🚀 go-pugleaf

werts@aardvark.cecs.csulb.edu (Michael Werts) wrote:

> I have never heard of "mu abstractions" before. Would somemone
> please post a short description?

As far as I understand it, a mu abstraction is to declarations as a
lambda abstraction is to values (expressions).  Consider this
hypothetical code:

  let
    val x = 1;
    mu (fn context => Context.add_value_binding (context, "y", 3))
  in
    x+y
  end

==> 4

The body of the mu is a piece of code which modifies the environment
by adding a binding of the symbol "y" to 3.

Clearly, the body of the mu must be executed during the _compilation_
of this code fragment, and thus mu abstractions provide a way to
escape upwards by one `level' into the compilation.  (There is no good
reason not to nest them, by the way.)

These things allow language extensions such as `import', to include
the result of compiling another file of declarations:

  fun import context source =
    <<stuff to add declarations from `source'>>

  let
    mu import "foo"
  in
    <<stuff using declarations from source `foo'>>   
  end

This is a bit wooly, I'm afraid.  I hope you get the idea.
---
Richard Brooksby <richard@harlqn.co.uk>
ML Project / Symbolic Processing Division / Harlequin
+44 223 872522 ext 50