[hail] matrix table tail

[iitalics]

stacked on #7386

• implements .tail method on MatrixTable

    def tail(self, n: Optional[int], n_cols: Optional[int] = None) -> 'MatrixTable':
        """Subset matrix to last `n` rows.

        ....

        Parameters
        ----------
        n : :obj:`int`
            Number of rows to include (all rows included if ``None``).
        n_cols : :obj:`int`, optional
            Number of cols to include (all cols included if ``None``).
        Returns
        -------
        :class:`.MatrixTable`
            Matrix including the last `n` rows and last `n_cols` cols.
        """

question: should the interface have the same backwards compatibility naming issue as .head, to preserve consistency? or should the keyword arguments be n_cols and n_rows?

[catoverdrive]

Can I ask you to add the same scan test here for rows and cols? This will mostly help protect against us trying to introduce an optimization that inadvertently breaks scans, which we've done in the past for e.g. filter intervals.

[iitalics]

of course. added.

[iitalics]

(rebased)

[johnc1231]

Can you explain what is going on here to me? Everything looks good to me other than this file, which is probably fine but confuses me because I've never looked at lowering stuff. From pattern matching against head it seems right though. I think if I understood what invoke("[*:]" meant that would clear this up enough for me to accept

[johnc1231]

Also have never seen that symbol literal thing before.

[iitalics]

invoke(name, ...) creates an apply IR to call one of the functions in the registry.

[*:], [:*], and [*:*] are the functions for slicing. for instance, [*:*] gets called when you do an expression like foo[start:end] in python to get a subsequence of foo. [:*] is when you leave out the start index (so it is implicitly 0), and [*:] is when you leave out the end index (so it is implicitly the length of the array).

if an index is negative, then it means that many steps away from the end of the sequence. for instance a[-1] is an idiom in python to get the last element of a. so here, i'm doing the slice [-n:] to go from -n (i.e., n steps away from the end) to the end of the array.

>>> "hello"[-2:]
"lo"

[johnc1231]

Thanks for adding tail