[MLton] Ad-hoc infix identifiers, Printf, and libraries

[Matthew Fluet]

> > I thought that was very cool.  I've been very jealous of Haskell's `id` 
> > infix syntax, and I think <\id\> is fairly concise.  I had also forgotten 
> > about the sectioning functions, which are also great. [...]
> 
> Thanks!  I'll take that as an invitation to add a page on the technique to
> the Wiki.

Please do.

> Introducing a top-level infix identifier in SML can easily break code far
> away.  Thus, mnemonic names (like `when', `by', `bind') are pretty much
> off-limits for infixing at the top-level (in a library).  For instance,
> different binding strengths can be useful for an identifier in different
> cases and having multiple top-level fixity declarations (with different
> binding strenghts) for an identifier might cause some rather subtle bugs.

Right.  This is where a principled use of the ML Basis sytem could be a 
real win.  I imagine something like:

foo.mlb:
  local
     libA.mlb
     libB.mlb
  in
     foo.sig
     foo.sml
  end

foo.sml:
  structure Foo : FOO =
    struct
       open A B

       ...
    end

As long as libA.mlb and libB.mlb export the right fixity directives for 
their respective libraries, then within the body of Foo, you will have 
them, but clients of Foo won't have them.

OTOH, I admit this might be too principled, entailing a .mlb, a .sig, and 
a .sml per module.  But, it means that one can very easily track down the 
binding occurences.

> Do/Can SML compilers issue warnings for incompatible fixity declarations?

I don't know any that do.  I don't think it would be hard to.

> You may be right.  Perhaps I'm worrying too much.  For application
> development top-level infix declarations are reasonably safe.  For
> library development the issue is more difficult.  Libraries defining
> mnemonic infix identifiers are likely to cause problems.

Again, I don't think it is that big a problem.  If you choose to import a 
library, it means you are interested in using it, so you want to have the 
recommended infix decls in scope.