[plt-scheme] Hygienic Macros

From: Matthias Felleisen (matthias at ccs.neu.edu)
Date: Thu Feb 12 08:57:22 EST 2009

The Hitchhiker's Guide to the Metauniverse used to advertise hygiene  
with a range of examples, starting from if/orelse and going to loop  
macros, especially for, which are notoriously to get right.

Story: After I had written the Hygiene paper Dan and I went to the  
MIT lab late one night and encountered an old Lisper (name withheld)  
hacking on a Lisp machine. My first. We asked him to write one of  
those macros. He wrote the worst possible version. So we wrote a  
client that messed up. He was actually stumped for a moment and  
didn't know why the thing didn't behave as expected. Then we went  
through the defensive spiel of all people who make mistakes in  

Lesson is: if you want to reason about the surface syntax of macro  
uses and if you want to have lots of programmers leverage the power  
of syntactic abstraction, you want hygiene, i.e., enforced lexical  
scoping. (Only superstars on their highest alert level get things  
right all the time.)

-- Matthias

On Feb 12, 2009, at 8:46 AM, Will Farr wrote:

> Paulo,
> Here's another one (probably Eli's examples are better, but I've been
> bitten by this one in practice):
> Suppose you define a macro do-range:
> (define-syntax do-range
>   (syntax-rules ()
>     ((do-range (i llow hhigh) body ...)
>      (let ((low llow)
>            (high hhigh))
>        (let loop ((i low))
>          (if (< i high)
>              (begin body ...
>                     (loop (add1 i)))
>              'done))))))
> And then try to use it in
> (define (vector-n-smallest vector n <)
>    (let* ((store (vector-copy vector 0 (+ n 1))))
>       (insert-sort! store <)
>       (do-range (i (+ n 1) (vector-length vector))
>          (vector-set! store n (vector-ref vector i))
>          (insert-sort! store <))
>       (vector-copy store 0 n)))
> Without hygiene, the '< in the do-range macro will refer to the '<
> that is passed into vector-n-smallest.  There's no way to avoid this
> without introducing hygiene (i.e. it's not a bug that can be fixed by
> simply using (gensym)).  And, it bit me in real life :).
> Will
> On Thu, Feb 12, 2009 at 7:07 AM, Paulo J. Matos  
> <pocmatos at gmail.com> wrote:
>> Hi all,
>> What would be a good simple example of an hygienic macro that would
>> not work as expected if the macro were not hygienic?
>> I would like an example that would be simple to understand to
>> programming language students that are non-schemers. All the ones I
>> have found from papers don't seem 'simple' enough.
>> Any suggestions?
>> --
>> Paulo Jorge Matos - pocmatos at gmail.com
>> Webpage: http://www.personal.soton.ac.uk/pocm
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