[MLton] Extending the SML Basis library (Library project)

[Stephen Weeks]

> >   signature ISO = sig
> >      type ('a, 'b) t
> [...]
> >      val make: ('a -> 'b) * ('b -> 'a) -> ('a, 'b) t
> [...]
> >   end
> 
> The major problem (sorry for the repetition if you already spotted this
> from my earlier post) with this design (making Iso.t an abstract type) is
> that one can no longer make polymorphic isomorphisms (courtesy of the
> value restriction).  For example, it would be impossible to implement a
> structure matching the following signature (from my earlier message):
> 
> signature VECTOR =
>    sig
>       include VECTOR
>       val list : ('a vector, 'a list) iso
>       val toList : 'a vector -> 'a list
>    end
> 
> So, IMO, the iso/emb types must be concrete. 

In my experience, relying on the value restriction, which is quite
syntactically sensitive, is often, although not always, a design
mistake.  It might be better to specify Vector.list as

  val list : unit -> ('a vector, 'a list) iso

which then allows one to do things (like call Iso.make) when building
the isomorphism, and is more robust to underlying implementation
changes.

> (BTW, I'm using Iso.iso rather than Iso.t, which I usually prefer, to be
> consistent with the basis library style.)

Makes sense (in a basis library extension).