }
+
+// RefShape splits a reference into its sigil and the dotted path after it.
+func RefShape(name string) (sigil, rest string, err error) {
+	switch {
+	case strings.HasPrefix(name, "/"):
+		sigil, rest = "/", name[1:]
+	case strings.HasPrefix(name, ".."):
+		sigil, rest = "..", name[2:]
+	default:
+		sigil, rest = ".", name[1:]
+	}
+	if rest == "" {
+		return "", "", fmt.Errorf("reference has no path")
+	}
+	if strings.HasPrefix(rest, "/") {
+		return "", "", fmt.Errorf("the path after %s starts at a name, not a /; write {%s%s}", sigil, sigil, rest[1:])
+	}
+	if strings.HasPrefix(rest, ".") {
+		return "", "", fmt.Errorf("a reference starts with / (the root), . (this folder) or .. (the folder above)")
+	}
+	segs, err := SplitPath(rest)
+	if err != nil {
+		return "", "", err
+	}
+	for _, seg := range segs {
+		if seg == "" {
+			return "", "", fmt.Errorf("path has an empty segment")
+		}
+	}
+	return sigil, rest, nil
+}