Seth Gordon wrote:
From a friendliness-to-newbies point of view, these error messages are a tremendous wart... Eeeww.
Neil Mitchell wrote:
If the interface for some feature requires rank-2 types I'd call that an abstraction leak in most cases.
As the original poster of this thread, the one who was bitten this time, let me point out that the use of rank-2 polymorphism here is actually really nice. It provides a strong safety guarantee for the ST monad at *compile time*. But the protection is a bit heavy-handed, so there are some painful side effects that need to be addressed. One is the confusion caused by the strange semantics to those not familiar with the theory. That should be fixed by simple, prominant, task-oriented documentation. ("You must always provide runST with an argument. So, for example, you cannot write "runST $" or "runST .".) And yes, perhaps the error messages in GHC could be improved for newbies, but that was never intended to be the strong point of GHC. I think Hugs is fine here. The other is awkwardness in extending the capabilites of ST. For that, I would propose that the function "unsafeRunST" be added to the library. Of course, if there is some way to improve both of these situations by compilers relaxing the restrictions on rank-2 types somewhat, that would be great. But that is probably for the future. -Yitz