file: job.t package: muf status: alpha
When writing nontrivial code, one sometimes needs to abort a fairly deeply nested set of function calls.
One way of handling this problem is by having each function return a flag saying "yes, I'm aborting," or "no, I'm not aborting," and then having every function-call check this flag an conditionally propagate the abort. However, code written this way is tedious to code, ugly to read, and prone to errors. Code with the single purpose of implementing a non-local exit winds up scattered all through the program, which is poor separation of concerns and just plain poor software design.
C solves this problem via setjmp() and
longjmp(). Lisp implements a similar but more
sophisticated utility via catch and throw,
recently adopted by ANSI Forth. Muq implements a
similar catch/throw pair:
anyconst catch{ ... [ anyargs | anyconst ]throw ... }
Catch accepts one anyconst argument: It only responds
to ]throws given a matching anyconst value.
(This makes selective ]throwing possible, in which a
]throw bypasses several catch clauses to reach
a preferred one deeper down on the execution stack.)
Catch then executes the brace-enclosed clause; If the
clause complete normally, catch returns a nil value
on top of an empty block.
If a matching ]throw is executed in the clause,
catch pushes a true value on top of the block
returned by ]throw.
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