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https://issues.apache.org/jira/browse/METRON-1277?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16223797#comment-16223797
]
ASF GitHub Bot commented on METRON-1277:
----------------------------------------
Github user ottobackwards commented on a diff in the pull request:
https://github.com/apache/metron/pull/814#discussion_r147567984
--- Diff: metron-stellar/stellar-common/README.md ---
@@ -100,6 +102,28 @@ In the core language functions, we support basic
functional programming primitiv
* `FILTER` - Filters a list by a predicate in the form of a lambda
expression. For instance `FILTER([ 'foo', 'bar'], (x ) -> x == 'foo' )`
returns `[ 'foo' ]`
* `REDUCE` - Applies a function over a list of input. For instance
`REDUCE([ 1, 2, 3], (sum, x) -> sum + x, 0 )` returns `6`
+### Stellar Language Match Expression
+
+Stellar provides the capability to write match expressions, which are
similar to switch statements commonly found in c like languages, but more like
+Scala's match.
+
+The syntax is:
+* `match{ logical_expression1 : evaluation expression1,
logical_expression2 : evaluation_expression2` : A match expression with no
default
+* `match{ logical_expression1 : evaluation expression1,
logical_expression2 : evaluation_expression2, default : default_expression}` :
A match expression with a default expression
+
+Where:
+
+* `logical_expression` is a Stellar expression that evaluates to true or
false. For instance `var > 0` or `var > 0 AND var2 == 'foo'`
--- End diff --
```bash
Please note that functions are loading lazily in the background and will be
unavailable until loaded fully.
[Stellar]>>> Functions loaded, you may refer to functions now...
[Stellar]>>> foo := 500
[Stellar]>>> match{ foo < 100 : THROW('oops'), foo > 200 : 'ok', default :
THROW('exception thrown') }
ok
[Stellar]>>>
```
I don't see that.
Also, if you check the last test in TestMatch you will see I have a test
for this kind of thing.
> 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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