Add support for f ∘ g

[pdeffebach]

This PR does not transform f(g(:x)) to (f ∘ g)(:x).

Rather, it ensures that the expression

:y = (f ∘ g)(:a, :b)

gets turned into

[:a, :b] => (f ∘ g ) => :y

which is a good first step for transforming f(g(x)).

I'm leaving this undocumented for now, since it's a behind-the-scenes transformation.

[pdeffebach]

@bkamins can I get an review? I have code ready based on this which will do the more substantive change of transforming f(g(:x, :y)) into (f ∘ g)(:x, :y).

[bkamins]

same here - I do not understand the benefit of @eval?

[pdeffebach]

As mentioned above, @eval is necessary because we need to evaluate in global scope to prevent caching the anonymous function in the method table.

[bkamins]

Do you think testing timing is enough? Maybe also generated code should be tested?

[pdeffebach]

We don't check generated code anywhere else. I think this should be enough.

[pdeffebach]

Nevermind it's easy to add. Added. Will merge after tests.

[bkamins]

Apart from the comments left I think it looks good. The code assumes that ∘ is not redefined by the user, but I think it is a valid assumption (though maybe it should be noted somewhere).

[pdeffebach]

No that's a good catch. Overall I'm not very good with macro hygiene in this package and should fix it.

[bkamins]

I am not sure who is good at macro hygene 😄 (e.g. I do not write macros often and in consequence it is really hard for me to do it properly when I do).

[pdeffebach]

Macro hygiene doesn't matter here, actually. Since the expression f ∘ g is just "moved" not changed in any way. And it is assumed to be callable both before and after the transformation.