How can I print polymorphic values in Standard ML?

Question

Is there a way to print polymorphic values in Standard ML (SML/NJ specifically)? I have a polymorphic function that is not doing what I want and due to the abysmal state that is debugging in SML (see Any real world experience debugging a production functional program?), I would like to see what it is doing with some good-ol' print's. A simple example would be (at a prompt):

fun justThisOnce(x : 'a) : 'a = (print(x); x);
justThisOnce(42);

Other suggestions are appreciated. In the meantime I'll keep staring the offending code into submission.

Update

I was able to find the bug but the question still stands in the hopes of preventing future pain and suffering.

Answer 1

No, there is no way to print a polymorphic value. You have two choices:

• Specialize your function to integers or strings, which are readily printed. Then when the bug is slain, make it polymorphic again.

• If the bug manifests only with some other instantiation, pass show as an additional argument to your function. So for example, if your polymorphic function has type

  'a list -> 'a list
  

  you extend the type to

  ('a -> string) -> 'a list -> 'a list
  

  You use show internally to print, and then by partially applying the function to a suitable show, you can get a version you can use in the original context.

  It's very tedious but it does help. (But be warned: it may drive you to try Haskell.)