Answer architecture round 1: a module's reads sits on its index entry or, for a source without an index, in the load that walks it, the note names default modules, the shipped modules are listed by name, datafiles drops its unused base, and item 195 files the builtins' second parse

resolve.go

@@ -3,18 +3,26 @@ package fejkdata
 import (
 	"errors"
 	"fmt"
-	"strings"
 )
 
-// templateSite is a template to resolve, with its folder and category, both empty for an inline
-// template, and eachNode's label for it.
+// templateSite is a template to resolve, with its folder and its category's path, both empty for an
+// inline template, and eachNode's label for it.
 type templateSite struct {
 	t        *template
 	folder   []string
 	label    string
-	category *categorySite
+	category string
 }
 
+// unresolvedRead is a reference of a category's template naming nothing.
+type unresolvedRead struct {
+	category string
+	err      error
+}
+
+func (u unresolvedRead) Error() string { return u.err.Error() }
+func (u unresolvedRead) Unwrap() error { return u.err }
+
 // resolveCategoryTemplates resolves every template of the categories, once all data is merged, so a
 // reference sees the override-resolved tree.
 func resolveCategoryTemplates(sites []categorySite, root map[string]node) error {
@@ -22,7 +30,7 @@ func resolveCategoryTemplates(sites []categorySite, root map[string]node) error
 	for _, s := range sites {
 		if err := eachNode(s.n, s.path, func(label string, n node) error {
 			if t, isTemplate := n.(*template); isTemplate {
-				ts = append(ts, templateSite{t: t, folder: s.dir, label: label, category: &s})
+				ts = append(ts, templateSite{t: t, folder: s.dir, label: label, category: s.path})
 			}
 			return nil
 		}); err != nil {
@@ -58,8 +66,12 @@ type resolvedNames struct {
 func resolveTemplates(ts []templateSite, root map[string]node) error {
 	for _, s := range ts {
 		refs, err := s.t.resolveRefs(s.folder, s.label, root)
+		var ne noEntry
+		if err != nil && s.category != "" && errors.As(err, &ne) {
+			return unresolvedRead{category: s.category, err: err}
+		}
 		if err != nil {
-			return s.withReads(err)
+			return err
 		}
 		s.t.refs = refs
 	}
@@ -90,16 +102,3 @@ func resolveTemplates(ts []templateSite, root map[string]node) error {
 	}
 	return nil
 }
-
-// withReads adds to err, where a reference of a category names nothing, the modules the
-// category's manifest names it reading by default, or that it names none.
-func (s templateSite) withReads(err error) error {
-	var ne noEntry
-	if s.category == nil || !errors.As(err, &ne) {
-		return err
-	}
-	if len(s.category.reads) == 0 {
-		return fmt.Errorf("%w, and the module holding %s names no module it reads by default", err, s.category.path)
-	}
-	return fmt.Errorf("%w; the module holding %s reads by default %s", err, s.category.path, strings.Join(s.category.reads, ", "))
-}