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

[Stanislav Blinov]

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`).