Move how the template language is written into internal/grammar

internal/grammar/name.go

@@ -0,0 +1,50 @@
+package grammar
+
+import (
+	"fmt"
+	"strings"
+)
+
+// reservedInName is what a category, folder or field name may not contain: a dot
+// separates the segments of a path, '|' the arms of a token, '(' opens a function
+// call, braces delimit the token, '/' starts a reference, and brackets and a quote
+// open a JSON value. A name carrying one is rejected where it is authored rather
+// than where it would be unreachable.
+const reservedInName = ".|({}/[]\""
+
+var reservedList = strings.Join(strings.Split(reservedInName, ""), " ")
+
+// CheckName rejects a name the dot path, {token} and JSON grammars cannot spell, or a struct
+// tag cannot read.
+// Both a category or folder and a field go through it, so there is one answer to
+// what a name may contain.
+func CheckName(name string) error {
+	if name == "" {
+		return fmt.Errorf("%q is empty, which is not a path segment, so List never offers it", name)
+	}
+	if name == "-" {
+		return fmt.Errorf(`%q is reserved: the struct tag fake:"-" leaves a field unfilled, so no tag could read it; rename it`, name)
+	}
+	if strings.Contains(name, AsWord) {
+		return fmt.Errorf("%q contains %q, which a token reads as binding a name; rename it", name, AsWord)
+	}
+	if i := strings.IndexAny(name, reservedInName); i >= 0 {
+		return fmt.Errorf("%q contains %q; a name may not use %s, which the dot path, {token} and JSON grammars reserve",
+			name, name[i:i+1], reservedList)
+	}
+	return nil
+}
+
+// CheckPathNames rejects a dotted path with a segment no name may be.
+func CheckPathNames(path string) error {
+	segs, err := SplitPath(path)
+	if err != nil {
+		return err
+	}
+	for _, seg := range NameSegments(segs) {
+		if err := CheckName(seg); err != nil {
+			return fmt.Errorf("path %w", err)
+		}
+	}
+	return nil
+}