[
https://issues.apache.org/jira/browse/METRON-1277?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16223586#comment-16223586
]
ASF GitHub Bot commented on METRON-1277:
----------------------------------------
Github user jjmeyer0 commented on a diff in the pull request:
https://github.com/apache/metron/pull/814#discussion_r147556583
--- Diff:
metron-stellar/stellar-common/src/main/java/org/apache/metron/stellar/common/StellarCompiler.java
---
@@ -105,59 +107,77 @@ public Object apply(ExpressionState state) {
Deque<Token<?>> instanceDeque = new ArrayDeque<>();
{
boolean skipElse = false;
+ boolean skipMatchClauses = false;
Token<?> token = null;
for (Iterator<Token<?>> it = getTokenDeque().descendingIterator();
it.hasNext(); ) {
token = it.next();
//if we've skipped an else previously, then we need to skip the
deferred tokens associated with the else.
- if(skipElse && token.getUnderlyingType() == ElseExpr.class) {
- while(it.hasNext()) {
+ if (skipElse && token.getUnderlyingType() == ElseExpr.class) {
+ while (it.hasNext()) {
token = it.next();
- if(token.getUnderlyingType() == EndConditional.class) {
+ if (token.getUnderlyingType() == EndConditional.class) {
break;
}
}
skipElse = false;
}
+ if (skipMatchClauses && (token.getUnderlyingType() ==
MatchClauseEnd.class
+ || token.getUnderlyingType() == MatchClauseCheckExpr.class))
{
+ while (it.hasNext()) {
--- End diff --
This will only skip one match clause after a true one is found. See below
test for a stellar statement that fails with a `ParseException`.
```java
@Test
@SuppressWarnings("unchecked")
public void moreThanTwoConsecutiveTrueCasesWillResultInFailure() {
Assert.assertEquals("ok", run("match{ foo < 100 : THROW('oops'), foo >
200 : 'ok', foo > 300 : 'ok', default : 'works' }",
new HashMap() {{
put("foo", 500);
}}));
}
```
> STELLAR Add Match functionality to language
> -------------------------------------------
>
> Key: METRON-1277
> URL: https://issues.apache.org/jira/browse/METRON-1277
> Project: Metron
> Issue Type: New Feature
> Reporter: Otto Fowler
> Assignee: Otto Fowler
>
> From dev list:
> ------------
> Hi All,
> It's high time that Stellar supports some form of conditional that is
> beyond if/then/else. Right now, the way to do fall-through conditionals is:
> if x < 10 then 'info' else if x >= 10 && x <= 20 then 'warn' else 'critical'
> That becomes non-scalable very quickly. I wanted to facilitate a
> discussion with the community on the syntax. I'll give a few options and
> you guys/gals can come up with your own suggestions too, but I wanted to
> frame teh conversation.
> *MAP-BASED SWITCH*
> With the advent of METRON-1254 (https://github.com/apache/metron/pull/801),
> we could enable (from a language perspective in Stellar) multi-part
> conditionals or switch/case style statements. To wit:
> MAP_GET(true, { x < 10 : 'info', x >= 10 && x <= 20 : 'warn', x > 20 :
> 'critical' })
> Or, with a convenience function:
> CASE( { x < 10 : 'info', x >= 10 && x <= 20 : 'warn', x > 20 : 'critical' }
> )
> The issue with this is that the last true condition wins because we're
> using a map.
> *LIST-BASED SWITCH*
> We could correct this by adding a list of pairs construction to stellar:
> CASE( [ x < 10 : 'info', x <= 20 : 'warn'], 'critical')
> This would enable us to allow the first true condition to win, so the
> second condition can be simpler and we could pass a default return value as
> the final argument.
> The downside to this, is that it requires a language enhancement (the list
> of pairs construction you see there).
> *LAMBDA FUNCTION-BASED SWITCH*
> Some of the problems with the previous statements are that every
> conditional has to be evaluated and there is no opportunity to short
> circuit. They're all evaluated at parse-time rather than execution time.
> We could, instead, construct a lambda function approach to this and support
> short-circuiting in even complex conditionals:
> CASE( real_variable_name, [ x -> x < 10 ? 'info', x -> x <= 20 ? 'warn' ],
> 'critical')
> or
> CASE( real_variable_name, [ x -> if x < 10 then 'info', x -> if x <= 20
> then 'warn' ], 'critical')
> This would require lessening ?: (if/then/else) syntax to support to enable
> just if without else conditions. This also has the benefit of allowing
> simplifying the expression due to lambda function variable renaming
> (real_variable_name can be much more complex (or even an expression) than
> 'x'.
> Creative other approaches to this are appreciated!
> Thanks,
> Casey
> ----------------
> and ->
>
> How about this:
> match(VAR_TO_VAL_ASSIGNMENT+) { BOOLEAN_STATEMENT(VALS) : LAMBDA(VALS),
> BOOLEAN_STATEMENT(VALS) : LAMBDA(VALS) , LAMBDA(VALS)}
> * match = new keyword
> * match takes variable number of assignments, where the val assigned to is
> available in the evaluation and the lambdas
> * match {} contains comma separated list of a statement that evaluates to a
> boolean and a lambda
> * LAMBDA is executed on match, and it’s value is returned
> * no matches returns null or return of optional final statement, which is a
> LAMBDA without a BOOLEAN_STATEMENT
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