Exclamation symbol "!" within functions syntax

[Paul Backus]

On Monday, 1 August 2022 at 11:35:25 UTC, pascal111 wrote:
> I noticed that filter is using the concept of templates but 
> this time it's with a lambda function, not with a data type, 
> how can we explain this? isn't supposed to use a data type 
> after the exclamation mark: "auto r = chain(a, b).filter!(a => 
> a > 0);"
>
> https://dlang.org/phobos/std_algorithm_iteration.html#.filter.filter

If you look at the documentation for the filter template [1], you 
will notice that the "predicate" parameter is marked with the 
keyword "alias":

     template filter(alias predicate)

Alias template parameters are special. They can accept not only 
types, but also values, names of variables, and even things like 
modules and packages as arguments. You can read more about them 
in the "More Templates" chapter of Ali's book [2], and in the 
official D language specification [3].

[1] https://dlang.org/phobos/std_algorithm_iteration.html#.filter
[2] 
http://ddili.org/ders/d.en/templates_more.html#ix_templates_more.alias,%20template%20parameter
[3] https://dlang.org/spec/template.html#aliasparameters