[C++] Compute: Why here arrow::compute has MetaFunction

[mapleFU]

Describe the usage question you have. Please include as many useful details as possible.

In arrow compute, MetaFunction means some "sepcial function", which "dispatch implementation to other impl. And the introduced pr is here: 199d089

However, some function like "CAST" also has correspond kernel, and it just works like a normal function. Besides, why should it has a MetaFunction rather rathan a function with some "Match Any" Kernels?

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[zanmato1984]

In arrow compute, MetaFunction means some "sepcial function", which "dispatch implementation to other impl. And the introduced pr is here: 199d089

As I understand it, a meta function gives you a structural way to bypass all the routines to actually invoke a regular function: executor creation, dispatching (kernel lookup), initialization, etc. So it's suitable for things like checking input "shape" and dispatch to concrete overloads (CC/CA/AC/etc.), or any other thing that you feel best to implement a function without using kernels.

However, some function like "CAST" also has correspond kernel, and it just works like a normal function.

IIUC, cast, as a meta function, has no kernel. What you saw might be subclasses of

arrow/cpp/src/arrow/compute/cast_internal.h

Line 35 in 383ffe0

class CastFunction : public ScalarFunction {

What is a arguably strange though, is that these concrete cast_xxx functions are not registered in the global function registry. Instead, cast meta function manages them using an internal registry-like structure:

arrow/cpp/src/arrow/compute/cast.cc

Line 49 in 383ffe0

std::unordered_map<int, std::shared_ptr<CastFunction>> g_cast_table;

Besides, why should it has a MetaFunction rather rathan a function with some "Match Any" Kernels?

Well, I would say that probably is technically doable. I think people just chose one.

[mapleFU]

I understand MetaFunction is "tiny function without kernel and not evolved in scalar eval", however, "cast" seems to be a special case, which also has kernel as internal implementation and would be bound in scalar eval. I don't know why it's design like that

[thisisnic]

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