Description:
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Usually we can get from user (from GET/POST parsers) anything.
Yes, usually we use validators.

But even validators have to know - what is it.

Is it an array? (usually building collections, iterate and so on -> that have numeric incremental keys ordered by natsort without misses)

Is it an list? (usually PUT requests to batch upload by keys -> that have numeric non-incremental keys non-ordered, can miss some keys)

Is it an assoc? (usually grouping with arguments unpacking - you can pass string, array, array of strings, assoc where strings is key and value are bool -> that have at least one string key)

Is it an dict? (usually building an Entity object -> that have only string keys)