README.md

@@ -627,7 +627,8 @@ tokens add cost in proportion to the output.
   its columns from a struct instead, because a Go caller has already written that
   schema: the fields name the columns and their types are the datatypes, so a tag
   says only what to draw, and a `datatype` in it would be a second spelling of the
-  type.
+  type. The Go type is a struct column's one datatype, so its items need not agree on
+  one among themselves; each must only hold that type.
 - **A struct's records follow Go's field access, and compile on first use.** The
   fields an embedded struct promotes are the struct's own — `e.First`, as
   `encoding/json` and SQL mappers read them — so they are columns of its record and

datatype.go

@@ -64,18 +64,41 @@ func datatypeOf(m map[string]any, pos position) (DataType, error) {
 	return 0, fmt.Errorf(`datatype takes "integer", "number" or "boolean", got %q`, name)
 }
 
-// checkColumns rejects a record column whose items hold different datatypes.
-func checkColumns(path string, n node) error {
+// checkColumns rejects a record column whose items hold different datatypes, checking a column
+// after the columns its items read, so a column read is named before its readers.
+func checkColumns(s nodeScope) error {
+	checked := map[node]bool{}
+	var check func(path, name string, column node) error
+	check = func(path, name string, column node) error {
+		if checked[column] {
+			return nil
+		}
+		checked[column] = true
+		items, _ := columnItems(column)
+		for _, it := range items {
+			if r := it.readsColumn; r != nil {
+				if err := check(r.a.key[1:], r.a.tail[0], r.column); err != nil {
+					return err
+				}
+			}
+		}
+		if _, err := columnDatatype(column); err != nil {
+			return fmt.Errorf("%s: field %q: %w", path, name, err)
+		}
+		return nil
+	}
+	return s(func(path string, n node) error {
 		t, ok := n.(*template)
 		if !ok || !t.record {
 			return nil
 		}
 		for _, name := range recordColumns(t) {
-		if _, err := columnDatatype(t.fields[name]); err != nil {
-			return fmt.Errorf("%s: field %q: %w", path, name, err)
+			if err := check(path, name, t.fields[name]); err != nil {
+				return err
 			}
 		}
 		return nil
+	})
 }
 
 // columnDatatype is the datatype a column's items hold. They must agree, since a
@@ -136,13 +159,15 @@ func columnItems(n node) (items []*template, nullable bool) {
 
 // disagreement names the fix for two items of one column holding different datatypes.
 func disagreement(a *template, da DataType, b *template, db DataType) error {
-	bare, want := b, da
+	bare, typed, want := b, a, da
 	if da == DataTypeString {
-		bare, want = a, db
+		bare, typed, want = a, b, db
 	}
 	switch {
 	case da != DataTypeString && db != DataTypeString:
 		return fmt.Errorf("its items hold %s and %s; a column holds one datatype", da, db)
+	case typed.datatype == DataTypeString && (&valueProof{}).columnItem(bare).not[want] != "":
+		return fmt.Errorf(`item %q is not %s, the datatype item %q takes from the column it reads; to read that column as text, write {"format":"{text}","text":%q}`, bare.format, dataTypeNouns[want], typed.format, typed.format)
 	case bare.fromString: // an object may carry a weight, which this spelling would drop
 		return fmt.Errorf(`item %q declares no datatype, and a column holds one; write it as {"format":%q,"datatype":%q}`, bare.format, bare.format, want)
 	}

graph.go

@@ -177,14 +177,19 @@ func inlineScope(n node, label string) nodeScope {
 	return func(fn func(path string, m node) error) error { return eachNode(n, label, fn) }
 }
 
-// checkScope runs the per-node fences over a scope, each over the whole scope
+// checkScope runs every fence over a scope: checkColumns, then checkRenders.
+func checkScope(s nodeScope) error {
+	if err := checkColumns(s); err != nil {
+		return err
+	}
+	return checkRenders(s)
+}
+
+// checkRenders runs the per-node fences over a scope, each over the whole scope
 // before the next, so which of several broken nodes is reported does not depend on
 // the walk. It runs after checkNoCycles, whose guarantee is what lets the walks
 // terminate.
-func checkScope(s nodeScope) error {
-	if err := s(checkColumns); err != nil {
-		return err
-	}
+func checkRenders(s nodeScope) error {
 	mem := reachMemo{}
 	if err := s(func(path string, n node) error { return repeatCheck(path, n, mem) }); err != nil {
 		return err

inline.go

@@ -32,7 +32,7 @@ func (f *Generator) NewTemplate(input string) (*Template, error) {
 	if err != nil {
 		return nil, fmt.Errorf("fejkdata: %w", err)
 	}
-	if err := bindInline(n, "template", f.categories); err != nil {
+	if err := bindInline(n, "template", f.categories, checkScope); err != nil {
 		return nil, fmt.Errorf("fejkdata: %w", err)
 	}
 	return &Template{g: f, n: n}, nil
@@ -133,14 +133,14 @@ func inputValue(input string) (any, error) {
 	return raw, nil
 }
 
-// bindInline links an inline node's references against root and runs the fences over it,
-// naming its nodes from label.
-func bindInline(n node, label string, root map[string]node) error {
+// bindInline links an inline node's references against root and runs check over it, naming its
+// nodes from label.
+func bindInline(n node, label string, root map[string]node, check func(nodeScope) error) error {
 	scope := inlineScope(n, label)
 	if err := linkNodeRefs(scope, root); err != nil {
 		return err
 	}
-	return checkScope(scope)
+	return check(scope)
 }
 
 // linkNodeRefs binds the references in an inline node's templates against the

record.go

@@ -217,7 +217,7 @@ func recordOf(n node) (*template, []Column, error) {
 	}
 	columns := make([]Column, len(names))
 	for i, name := range names {
-		datatype, _ := columnDatatype(t.fields[name]) // checkColumns refused a column whose items disagree
+		datatype, _ := columnDatatype(t.fields[name]) // checkColumns refused items that disagree wherever DataType is read
 		columns[i] = Column{Name: name, DataType: datatype}
 	}
 	return t, columns, nil

struct.go

@@ -258,7 +258,7 @@ func (s *structShape) compileRecord(root map[string]node, t reflect.Type, label
 	if err != nil {
 		return fmt.Errorf("%s: %w", label, err)
 	}
-	if err := bindInline(n, label, root); err != nil {
+	if err := bindInline(n, label, root, checkRenders); err != nil {
 		return err
 	}
 	record, columns, err := recordOf(n)