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

["zanmato1984 (via GitHub)" <gi...@apache.org>]

zanmato1984 commented on issue #41926:
URL: https://github.com/apache/arrow/issues/41926#issuecomment-2162943019

   > In arrow compute, `MetaFunction` means some "sepcial function", which "dispatch implementation to other impl. And the introduced pr is here: [199d089](https://github.com/apache/arrow/commit/199d089e6343df1a1aa95f75b9e99e05a2702257)
   
   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
   https://github.com/apache/arrow/blob/383ffe06075c5b66e2c7a37bfaf91a84aeb453d2/cpp/src/arrow/compute/cast_internal.h#L35
   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:
   https://github.com/apache/arrow/blob/383ffe06075c5b66e2c7a37bfaf91a84aeb453d2/cpp/src/arrow/compute/cast.cc#L49
   
   >  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.


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