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Splicing binding constructs for syntactic keywords are versions
of let-syntax and letrec-syntax that can
be used in a definition context in the same way
as begin.
The R7RS defines two binding constructs for syntactic keywords,
namely let-syntax and letrec-syntax. Both
are equivalent to their R5RS versions and are expressions.
The R6RS versions of let-syntax
and letrec-syntax are equivalent to the R5RS/R7RS
version when they are not in a definition context. In a definition
context, however, the R6RS versions splice their body forms in the
enclosing definition context.
In some sense, the R6RS splicing versions are more fundamental than the
R5RS/R7RS non-splicing versions as one can easily define the
non-splicing versions by way of a syntax-rules macro in
terms of the splicing versions. Moreover, the non-splicing versions
can also be defined by way of a syntax-rules macro in
terms of the local define-syntax.
On the other hand, the splicing versions cannot be defined through
the non-splicing versions or define-syntax.
The usefulness of the splicing versions seems to be
non-controversial. For
example, SRFI
148's em syntax would become unnecessary.
Thus, this SRFI proposes splicing versions
of let-syntax and letrec-syntax for R7RS
under a different name. Note, however, that, in some corner cases,
the splicing versions cannot be a drop-in replacement for the
non-splicing forms (only that the non-splicing forms can be defined
as derived syntax in terms of the splicing forms).
The following examples show the difference between the non-splicing and splicing behavior.
This expression evaluates to let-syntax:
(let ((x 'let-syntax))
(let-syntax ()
(define x 'splicing-let-syntax)
#f)
x)
This expression evaluates to splicing-let-syntax:
(let ((x 'let-syntax))
(splicing-let-syntax ()
(define x 'splicing-let-syntax)
#f)
x)
For more examples, see the test suite that comes bundled with this specification.
(splicing-let-syntax ((〈keyword〉 〈transformer spec〉) ...)
〈form〉 ...)
Like (let-syntax ((〈keyword〉 〈transformer spec〉) ...)
〈form〉 ...) except that in a definition context, the forms
〈form〉 ... are spliced into the enclosing definition context (in the
same way as for begin).
Note: The R6RS version is a bit more restricted. There, in a definition context,
the forms 〈form〉 ... all have to be definitions and, in an expression
context, the forms 〈form〉 ... all have to expressions.
(splicing-letrec-syntax ((〈keyword〉 〈transformer spec〉) ...)
〈form〉 ...)
Like (letrec-syntax ((〈keyword〉 〈transformer spec〉) ...)
〈form〉 ...) except that in a definition context, the forms
〈form〉 ... are spliced into the enclosing definition context (in the
same way as for begin).
Note: The R6RS version is a bit more restricted. There, in a definition context,
the forms 〈form〉 ... all have to be definitions and, in an expression
context, the forms 〈form〉 ... all have to expressions.
A portable R7RS implementation of the binding constructs described here is not possible.
The splicing binding constructs as described here are at least provided by Chibi Scheme, Chez Scheme, and Racket.
A Chibi Scheme implementation (.tgz, Github)
together with a SRFI 64 test suite is bundled with this
specification.
The constructs described here aren't new but go back to the R4RS/R5RS (in the non-splicing version) and the R6RS (in the splicing version). Thanks go to the editors of the (revised) reports on the algorithmic language Scheme.
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