As a language and runtime environment, occam provides low-cost switching
between multiple processes. Continuations -- such as those provided by
Scheme's call/cc -- are used to implement this in other languages. We
could thus experiment with providing continuations in occam; this should
be relatively low-cost, at least for a subset implementation.
An obvious use would be to avoid deeply-nested control structures. For example, it's not uncommon to see code like this in bits of RMoX that are trying to obtain multiple resources:
try.to.obtain (resource.1, ok)
IF
ok
SEQ
try.to.obtain (resource.2, ok)
IF
ok
SEQ
...
TRUE
die ("resource.2 failed")
TRUE
die ("resource.1 failed")
With an extremely limited form of continuations -- equivalent
approximately to C's setjmp -- we could do this:
MOBILE []BYTE var:
CONTINUE cont TO var
SEQ
try.to.obtain (resource.1, ok)
IF
ok
SKIP
TRUE
cont ("resource.1 failed")
try.to.obtain (resource.2, ok)
IF
ok
SKIP
TRUE
cont ("resource.2 failed")
die (var)
The CONTINUE block has two children: a block to run first, and another
to run if the continuation is called. This would of course be cleaner
with a more concise one-branch IF syntax, but it does at least avoid the
deep nesting when many such resources must be obtained.
The one-branch IF would look like this:
IF condition
process