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This SRFI defines syntax to create SRFI 121/158 coroutine generators conveniently and in the flavor of Python generator functions.
In some sense, the most fundamental generator is the coroutine
generator. SRFI 121/158 allow the easy creation of a coroutine
generator by calling the
procedure make-coroutine-generator.
For example, the following evaluates to a generator that produces the series 0, 1, and 2:
(make-coroutine-generator
(lambda (yield)
(do ((i 0 (+ i 1)))
((<= 3 i))
(yield i)))))
These generators are similar to the generator functions of Python.
When using make-coroutine-generator, though, the
programmer has to wrap the actual coroutine body in a lambda and
has to explicitly bind the name of the yielding procedure.
In this SRFI, a syntax that evaluates to coroutine generators is defined that simplifies the creation of coroutine generators and avoids the need of binding the yielding procedure to a name.
Using the syntax of this SRFI, the above example becomes:
(coroutine-generator
(do ((i 0 (+ i 1)))
((<= 3 i))
(yield i)))
The yielding procedure can be passed to helper procedures.
(let ((yield-square (lambda (yield i) (yield (* i i)))))
(coroutine-generator
(do ((i 0 (+ i 1)))
((<= 3 i))
(yield-square yield i))))
An equivalent version using a local macro would be:
(let-syntax ((yield-square (syntax-rules () ((_ i) (yield (* i i))))))
(coroutine-generator
(do ((i 0 (+ i 1)))
((<= 3 i))
(yield-square i))))
A Python generator function is not completely equivalent to a coroutine generator in the sense of SRFI 121/158 and this SRFI as a Python generator function has to be called first to return a generator. This SRFI includes convenience syntax to define a generator function in the sense of Python.
The following defines a procedure that returns a generator producing the series 0, 1, …, n − 1.
(define-coroutine-generator (g n)
(do ((i 0 (+ i 1)))
((<= n i))
(yield i)))
All syntax defined in this SRFI is hygienic. In particular, the
identifier yield is exported (and bound to syntax) by
any library implementing this SRFI.
While the sample implementation uses syntax parameters and
identifier macros, a specific implementation that is being shipped
by a particular Scheme system doesn't have to. In fact, this is a
reasonable thing for those systems for which
make-coroutine-generator from SRFI 121/158 is slow
because call/cc is slow in these systems (unfortunately,
such systems exist). As coroutine-generator SRFI 190 is
just syntax and offers no more than Python's generator functions (note
that Python doesn't have call/cc), those systems can
translate it directly into fast code that does not have to rely on
first-class continuations. For this translation to be easy (and no
more difficult than that what CPython has to do), it is crucial that
yield is lexically scoped (and thus confined to the
expanded body of the coroutine) and not dynamically bound.
(coroutine-generator 〈body〉)
Creates a generator from a coroutine. When evaluated, immediately
returns a generator g. When g is called, the
definitions and expressions in 〈body〉 are evaluated
until the yielding procedure of the coroutine generator is called.
Calling the yielding procedure of the coroutine generator causes the
evaluation of 〈body〉 to be suspended,
and g returns the value passed to yield.
Whether this generator is finite or infinite depends on the
behavior of 〈body〉. If the last expression
in 〈body〉 returns, it is the end of the sequence —
g
returns an end-of-file object from then on.
yield
Evaluates to the yielding procedure in the (expansion of the)
〈body〉 of a coroutine generator.
It is an error to evaluate yield outside the body of
a coroutine generator.
(define-coroutine-generator 〈name〉 〈body〉)
(define-coroutine-generator (〈name〉 . 〈formals〉) 〈body〉)
Expands into
(define 〈name〉 (coroutine-generator 〈body〉))and
(define (〈name〉 . 〈formals〉) (coroutine-generator 〈body〉))
respectively.
The following implementation based on SRFI 121/158 makes use of syntax parameters as defined in SRFI 139 and identifier syntax as defined in the R6RS.
(define-syntax-parameter yield
(lambda (stx)
(syntax-case stx ()
(_ (error "yield used outside coroutine generator" stx)))))
(define-syntax coroutine-generator
(lambda (stx)
(syntax-case stx ()
((_ . body)
#'(make-coroutine-generator
(lambda (%yield)
(syntax-parameterize ((yield (identifier-syntax %yield)))
. body)))))))
(define-syntax define-coroutine-generator
(lambda (stx)
(syntax-case stx ()
((_ name . body) #'(define name (coroutine-generator . body))))))
Any implementation will define the identifiers yield,
define-coroutine-generator,
and coroutine-generator in the library (srfi
190).
Credits go to the authors of SRFI 121 and SRFI 158, Shiro Kawai, John Cowan, and Thomas Gilray.
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