ok
From the SETL Wiki
| ok | |
|---|---|
| Category | control |
| Syntax | nullary |
| Compatibility | |
| CIMS SETL | yes |
| SETL-S | no |
| SETL2 | no |
| GNU SETL | no |
Contents |
Purpose
Make a non-deterministic boolean choice
Synopsis
ok
Description
Simulates a non-deterministic choice (between returning true or false.)
ok works by effectively forking the current process into two sub-processes, with ok returning true in one and false in the other.
The two sub-processes are executed in sequence rather than in parallel. At first only the true branch executes immediately, with the false branch initially suspended. If control later reaches a fail statement, the true branch is terminated and the false branch begins executing.
These sub-processes are similar to unix processes in the fact that they both begin execution at the same point in the program, with copies of the same procedure call stack. However they are like threads in that all memory is shared between the two sub-processes by default (this can be overridden with a back declaration on a variable, causing each sub-process to get a private copy of that variable.)
ok is very similar to the AMB special form
Examples
- Generate single-word anagrams
dictionary := {'ascertain', 'ascertained', 'ascending', 'cartesian', 'cartwheel', 'certain', 'sect', 'sectarian', 'section'}; letters := 'aaceinrst'; words := {}; (while ok) word := ; (while exists letter = letters(i) | ok) word +:= letter; end; if letters = '' and word in dictionary then words with:= word; end if; fail; end; print(words);Output:
{ascertain cartesian sectarian}
Notes
Possible methods for implementing ok in a SETL compiler or interpreter include:
- Using the unix fork function, with the child process initially waiting until the parent has exited before it resumes execution. Explicitly allocating shared memory for variables that require it.
- Using continuations, either by translating to a target language that natively supports them or by translating SETL procedures into continuation passing style. Variables declared back would be pushed onto a special stack with each invocation of ok and then either restored on the following invocation of fail or discarded on invocation of succeed.
