[e-lang] Break, Continue, and Return

[Mark S. Miller]

Another draft note of a resolution for the next release:

                        Break, Continue, and Return

Prior to this release, the for and while loop shorthands would define 
variables named "break" and "continue" as ejectors for exiting the loop, or loop 
body respectively. By convention, when E programs needed an ejector for 
exiting a method body, they would name the variable holding this ejector 
"return". Code explicitly invoking these ejectors would obtain the 
conventional effect, but as Ping explains (see example #4 of 
http://www.eros-os.org/pipermail/e-lang/2002-June/006704.html ), a 
C-syntax-tradition programmer would expect that simply using any of these 
names without a following open paren would still cause the corresponding 
non-local exit. Of course, when these were just variable names, this did not 
occur.

As of this release, these three names are now keywords, to be used as 
shorthand for invocations of the corresponding variables __break, 
__continue, and __return. For example, the for loop

for i in 1..5 {
    println(i)
    break
}

now expands to 

for i in 1..5 {
    println(i)
    __break()
}

whose further expansion includes an "escape __break" which defines the 
ejector invoked above.

To support conventional syntax for return, any of these ejector keywords can 
be invoked with an argument without having to enclose the argument in 
parentheses. An example from E's libraries: 

/**
 * Is the predicate true for any member of the list?
 */
def anyOf(list, pred) :boolean {
    escape __return {
        for v in list {
            if (pred(v)) {
                return true
            }
        }
        false
    }
}

When exiting a loop with "break expr", the loop as a whole evaluates to the 
value of the expression.

Zooko and Dean have each made suggestions that would allow the programmer to 
omit the explicit "escape __return", but this release does not yet 
incorporate any such proposals. Some of these proposals may likewise require 
one to write "return false" rather than "false", but both work in this release.


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Text by me above is hereby placed in the public domain

        Cheers,
        --MarkM