[jira] [Commented] (ARROW-13167) [C++] Type determination kernels ("type", "type_id")

["Antoine Pitrou (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ARROW-13167?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17368974#comment-17368974 ] 

Antoine Pitrou commented on ARROW-13167:
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I don't understand why this needs to be a kernel. This seems to be an abuse of the general idea of compute functions.

> [C++] Type determination kernels ("type", "type_id")
> ----------------------------------------------------
>
>                 Key: ARROW-13167
>                 URL: https://issues.apache.org/jira/browse/ARROW-13167
>             Project: Apache Arrow
>          Issue Type: Improvement
>          Components: C++
>            Reporter: Ian Cook
>            Priority: Major
>
> The Arrow C++ library exposes an API for determining the data type of an expression, but it is exposed as a method of the expression class and it requires that the user pass a schema as an argument to the method. This is inconvenient; for example, we have had to write some inconsistent code in the R bindings to make expression objects carry schemas along with them and then pass the schemas to derivative expressions, unifying schemas as needed for derivative expressions that take 2+ expressions as arguments.
> This would be much cleaner if we could use the kernel function calling interface to call a unary {{type_id}} function that would simply determine the type of its input datum and return a scalar integer value from the data type enum indicating the its data type. It would be convenient to also have a version of this that returned the string description of the data type; I think this could be named {{type}}.



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