As I've mentioned a few times, this term, I find myself teaching OCaml 
to second year students. One of the things they have trouble 
understanding is the role of the type system: in their mind, type errors 
tend to be "something that OCaml won't let them do" rather than "a 
likely error detected by OCaml in their design". In turn, I extrapolate 
that this this is one of the reasons for the success of 
dynamically-typed (if not fully untyped) languages: programmers don't 
feel constrained by an error checker they don't understand.


After some thought, I believe that one good way of getting students to 
understand why the type system is good for them would be to let 
themselves get burnt by type errors a few time. In turn, if this is to 
be part of teaching OCaml, this would require a dynamically typed 
"simulator" for the language: an interpreter accepting any syntactically 
correct expression and executing it until a manifest type error, then 
displaying a nice, detailed explanation of what that error is all about. 
In other words, something like a Scheme interpreter with a nice 
pre-processor/pretty-printer.

Does anyone know if such a tool already exists ?

Thanks,
 David