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srfi-45

Primitives for Expressing Iterative Lazy Algorithms

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srfi-45
Documentation
1. lazy
2. lazy-strict
3. eager
4. force
5. delay
6. lazy-promise?
7. eager-promise?
8. recursive-promise?
9. promise?
Usage
Notes
Requirements
Author
Version history
License

Documentation

Extends the interpretation of a promise to include lazy and eager promises.

For more information see SRFI 45.

lazy

[syntax] (lazy EXPRESSION) => promise

Returns a promise for EXPRESSION, which must evaluate to a SRFI 45 promise. (See lazy-strict.}

The promise will be iteratively forced, overwriting the promise at each step before the next iteration, so as to avoid storage leaks.

lazy-strict

[procedure] (lazy-strict [STRICT?]) => boolean

The body EXPRESSION of a lazy promise must produce a result of type promise according to SRFI 45. This is the default behavior of this implementation. To accept a more lenient interpretation call with a value of #f for STRICT?.

The "improper use of `lazy'" error from force serves notice that a lazy expression was not of the correct type.

eager

[syntax] (eager EXPRESSION) => promise

Returns a promise but immediately evaluates EXPRESSION.

force

[procedure] (force PROMISE) => OBJECT

Returns the result of the evaluation of PROMISE. When PROMISE is an R5RS promise an R5RS force is performed and when PROMISE is not a promise it is the result.

delay

[syntax] (delay EXPRESSION) => promise

Returns a SRFI-45 promise, a delayed evaluation of EXPRESSION.

lazy-promise?

[procedure] (lazy-promise? OBJECT) => boolean

eager-promise?

[procedure] (eager-promise? OBJECT) => boolean

recursive-promise?

[procedure] (recursive-promise? OBJECT) => boolean

Is the OBJECT a recursive-promise; an eager or lazy promise?

promise?

[procedure] (promise? OBJECT) => boolean

Some kind of promise? An R5RS promise or a recursive-promise.

Usage

(module foo (...)
  ; Allow access to the original API and stop those annoying
  ; redefined messages.
  (import
    (rename scheme (force r5rs:force) (delay r5rs:delay))
    (rename chicken (promise? r5rs:promise?))
    ...)
  (use srfi-45)
  ... code that can use R5RS and SRFI 45 promises ...
)

Notes

Supports multiple values for an eager promise and when (lazy-strict) is #f for a lazy promise.
If compiled with the feature srfi-45-paranoia defined then extra sanity checks are enabled.
The built-in routines are not rebound. This is purely a module implementation. As such the built-in promise? will not detect a recursive-promise as a promise. Do not allow these abstractions to leak into unsuspecting contexts.

Requirements

None

Author

Kon Lovett

Version history

3.1.0: Added lazy-strict and compile-time feature srfi-45-paranoia. Better R5RS promise support by lazy.
3.0.0: Redefines delay. Removed d-lay and recursive-delay.
2.2.0: Bug fix for lazy R5RS promise in force. [Reported by Derrell Piper]
2.1.0: Bug fix for too eager force. Removed "box" extension dependency.
2.0.0: Chicken 4 release.

License

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 shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED ASIS, 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.