Implement string list concatenation

[ttnghia]

Given a lists column of strings (each row is a list of strings), this PR facilitates the concatenation of strings within each list.

For example:

s = [ {'aa', 'bb', 'cc'}, null, {null, 'dd'}, {'ee', 'ff'} ]
r = strings::concatenate_list_elements(s, '+++') 
r is ['aa+++bb+++cc', null, null, 'ee+++ff']

This PR is similar to Spark's concat_ws, and closes #7727.

[harrism]

Given a lists column of strings (each row is a list of strings), this PR facilitates the concatenation of strings within each list.

For example:

s = [ {'aa', 'bb', 'cc'}, null, {null, 'dd'}, {'ee', 'ff'} ]
concatenate_rows(s, "+++") = ['aa+++bb+++cc', null, null, 'ee+++ff']

This PR also partially supports Spark's concat_ws (closes #7727).

I think the name concatenate_rows is wrong for this API behavior. The behavior demonstrated above is concatenating the strings within each row into a single string. It is not concatenating multiple rows of a single column into one row, or concatenating corresponding rows of multiple columns into one row of an output column. Either of those might be called concatenate_rows.

I would call this concatenate_strings or concatenate_strings_list or something.

[ttnghia]

I think the name concatenate_rows is wrong for this API behavior. The behavior demonstrated above is concatenating the strings within each row into a single string. It is not concatenating multiple rows of a single column into one row, or concatenating corresponding rows of multiple columns into one row of an output column. Either of those might be called concatenate_rows.

I would call this concatenate_strings or concatenate_strings_list or something.

Thanks Mark. The name concatenate_strings_list sounds good to me. Jake has also suggested the name concatenate_list_elements and I have adopted it (sorry that I didn't update the API name in the example).

So, now we have one more name suggestion. For me, concatenate_strings_list would sound better. Unless somebody objects to it, I'm going to change the API name to it.

[harrism]

concatenate_list_elements works for me and I like it better than my suggestion. However, will there ever be a type stored in a list other than strings that can be concatenated?

[ttnghia]

concatenate_list_elements works for me and I like it better than my suggestion. However, will there ever be a type stored in a list other than strings that can be concatenated?

Here the API is part of the strings namespace and it only works for strings type.

However, it seems that concatenate_list_elements is a good fit (so I will keep it), because in the future we may have other concatenate_list_elements APIs in other namespaces, such as an API for concatenating lists within a list (something that is similar to list unravel here https://nvidia.slack.com/archives/CB9GTQZAQ/p1618926053144000).

[davidwendt]

This is looking good.

[ttnghia]

@gpucibot merge