In their paper "The First Report on Scheme Revisited", the authors Sussman and Steele remarked
"We also now believe that Carl Hewitt was right: we would have been better off to have introduced cells as a separate, primitive kind of object, rather than allowing assignment to any and every lambda-bound variable."
This module implements the cell datatype as a procedure of zero or one argument. Using it instead of set! set-car! and set-cdr! you have what Hewitt proposed ...
- (simple-cells sym ..) procedure
Documentation procedure. Without argument, returns the list of exported symbols, with argument the documentation of that symbol.
- (cell var . tests) procedure
Constructor. Creates a cell as a procedure of zero or one argument, initialising its content to var provided all tests succedd. Without argument returns two values, the content of the cell and the list of test predicates. With argument, replaces the cell's content with that very argument, provided all tests succead.
- (cell? xpr) procedure
(use simple-cells simple-exceptions) (define o% (cell 5 number? odd?)) (cell? o%) ; -> #t (o%) ; -> 5 (condition-case (o% 4) ((exn arguments) #f)) ; -> #f (o%) ; -> 5 (define n% (cell 4 (named-lambda 3<=? (x) (<= 3 x)))) (n%) ; -> 4 (receive (val checks) (n%) checks) ; -> (<procedure (3<=? x)>) (n% 20) (n%) ; -> 20 (o%) ; -> 5 (receive (val checks) (o%) checks) ; -> (<procedure (number? x)> <procedure (odd? x)>)
Feb 17, 2017
Copyright (c) 2017, Juergen Lorenz All rights reserved.
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- dependency on simple-exceptions updated
- now working with simple-exceptions 0.5
- cells now return old content on change
- initial import