Predicates Within Strings

[jfondren]

On Monday, 14 June 2021 at 15:01:01 UTC, Justin Choi wrote:
> Could somebody explain or point me to documentation that helps 
> to explain the usage of strings in predicates?
> My main question is how D infers the omitted variable 
> specifications given otherwise - for example:
> `filter!(a => a < 3)(arr);`
> and
> `filter!"a < 3"(arr);`
> produce the same result.

The strings are just strings. If you look at the implementation 
for
filter you can trace the strings getting to 
std.functional.unaryFun
and friend, which pulls the string apart at compile-time to
construct a function. I think this system is pretty cool (terse 
and
clear in usage, and a good demonstration of static parameters in
templates) but they have confusing limits since the function is
built inside std.functional and not where the user is.

That has nothing to do with your main question though. Function
literals are documented in
https://dlang.org/spec/expression.html#FunctionLiteral which just
these remarks:

- If the type of a function literal can be uniquely determined 
from
its context, the parameter type inference is possible.

- If the function literal is assigned to an alias, the inference 
of
the parameter types is done when the types are needed, as the
function literal becomes a template.

I don't see a lot of detail about the type inference itself.