# Title

Coroutine Generators

# Author

Marc Nieper-Wißkirchen

# Status

This SRFI is currently in final status. Here is an explanation of each status that a SRFI can hold. To provide input on this SRFI, please send email to `srfi-190@nospamsrfi.schemers.org`. To subscribe to the list, follow these instructions. You can access previous messages via the mailing list archive.

• Received: 2020-03-30
• Draft #1 published: 2020-03-30
• Finalized: 2020-06-11

# Abstract

This SRFI defines syntax to create SRFI 121/158 coroutine generators conveniently and in the flavor of Python generator functions.

# Rationale

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.

# Specification

## Syntax

`(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.

# Implementation

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)```.

# Acknowledgements

Credits go to the authors of SRFI 121 and SRFI 158, Shiro Kawai, John Cowan, and Thomas Gilray.

# Copyright

Copyright © Marc Nieper-Wißkirchen (2020).

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice (including the next paragraph) shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Editor: Arthur A. Gleckler