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Fri Aug 7 13:20:51 CEST 2009

Two Kinds of Primitives

Due to restart at GC, the interpreter can support two kinds of
primitives:

     * RESTARTABLE: Pure functions operating on Scheme data structures
       (or impure functions that do not perform allocation _after_
       mutation).

     * ABSTRACT: C code that does not refer to any Scheme data.

The disadvantage of not being able to access Scheme data from impure C
code can be largely removed by providing a suspension mechanism for
primitives.  In this case the C code could behave as a coroutine,
which allows the use of enumerators / iterators instead of
construction of intermediate data.

Impure functions performing allocation before mutation are hard to
write, and are best limited to the implementation of internal
primitives supporting mutation (like `set!' and `define') and adaptor
code that bridge Scheme and C datastructures for abstract functions.

In short: use either purely functional programming or concurrency
oriented programming, and you get a very simple interpreter and
application structure.



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