The following program does not compile:

>import std.stdio;
>import std.algorithm;
>
>struct A
>{
>    struct S
>    {
>        int x;
>    }
>
>    const bool foo(in S s, in int k)
>    {
>        return s.x<k;
>    }
>
>    void bar()
>    {
>        [S(1),S(2),S(3),S(4),S(5)].filter!(a=>a.foo(3)).writeln;
>    }
>}
>
>void main()
>{
>    A().bar();
>}

I get (using rdmd):

>test.d(18): Error: no property foo for type S
>/usr/include/dmd/phobos/std/algorithm/iteration.d(1108):        instantiated from here: >FilterResult!(__lambda1, S[])
>test.d(18):        instantiated from here: filter!(S[])

When I replace the UFCS-Syntax in the lambda by a function call it works:

> [S(1),S(2),S(3),S(4),S(5)].filter!(a=>foo(a,3)).writeln;

Where is my mistake?

On Sunday, 16 September 2018 at 09:46:15 UTC, berni wrote:
> Where is my mistake?

Lambdas are not the issue here. The problem is more general: you can only use top-level symbols in UFCS.

You can use an identity alias template to bypass this:
https://blog.thecybershadow.net/2015/04/28/the-amazing-template-that-does-nothing/
(search for UFCS in the page).

> The problem is more general: you can only use top-level symbols in UFCS.
>
> You can use an identity alias template to bypass this:
> https://blog.thecybershadow.net/2015/04/28/the-amazing-template-that-does-nothing/
> (search for UFCS in the page).

Good to know. :-)

On Sunday, 16 September 2018 at 10:55:43 UTC, berni wrote:
>> The problem is more general: you can only use top-level symbols in UFCS.
>>
>> You can use an identity alias template to bypass this:
>> https://blog.thecybershadow.net/2015/04/28/the-amazing-template-that-does-nothing/
>> (search for UFCS in the page).
>
> Good to know. :-)

Worth noting, this is now included in Phobos as `std.meta.Alias`.