Consider change udf function to lambda style

[sundy-li]

Current UDF style: CREATE FUNCTION cal = '@0 + @2 * (@4 / @1) - @3';

What if there is String literal inside?

CREATE FUNCTION cal = 'concat($0, \'33\' ) ';

Need escape.

What if we introduce lambda functions to traverse array or maps in the future?

SELECT arrayMap(x -> (x + 2), [1, 2, 3]) as res;

What's more, this function did not have function propertype.

In rust, |a, b| a + b + 3 .

We can have a similar function like Rust/ClickHouse.

CREATE FUNCTION name AS |parameter0, ...| -> expression

Or

CREATE FUNCTION name AS (parameter0, ...) -> expression

What's your opinion? cc @lianghanzhen

[BohuTANG]

Good idea, this will unify our udf and higher order function on usage for users.
By the way, we already have a plan to support Lambda functions in Roadmap2021 #746

[lianghanzhen]

As we discussed before, the only difference between ClickHouse and current implementation is implicit/explicit parameters. But I do agree that the explicit one would be much more intuitive. Here is the plan to achieve that:

1. Fix the escape issue, may be better parse an expression instead of literal string;
2. Support Lambda function style;

Please let me know what your thought is.

[sundy-li]

It's ok to go on as you suggest.

[lianghanzhen]

@sundy-li @BohuTANG Can you help review this MR of first step: #3684

[lianghanzhen]

@sundy-li @BohuTANG This MR should cover all steps.

[BohuTANG]

Sure, thank you!