by John Cowan (text), Arvydas Silanskas (implementation)
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This is a set of convenience routines for generators and accumulators
intended to blend in with
The authors recommend that they be added to the
(srfi 158) library provided by
users or implementations.
If they are approved by the R7RS-large process,
they can also be added to
The procedures of this SRFI don't particularly cohere together, being just a collection of procedures devised since SRFI 158 was finalized. Consequently, this section provides a rationale for each procedure independently.The
gcompose-rightprocedures, in cooperation with generator constructors and operations (that is, procedures that accept one or more generators and return a generator) from SRFI 158 or elsewhere, make it easier to set up pipelines of generators, starting with an initial generator and adding generator operations to it to produce a desired result.
procedure captures the output of a generator
and passes it directly to an accumulator
until the generator is exhausted. The
generator can be a
which allows arbitrary transformations.
genumerate procedure attaches increasing
non-negative exact integers to the outputs of a generator.
Filtering the stream will still allow the original indices
to be recovered at a later stage.
gchoice operation uses one generator to dictate
which one of a number of other generators will be drawn on to
provide the next value. If used with the
make-categorical-generator procedure of
as the first argument, it can choose from the other generators
using a weighted probability.
conversion between generators and
SRFI 41 streams,
and indirectly between streams and
SRFI 127 lazy sequences,
which are either lists or improper lists
with a generator in their tails.
(gcompose-left constructor operation ...
(gcompose-right operation ... constructor
Creates a generator from a generator constructor plus
a chain of generator operations. The first/last argument is
called with no arguments, the remaining arguments with
the result of calling the previous/following argument.
(gcompose-left make-gen gfoo gbar) returns the same
(gbar (gfoo (make-gen))),
(gcompose-right gbar gfoo make-gen).
Since it is typically necessary to pass additional arguments to the constructor and the operations, the arguments will usually be lambda expressions. For example:
(gcompose-left (lambda () (make-iota-generator 100)) (lambda (g) (gfilter even? g)) (lambda (g) (ggroup g 5)))
returns a generator that outputs the values
(0 2 4 6 8),
(10 12 14 16 18), ...
(90 92 94 96 98).
(gcompose-right (lambda (g) (ggroup 5 g))) (lambda (g) (gfilter even? g)) (lambda () (make-iota-generator 100)).
Such calls can be written more compactly using the
cut macro from
(gcompose-left (cut make-iota-generator 100) (cut gfilter even? <>) (cut ggroup 5 <>))
Invokes generator, passing the value
it returns to accumulator. If the value was an end-of-file
accumulate-generated-values returns whatever the
accumulator returned. Otherwise, the process is repeated.
Creates a generator that returns pairs. The car of each pair is an exact integer counting up by 1 from 0. The cdr of each pair is the result of gen.
(gchoice choice-gen source-gen ...
Returns a generator g that behaves as follows:
When g is invoked, it first invokes choice-gen to return an index value i. It is an error if i is not an exact integer between 0 (inclusive) and the number of source-gens (exclusive). Then the ith source-gen is invoked and its value returned by g.
If choice-gen is exhausted, g returns an end-of-file object. If the chosen source-gen is exhausted, g remembers that fact and invokes choice-gen until it returns the index of a source-gen that has not been exhausted. When all source-gens are exhausted, g returns an end-of-file object.
from SRFI 158
performs round-robin selection of the source-gens.
Returns a generator that steps through the elements of a SRFI 41 stream.
Returns a SRFI 41 stream containing the values of a generator.
The sample implementation of this SRFI is in the repository for it.
Thanks to all who participated on the SRFI mailing list.
© 2020 John Cowan (text), Arvydas Silanskas (implementation).
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