[MLton] combining signatures

Michael Norrish Michael.Norrish at nicta.com.au
Sun Jun 5 17:28:14 PDT 2011

If I have

 signature One =
   eqtype foo
   val op1 : foo -> ...
   val op2 :
   val op3 :

 signature Two =
   eqtype foo
   type bar
   val op4 : bar -> foo
   val op5 : ...

is it possible to create a signature that merges One and Two without retyping all of the bindings?

It'd be nice to be able to do

  signature Combined =
    include One
    include Two

but, reasonably, there is a complaint about duplicate bindings of foo.

In my situation, it would be OK to have a combined signature where a particular instance of foo was already decided on, but

  signature Combined =
    include One where type foo = int
    include Two where type foo = int

is also rejected.

I know I can do

  signature Combined =
    structure One : One
    structure Two : Two where type foo = One.foo

but this isn't quite the same.  Is there any way of lifting the internal One and Two bindings out to the top-level to create the signature I really want?

If there's some not-too-destructive way of rewriting the One and Two signatures to make subsequent combination work, I'd be on for that too.


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