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The static delimited continuation operators shift and reset.

Shift/Reset - Dynamically scoped shift/reset

(import shift-reset)

A "stuck on control" situation, a 'shift' without an enclosing 'reset', is an error.

reset

(reset EXPRESSION ...)syntax: Evaluate the body EXPRESSION ... with a delimited continuation. The body will contain one or more instances of (shift ...).

shift

(shift PC-TAG EXPRESSION)syntax: Within the scope of EXPRESSION PC-TAG is bound to the reified partial continuation delimited by the enclosing (reset ...). Provide a value to the partial continuation using the form (PC-TAG <something>).

reset-values

(reset-values EXPRESSION ...)syntax: Multiple value return version of (reset ...). The body will contain one or more instances of (shift-values ...).

shift-values

(shift-values PC-TAG EXPRESSION)syntax: Multiple value return version of (shift ...). Provide a value to the partial continuation using the form (PC-TAG <something> ...).

BShift/BReset - Statically scoped shift/reset

(import bshift-breset)

Invalid delimited continuations, what RC-TAG below represents, and stuck on control will generate an error

breset

(breset RC-TAG EXPRESSION ...)syntax: Evaluate the body EXPRESSION ... with a delimited continuation named RC-TAG. The body will contain one or more instances of (bshift RC-TAG ...).

bshift

(bshift RC-TAG PC-TAG EXPRESSION)syntax: Within the scope of EXPRESSION PC-TAG is bound to the reified partial continuation delimited by the enclosing (breset RC-TAG ...). Provide a value to the partial continuation using the form (PC-TAG <something>).

breset-values

(breset-values RC-TAG EXPRESSION ...)syntax: Multiple value return version of (breset ...). The body will contain one or more instances of (bshift-values RC-TAG ...).

bshift-values

(bshift-values RC-TAG PC-TAG EXPRESSION)syntax: Multiple value return version of (bshift ...). Provide a value to the partial continuation using the form (PC-TAG <something> ...).

Range

(import range)

range

(range RC-TAG FROM VALUE STEP TO?)syntax

The value of the delimited continuation RC-TAG ranges over the set of values specified by the state generation procedure suite. For use with (breset ...)

FROM: Zero argument procedure, returning the initial state
VALUE: Single argument procedure, of the state, returning the value of the state
STEP: Single argument procedure, of the state, returning the next state
TO?: Single argument procedure, of the state, returning {{#t) when the range is complete

range

(range RC-TAG FROM [STEP] TO)syntax: The value of the delimited continuation RC-TAG ranges over the number interval [FROM TO], by STEP. The increment is 1 when missing. For use with (breset ...).

Reflect/Reify - Monads

(import reflect-reify)

The Monad example from Filinski, POPL '94

define-unit

(define-unit KIND BODY ...)syntax: Expands to (define (KIND-unit obj) BODY ...).

define-bind

(define-bind KIND BODY ...)syntax: Expands to (define (KIND-bind monad func) BODY ...).

reflect

(reflect KIND MONAD)syntax: Extract value from MONAD. Plays the role of Haskell '<-'.

reflect-values

(reflect-values KIND MONAD)syntax: Extract value from MONAD. Plays the role of Haskell '<-'.

reify

(reify KIND EXPRESSION)syntax: Return result of EXPRESSION as a monad.

reify-values

(reify-values KIND EXPRESSION)syntax: Return result of EXPRESSION as a monad.

GShift/GReset - Generalized shift/reset

(import gshift-greset)

The generalized shift and reset operator family from How to remove a dynamic prompt: static and dynamic delimited continuation operators are equally expressible

greset

(greset HR E)syntax: Reset parameterized by the H Reset procedure HR.

gshift

(gshift HS F E)syntax: Shift parameterized by the H Shift procedure HS.

hr-stop

hr-stop Vprocedure: H Reset Stop.

hs-stop

hs-stop Vprocedure: H Shift Stop

hr-prop

hr-prop Vprocedure: H Reset Propagate.

hs-prop

hs-prop Vprocedure: H Shift Propagate.

h-compose

h-compose F Xprocedure: Returns the composition of F and X as an h-datatype.

h-value

h-value Vprocedure: Returns the value of V as an h-datatype.

h-datatype?

h-datatype? OBJECTprocedure: Is OBJECT an h-datatype?

h-cases

(h-cases E ((F X) ON-h-EXPR) (V ON-V-EXPR))syntax: Deconstructs the h-datatype E, binding F & X for an evaluation of the ON-h-EXPR and V for an evaluation of the ON-V-EXPR.

Delimited Control

(import delimited-control)

Generalized shift/reset implementations of some control operators.

Within the scope of EXPRESSION PP is bound to the reified partial continuation delimited by the enclosing (prompt/prompt0/reset/reset0 ...). Provide a value to the partial continuation using the form (PP <something>).

Examples

(import shift-reset srfi-41 srfi-45)

(define (iteration-procedure->stream iteration-procedure)
  (reset
    (iteration-procedure
     (lambda (element)
       (shift continue-iteration
         (stream-cons element (lazy (continue-iteration (void)))))))
    stream-null))

(define (my-list->stream list)
  (iteration-procedure->stream
   (lambda (receiver)
     (for-each receiver list))))

Notes

The reflect-reify module (Monads) is an example of the use of shift/reset. Not a general purpose implementation for use in Scheme monadic programming.
The distinction between the single-valued return & multi-valued return versions of the control operators, ex: shift & shift-values, is probably confusing.

Requirements

box datatype miscmacros

Bugs and Limitations

Single Thread Only
Not a direct implementation of partial continuations. Simulated using full continuations. As such may be prey to the issues elaborated by "call/cc implements shift? A good question".
The distinction between the single-valued return & multi-valued return versions of the control operators, ex: shift & shift-values. Unlike the CHICKEN 4 egg, the current single-valued syntax cannot handle multi-value returns, due to the type signature.

Author

Kon Lovett

Version history

4.1.0: Added delimited-control module.
4.0.0: CHICKEN 5 release.
3.0.0: Remove %reset, %shift, %reset-values, %shift-values, %breset, %bshift, %breset-values, %bshift-values, %range, %reflect, & %reify.
2.0.4: Has argc-checks.
2.0.3: Use er-macro-transformer.
2.0.2: Has lambda-info.
2.0.1: Added miscmacros as a dependency [Ivan Raikov].
2.0.0: Initial 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.

Does not supercede any restrictions found in the source code.

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