Add another example to todo-better-errors list

todo-better-errors.md

@@ -1,5 +1,26 @@
 # Error messages that could be better
 
+## Expected to see type constructor
+
+**Problem**: The parser sees `(..., ...)` as the beginning of a type, and
+thinks that there is a missing type constructor. It's likely however that
+the programmer forgot to use product syntax (with `*`) for tuple types.
+
+**Ideal Solution**: Highlight the `(..., ...)` with an appropriate error
+message. Something like: "Invalid or incomplete type". Explain the `*`
+syntax in below the source reference.
+
+```
+Unexpected token.
+
+foo.sml
+  | 
+1 | val x: (int, int) = (42, 15210)
+  |                   ^
+
+Expected to see a type constructor.
+```
+
 ## Equal-sign looks like identifier
 
 **Problem**: The parser tries to use '=' as the name of a variable, but gets