[gambit-list] define-prim, ##, and macro-

Marc Feeley feeley at iro.umontreal.ca
Mon Sep 21 15:02:21 EDT 2009

On 21-Sep-09, at 1:11 PM, Taylor R Campbell wrote:

> Why is Gambit's source code so completely full of DEFINE-PRIM, and
> names beginning with ## or MACRO-, some of which are macros and some
> of which are essentially just procedures implemented as macros?  This
> makes the code very hard to follow, because it is visually distracting
> for every nth token to be ## or MACRO and because so many macros make
> it hard to remember what constructs follow different evaluation rules.

Have you never heard of "security through obfuscation"?

Seriously, the "##" prefix is Gambit's "internal" namespace.  It has  
the advantage that it is illegal in Scheme to begin a symbol that way,  
so if you see it in some Scheme code you are reminded that it is not  
portable.  I believe PLT Scheme has something similar... "#%" if I  
remember correctly.  Chicken also does this.

The form (define-prim NAME BODY) is more-or-less equivalent to

   (define NAME (let () (declare (extended-bindings) ...) BODY))

which makes the code less dense and less brittle than having the  
"(declare ...)" everywhere.  Conceptually it is just a "define", but  
it adds the annotations that are appropriate for a runtime procedure.

Many of the "macro-..." forms could be procedures.  For performance  
reasons they are defined as macros (they are often used in several  
files, and procedure inlining across file boundaries has its  
problems).  They are prefixed with "macro-" to easily identify them.   
Some time ago the prefix was not used and it was hard to analyze the  
code (for a human, i.e. me) to verify that no interrupt checking is  
performed in some critical places.  The "macro-" prefix convention  
gives me that information.

For the record I do think the code could be cleaned up and  
modularized.  But as with most big systems that grow over many years  
this is a task that must be done carefully, and it is not at the top  
of my TODO list.  If you do feel the urge to improve Gambit then  
please take a crack at it.


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