Name the typed spelling for a column read whose values hold the datatype beside it, and say holding for a datatype taken from a column read

datatype.go

@@ -171,22 +171,34 @@ func disagreement(a *template, da DataType, b *template, db DataType) error {
 	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)
 	}
-	return fmt.Errorf(`item %q declares no datatype beside one declaring %s; a column holds one, so give it "datatype": %q`, bare.format, want, want)
+	return fmt.Errorf(`item %q declares no datatype beside one holding %s; a column holds one, so give it "datatype": %q`, bare.format, want, want)
 }
 
-// bothTyped names the fix for two items holding different datatypes: reading one that takes its
+// bothTyped names the fix for two items holding different datatypes: the one taking its datatype
-// datatype from the column it reads as text, since only a declared datatype can be edited away.
+// from the column it reads is typed as the other where its values prove it, else read as text.
 func bothTyped(a *template, da DataType, b *template, db DataType) error {
-	read := a
+	read, other := a, db
 	if a.datatype != DataTypeString {
-		read = b
+		read, other = b, da
 	}
-	if read.datatype != DataTypeString {
+	switch {
+	case read.datatype != DataTypeString:
 		return fmt.Errorf("its items hold %s and %s; a column holds one datatype", da, db)
+	case (&valueProof{}).columnItem(read).not[other] == "":
+		return fmt.Errorf("its items hold %s and %s; a column holds one datatype, so %s", da, db, typedAs(read, other))
 	}
 	return fmt.Errorf("its items hold %s and %s; a column holds one datatype, so to read %q as text, %s", da, db, read.format, asText(read))
 }
 
+// typedAs names the spelling giving a column-read item datatype d, keeping the other keys an object
+// item carries.
+func typedAs(t *template, d DataType) string {
+	if t.fromString {
+		return fmt.Sprintf(`write %q as {"format":%q,"datatype":%q}`, t.format, t.format, d)
+	}
+	return fmt.Sprintf(`give %q "datatype": %q`, t.format, d)
+}
+
 // asText names the spelling that reads the column a column-read item reads as text, keeping the
 // other keys an object item carries.
 func asText(t *template) string {

datatype_test.go

@@ -53,6 +53,7 @@ func TestDatatypeRejectsAValueItsTypeRejects(t *testing.T) {
 		{"a datatype over a typed column", `{"format":"{/src.score}","datatype":"integer"}`, `{/src.score} takes datatype integer from the column it reads; drop "datatype"`},
 		{"a datatype a typed column's values reject", `{"format":"{/src.score}","datatype":"boolean"}`, "{int(1,9)} prints an integer, not a boolean"},
 		{"two typed columns it reads", `["{/src.score}","{/src.flag}"]`, `so to read "{/src.score}" as text, write {"format":"{text}","text":"{/src.score}"}`},
+		{"a typed column read that holds the datatype declared beside it", `[{"format":"1.5","datatype":"number"},"{/src.score}"]`, `so write "{/src.score}" as {"format":"{/src.score}","datatype":"number"}`},
 		{"text beside a weighted typed column read", `[{"format":"{/src.score}","weight":3},"n/a"]`, `to read that column as text, set its "format" to "{text}" and add "text": "{/src.score}"`},
 		{"a value of the column it reads", `{"format":"{/src.code}","datatype":"integer"}`, `"2x" is not an integer`},
 		{"a null read into text", `{"format":"{x}","x":"{/src.score}","datatype":"integer"}`, "reads a null"},