Otto Fowler created METRON-1277:
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Summary: 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:
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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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