[MLton] mlb support

[Matthew Fluet]

> You  talk  about multiple paths leading to the same file because of symlinks.

Yes, that was the case that immediately came to mind.

> There are other reasons that this can happen: you can  mount  the  same  file
> system  multiple  times  (hopefully  read-only).

I guess that is another case.  But then the OS doesn't even necessarily
think they are the same file.

> Also, much more typically,
> there is the issue of paths which aren't full (i.e.,  do  not  begin  with  a
> slash).   I  am  assuming  that  the  code will handle this last common case,
> right?

Yes.

> Speaking of the error messages I mentioned before, I still  am  confused  why
> the  first  message  wasn't  about the lexically earlier error.

Well, the lexically earlier error was a elaboration error, while the later
(reported) error was a syntax error.

> It certainly would be a bit better if we could have errors reported in
> this way since it is more what people expect.

Not sure what you are asking for here?

> Also once you get past the first error, you are
> in a program which is not what was read (because that isn't a program) but is
> in  what  the  error  correction  envisioned  as being what you meant, so the
> errors are going to be stranger.

Arguably, MLton goes one step further.  We don't even attempt to ascribe
meaning to a program that has any lexical or syntax errors.