Leave column agreement to the records that read DataType, name the text spelling beside a typed column read, and check a column after the columns it reads

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 {
-	t, ok := n.(*template)
-	if !ok || !t.record {
-		return nil
-	}
-	for _, name := range recordColumns(t) {
-		if _, err := columnDatatype(t.fields[name]); err != nil {
+// 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 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 := 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)
 	}

datatype_test.go

@@ -47,8 +47,9 @@ func TestDatatypeRejectsAValueItsTypeRejects(t *testing.T) {
 		{"an item beside a typed one", `[{"format":"1","datatype":"integer"},"x"]`, `write it as {"format":"x","datatype":"integer"}`},
 		{"a weighted item beside a typed one", `[{"format":"1","datatype":"integer"},{"format":"2","weight":3}]`, `give it "datatype": "integer"`},
 		{"items of two datatypes", `[{"format":"1","datatype":"integer"},{"format":"true","datatype":"boolean"}]`, "a column holds one datatype"},
-		{"an item beside a typed column it reads", `["{/src.score}","x"]`, `write it as {"format":"x","datatype":"integer"}`},
+		{"an item beside a typed column it reads", `["{/src.score}","5"]`, `write it as {"format":"5","datatype":"integer"}`},
 		{"a typed item beside a string column it reads", `["{/src.code}",{"format":"1","datatype":"integer"}]`, `write it as {"format":"{/src.code}","datatype":"integer"}`},
+		{"text beside a typed column it reads", `["{/src.score}","n/a"]`, `to read that column as text, write {"format":"{text}","text":"{/src.score}"}`},
 		{"a datatype over a typed column", `{"format":"{/src.score}","datatype":"integer"}`, `{/src.score} takes datatype integer from the column it reads; drop "datatype"`},
 		{"another datatype over a typed column", `{"format":"{/src.score}","datatype":"number"}`, `{/src.score} takes datatype integer from the column it reads; drop "datatype"`},
 		{"a value of the column it reads", `{"format":"{/src.code}","datatype":"integer"}`, `"2x" is not an integer`},
@@ -98,6 +99,13 @@ func TestDatatypeRejectsAValueItsTypeRejects(t *testing.T) {
 			t.Errorf("%s: NewTemplate = %v, want the inline template refused the same way", c.name, err)
 		}
 	}
+	_, err := New(WithoutShippedData(), WithDataPath(writeData(t, map[string]string{
+		"a": `{"format":"","c":["{/b.x}",{"format":"1","datatype":"integer"}]}`,
+		"b": `{"format":"","x":["2",{"format":"1","datatype":"integer"}]}`,
+	})))
+	if want := `b: field "x": item "2"`; err == nil || !strings.Contains(err.Error(), want) {
+		t.Errorf("a column reading one whose items disagree: New = %v, want the column read reported first, containing %q", err, want)
+	}
 }
 
 var jsonInteger = regexp.MustCompile(`^(0|-?[1-9][0-9]*)$`)

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

record_test.go

@@ -385,7 +385,7 @@ func TestRecordBareReferenceStaysIndependent(t *testing.T) {
 func TestRecordColumnOfOneReferenceIsTheColumnItReads(t *testing.T) {
 	dir := writeData(t, map[string]string{
 		"mid": `{"format":"","score":"{/src.score}"}`,
-		"row": `{"format":"","chain":"{/mid.score}","code":{"format":"{/src.code}","datatype":"integer"},"label":"n={/src.score}","same":"{/src.score}","score":"{/src.score}","text":"{/src.code}"}`,
+		"row": `{"format":"","chain":"{/mid.score}","code":{"format":"{/src.code}","datatype":"integer"},"dot":"{.src.score}","label":"n={/src.score}","mixed":[{"format":"{text}","text":"{/src.score}"},"n/a"],"pick":["{/src.score}",{"format":"7","datatype":"integer"}],"same":"{/src.score}","score":"{/src.score}","text":"{/src.code}"}`,
 		"src": `{"format":"","code":[null,"200","404"],"score":[null,{"format":"{int(1,9)}","datatype":"integer"}]}`,
 	})
 	f := newGenerator(t, dir, WithSeed(1))
@@ -408,22 +408,28 @@ func TestRecordColumnOfOneReferenceIsTheColumnItReads(t *testing.T) {
 			return cols[name].Null == want.Null && cols[name].Value == want.Value
 		}
 		switch {
-		case score.DataType != DataTypeInteger || cols["chain"].DataType != DataTypeInteger || code.DataType != DataTypeInteger || cols["label"].DataType != DataTypeString || cols["text"].DataType != DataTypeString:
-			t.Fatalf("%s: want score, chain and code integer columns, label and text string ones", r.JSON())
+		case score.DataType != DataTypeInteger || cols["chain"].DataType != DataTypeInteger || code.DataType != DataTypeInteger || cols["pick"].DataType != DataTypeInteger || cols["label"].DataType != DataTypeString || cols["mixed"].DataType != DataTypeString || cols["text"].DataType != DataTypeString:
+			t.Fatalf("%s: want score, chain, code and pick integer columns, label, mixed and text string ones", r.JSON())
 		case score.Null == (len(score.Value) == 1 && score.Value >= "1" && score.Value <= "9"):
 			t.Fatalf("score = %+v, want null or a digit from src.score", score)
 		case code.Null == (code.Value == "200" || code.Value == "404"):
 			t.Fatalf("code = %+v, want null or a code from src.code", code)
-		case !agree("same", score) || !agree("chain", score) || !agree("text", code):
+		case !agree("same", score) || !agree("chain", score) || !agree("dot", score) || !agree("text", code):
 			t.Fatalf("%s: want every read of one src column one draw, through mid too", r.JSON())
+		case cols["pick"].Value != "7" && !agree("pick", score), cols["mixed"].Null || cols["mixed"].Value != "n/a" && cols["mixed"].Value != score.Value:
+			t.Fatalf("pick = %+v, mixed = %+v beside score %+v: want pick 7 or the score's draw, mixed n/a or the score's text", cols["pick"], cols["mixed"], score)
 		case cols["label"].Null || cols["label"].Value != "n="+score.Value:
 			t.Fatalf("label = %+v beside score %+v, want the text of the read, a null as \"\"", cols["label"], score)
 		case strings.Contains(r.JSON(), `"score":null`) != score.Null:
 			t.Fatalf("JSON() = %s, want a null score written null", r.JSON())
 		}
-		ic := inline.Fake().Columns()[0]
-		if ic.DataType != DataTypeInteger || ic.Null == (len(ic.Value) == 1) {
-			t.Fatalf("inline score = %+v, want an integer column, null or a digit", ic)
+		ir := inline.Fake()
+		ic, sqlValue := ir.Columns()[0], ir.Columns()[0].Value
+		if ic.Null {
+			sqlValue = "NULL"
+		}
+		if ic.DataType != DataTypeInteger || ic.Null == (len(ic.Value) == 1) || ir.SQLInsert("t") != `INSERT INTO "t" ("score") VALUES (`+sqlValue+`);` || ir.CSVLine() != ic.Value {
+			t.Fatalf("inline score = %+v written %s and %q, want an integer column, NULL and an empty field or a bare digit", ic, ir.SQLInsert("t"), ir.CSVLine())
 		}
 		for name, c := range map[string]Column{"code": code, "inline": ic, "score": score} {
 			if c.Null {

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)

struct_test.go

@@ -54,7 +54,8 @@ func structData(t *testing.T) *Generator {
 	return newGenerator(t, writeData(t, map[string]string{
 		"person": `[{"format":"{first} {last}","first":"Ada","last":"Lovelace"},{"format":"{first} {last}","first":"Bo","last":"Ek"}]`,
 		"place":  `[{"format":"{city}","city":"Stockholm","zip":"111 22"},{"format":"{city}","city":"Tranås","zip":"573 31"}]`,
-		"src":    `{"format":"","code":[null,"200","404"],"score":[null,{"format":"{int(1,9)}","datatype":"integer"}]}`,
+		"mid":    `{"format":"","score":["{/src.score}",{"format":"5","datatype":"integer"}]}`,
+		"src":    `{"format":"","code":[null,"200","404"],"del":null,"score":[null,{"format":"{int(1,9)}","datatype":"integer"}]}`,
 		"trip":   `{"format":"","leg":[{"format":"{to}","to":"Oslo"},{"format":"{to}","to":"Rome"}]}`,
 	}), WithSeed(1))
 }
@@ -62,15 +63,39 @@ func structData(t *testing.T) *Generator {
 func TestFakeStructFieldTaggedWithAColumnIsThatColumn(t *testing.T) {
 	f := structData(t)
 	nils := map[string]int{}
+	show := func(p any) string {
+		switch p := p.(type) {
+		case *string:
+			if p != nil {
+				return *p
+			}
+		case *int64:
+			if p != nil {
+				return strconv.FormatInt(*p, 10)
+			}
+		}
+		return "nil"
+	}
 	for i := 0; i < 200; i++ {
 		var v struct {
 			Code  *string `fake:"src.code"`
+			Codes *string `fake:"[\"{/src.score}\",\"{/src.code}\"]"`
+			Del   *int64  `fake:"src.del"`
 			Label string  `fake:"n={/src.score}"`
+			Mixed *string `fake:"[\"{/src.score}\",\"x\"]"`
+			Pair  *int64  `fake:"[\"{/src.score}\",\"5\"]"`
 			Score *int64  `fake:"src.score"`
 		}
 		if err := f.FakeStruct(&v); err != nil {
 			t.Fatal(err)
 		}
+		score, code := show(v.Score), show(v.Code)
+		switch {
+		case show(v.Del) != "nil":
+			t.Fatalf("Del = %s, want nil from a column only ever null", show(v.Del))
+		case show(v.Mixed) != "x" && show(v.Mixed) != score, show(v.Pair) != "5" && show(v.Pair) != score, show(v.Codes) != score && show(v.Codes) != code:
+			t.Fatalf("Mixed %s, Pair %s, Codes %s beside score %s and code %s: want each the literal or the draw of the column it reads", show(v.Mixed), show(v.Pair), show(v.Codes), score, code)
+		}
 		switch {
 		case v.Code == nil:
 			nils["code"]++
@@ -209,6 +234,9 @@ func TestFakeStructErrors(t *testing.T) {
 		{&struct {
 			A *bool `fake:"src.score"`
 		}{}, ".A (*bool): {int(1,9)} prints an integer, not a boolean"},
+		{&struct {
+			A int64 `fake:"mid.score"`
+		}{}, "can draw null, which int64 cannot hold; make it *int64"},
 		{&struct {
 			A int `fake:"{digits(3)}"`
 		}{}, ".A (int): {digits(3)} prints text, not an integer"},