[R6RS] syntax-case semantics
Anton van Straaten
anton at appsolutions.com
Fri Mar 17 05:49:19 EST 2006
> Whether there's a hygiene issue here is debatable. I say there's not one,
> because in my mind, hygiene issues involve interactions between macros,
> not within a single macro.
I agree that the terminology isn't quite right. It could be called a
meta-hygiene issue, though.
At least in theory, it means that writing reusable and reliable
syntax-generating procedures for use by macros should be trickier, since
such a procedure has to take special steps to avoid having variables it
introduces capture variables from the invoking macro's syntactic
Such procedures are potentially unhygienic with respect to the invoking
macro's syntactic environment, in that they can perform unintended
capture. This is analogous to the unhygienic relationship between e.g.
a defmacro macro and the normal Scheme environment -- it's just
occurring higher up the tower.
> While the SRFI 72 semantics is slightly simpler, the traditional semantics
> is easier to work with. (There is a tacit admission of this fact in the
> SRFI exception that commonizes identifiers in unquoted nested quasisyntax
Agreed. The "meta-unhygiene" is often desirable, since the procedural
code may deliberately want to deal with a variable that exists elsewhere
in the same macro.
However, this is only likely to apply when a macro is fairly
self-contained. If a macro depends on a procedure defined elsewhere,
the traditional semantics for SYNTAX starts to seem less appropriate:
it's relying on a kind of dynamic environment to manipulate lexical
variables, and that doesn't really seem to scale across procedure
Essentially, the SYNTAX form should probably not be used to return much
more than trivial syntax from stand-alone procedures.
> I might buy the "hygiene issue" or whatever you want to call it if
> I had ever run into it or seen anyone else run into it, but I haven't. So
> I prefer the traditional semantics.
I assume that in practice, macros don't often rely on syntax-generating
procedures of the sort which would trigger this issue. One reason for
that is probably that such procedures in the context of syntax-case
would be likely to be written as macros themselves, and a separate macro
introduces its own environment, which solves the problem.
Might it make sense to add to traditional syntax-case something like a
FRESH-SYNTAX form, which uses the SRFI 72 semantics? That would allow
the sorts of problem examples given in the SRFI to be written safely and
Of course, if this really would be solving a non-existent problem, then
there'd be no point. However, I'm not so sure that it's non-existent.
It seems likely to me that this could come up when refactoring
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