what's meaning of "(a) =>"

[step8]

On Saturday, 5 February 2022 at 15:17:05 UTC, Stanislav Blinov 
wrote:
> On Saturday, 5 February 2022 at 15:10:19 UTC, step8 wrote:
>> I'm trying to study D programming
>> Following is code from vibe's example(web_ajax) code:
>> ~~~~
>> void getDataFiltered(Fields field, string value)
>> {
>> 	auto table = users.filter!((a) => value.length==0 || 
>> a[field]==value)().array();
>> 	render!("createTable.dt", table)();
>> }
>> ~~~~
>> I can't understand the expression　"(a) =>"，there is no 
>> defination of "a",how does this work?
>> thanks for help
>
> `(a) => value.length==0 || a[field]==value`
>
> is a https://dlang.org/spec/expression.html#FunctionLiteral 
> (see e.g. #10 there).
>
> This one is polymorphic, somewhat equivalent to defining a 
> function such as
>
> `bool func(T)(T a) { return value.length==0 || a[field]==value; 
> }`
>
> (assuming `value` is accessible to `func`).

Thanks very much.I think i kind of get that now.