Add spy-arg-matcher for more complex matching of spy arguments

[Fuco1]

More matchers to add for inspiration for example from Mockery: http://docs.mockery.io/en/latest/reference/argument_validation.html?highlight=matchers#additional-argument-matchers

[snogge]

I like the idea, but I don't like the syntax. It would be cool if we could use pcase patterns for this. That would probably require a new matcher that receives the argument patterns in a list. The hardest part is probably to figure out a good name for the new matcher 😃

[Fuco1]

I would argue against pcase or at least that we provide other syntax. Many people including myself find the pcase syntax quite complicated to read.

That would probably require a new matcher that receives the argument patterns in a list.

Something like adding :args-to-match-pattern ? I kept the existing matcher for "backward" compatibility so it's completely opt-in. A new matcher would also achieve that of course.

[snogge]

The current :to-have-been-called-with expects the arguments as &rest:

(expect 'foo :to-have-been-called-with 1 2 3)

That makes it difficult to make a pattern matching matcher, how do I see that

(expect 'foo :to-have-been-called-with 1 'predfunc 3)

is supposed to use predfunc as a predicate instead of expecting that symbol as an argument? Hard to change in a backward compatible way.
A new matcher could use

(expect 'foo :to-have-been-called-matching '(1 2 3))
(expect 'foo :to-have-been-called-matching 'predfunc)

and easily see that predfunc shall be used as a matching predicate. If the single argument to :to-have-been-called-matching is assumed to be a pattern (when it is a list) we could do more with less verbose syntax.

I agree that pcase has an ... interesting pattern language, but it is there to use which is a plus.

[Fuco1]

Well, you wouldn't say 'predfunc you would say (spy-arg-matcher #'predfunc). That's why I wrap the arguments in the spy-arg-matcher wrapper. If you pass anything which is not this structure, it is taken as a value to equal with. Otherwise the matcher's predicate is called on that position's argument. The mockery docs explain this in more detail, it's basically the same API.