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Remove section on 'note' expressions, they won't exist.

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Graydon Hoare 2012-10-09 20:05:10 -07:00
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@ -2182,54 +2182,6 @@ the *failing* state, a task unwinds its stack, destroying all frames and
freeing all resources until it reaches its entry frame, at which point it
halts execution in the *dead* state.
### Note expressions
~~~~~~~~{.ebnf .gram}
note_expr : "note" expr ;
~~~~~~~~
**Note: Note expressions are not yet supported by the compiler.**
A `note` expression has no effect during normal execution. The purpose of a
`note` expression is to provide additional diagnostic information to the
logging subsystem during task failure. See [log
expressions](#log-expressions). Using `note` expressions, normal diagnostic
logging can be kept relatively sparse, while still providing verbose
diagnostic "back-traces" when a task fails.
When a task is failing, control frames *unwind* from the innermost frame to
the outermost, and from the innermost lexical block within an unwinding frame
to the outermost. When unwinding a lexical block, the runtime processes all
the `note` expressions in the block sequentially, from the first expression of
the block to the last. During processing, a `note` expression has equivalent
meaning to a `log` expression: it causes the runtime to append the argument of
the `note` to the internal logging diagnostic buffer.
An example of a `note` expression:
~~~~{.xfail-test}
fn read_file_lines(path: ~str) -> ~[~str] {
note path;
let r: [~str];
let f: file = open_read(path);
lines(f) |s| {
r += ~[s];
}
return r;
}
~~~~
In this example, if the task fails while attempting to open or read a file,
the runtime will log the path name that was being read. If the function
completes normally, the runtime will not log the path.
A value that is marked by a `note` expression is *not* copied aside
when control passes through the `note`. In other words, if a `note`
expression notes a particular `lval`, and code after the `note`
mutates that slot, and then a subsequent failure occurs, the *mutated*
value will be logged during unwinding, *not* the original value that was
denoted by the `lval` at the moment control passed through the `note`
expression.
### Return expressions