[SISC-devel] psyntax and compiler extension required

[Matthias R. <mat...@so...>]

In my attempts to define the macros for srfi-9 I encountered the 
following problem:

define-values cannot be defined (in R5RS, or even using syntax-case) in 
a way that would make it work in internal definitions such as

(let ()
   (define a 1)
   (define-values (b c) (values 2 3))
   (define d 4)
   (+ a b c d))

The same problem, expressed differently, is
(letrec-values
    ((a 1)
     ((b c) (values 2 3))
     (d 4))
   (+ a b c d))

(**)

This is a known problem. There are some Schemes, notably MzScheme, that 
solve the problem by providing native support for define-values / 
letrec-values(*). Doing this in SISC shouldn't be too hard. My 
suggestion is to:

* Change the syntax of letrec to cover the letrec-values case.

* Change the macro expander to handle the extended letrec syntax.

* Change the macro expander to use the extended letrec syntax for 
expanding internal define-values.

* Change the compiler / LetrecExp to handle the extended letrec syntax.

* [optional] Change the macro expander to recognize top-level 
define-values as native syntax.

* [optional] Change the compiler to produce DefineValuesExpr for 
top-level define-values.

* Change the optimizer to handle any new native syntax we introduce.

A variation on the above is to leave letrec alone and instead introduce 
a native letrec-values. The macro expander would then have to either 
always use letrec-values for internal definitions or, if it tries to be 
smarter, use letrec if there are no internal define-values and use 
letrec-values otherwise.


Matthias
(*) As far as I can tell from the docs MzScheme handles this not quite 
right - it's not possible to intermix define and define-values in a 
mutually recursive way.
(**) The usual syntax for letrec-values does not specifically cater for 
the non-values case. I am using the extended syntax because it allows us 
to make the non-values case as efficient as letrec.