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Diffstat (limited to 'doc')
| -rw-r--r-- | doc/ref/api-macros.texi | 38 |
1 files changed, 38 insertions, 0 deletions
diff --git a/doc/ref/api-macros.texi b/doc/ref/api-macros.texi index ef0621415..7fa62e3d6 100644 --- a/doc/ref/api-macros.texi +++ b/doc/ref/api-macros.texi @@ -791,6 +791,44 @@ Return the source properties that correspond to the syntax object @var{x}. @xref{Source Properties}, for more information. @end deffn +And now, a bit of confession time. Guile's syntax expander originates +in code from Chez Scheme: a version of the expander in Chez Scheme that +was made portable to other Scheme systems. Way back in the mid-1990s, +some Scheme systems didn't even have the ability to define new abstract +data types. For this reason, the portable expander from Chez Scheme +that Guile inherited used tagged vectors as syntax objects: vectors +whose first element was the symbol, @code{syntax-object}. + +At the time of this writing it is 2017 and Guile still has support for +this strategy. It worked for this long because no one ever puts a +literal vector in the operator position: + +@example +(#(syntax-object ...) 1 2 3) +@end example + +But this state of affairs was an error. Because syntax objects are just +vectors, this makes it possible for any Scheme code to forge a syntax +object which might cause it to violate abstraction boundaries. You +can't build a sandboxing facility that limits the set of bindings in +scope when one can always escape that limit just by evaluating a special +vector. To fix this problem, Guile 2.2.1 finally migrated to represent +syntax objects as a distinct type with a distinct constructor that is +unavailable to user code. + +However, Guile still has to support ``legacy'' syntax objects, because +it could be that a file compiled with Guile 2.2.0 embeds syntax objects +of the vector kind. Whether the expander treats the special tagged +vectors as syntax objects is now controllable by the +@code{allow-legacy-syntax-objects?} parameter: + +@deffn {Scheme Procedure} allow-legacy-syntax-objects? +A parameter that indicates whether the expander should support legacy +syntax objects, as described above. For ABI stability reasons, the +default is @code{#t}. Use @code{parameterize} to bind it to @code{#f}. +@xref{Parameters}. +@end deffn + Guile also offers some more experimental interfaces in a separate module. As was the case with the Large Hadron Collider, it is unclear to our senior macrologists whether adding these interfaces will result |