\expandafter

yex.keyword.Expandafter(parser)

Process the argument, then give the result to the previous control.

For example, in

\def\spong{spong}
\uppercase{\spong}

firstly \uppercase runs on "\spong", which gives "\spong" (since it's a control, not a series of letters). Then \spong is run, so we end up with "spong".

But in

\def\spong{spong}
\uppercase\expandafter{\spong}

\expandafter runs "\spong" and gets "spong", then passes it to \uppercase. So we end up with "SPONG".

Source code in yex/keyword/other.py

@yex.decorator.control(
        expandable = True,
        even_if_not_expanding = True,
        )
def Expandafter(parser):
    r"""
    Process the argument, then give the result to the previous control.

    For example, in

        \def\spong{spong}
        \uppercase{\spong}

    firstly \uppercase runs on "\spong", which gives "\spong" (since it's a
    control, not a series of letters). Then \spong is run, so we end up
    with "spong".

    But in

        \def\spong{spong}
        \uppercase\expandafter{\spong}

    \expandafter runs "\spong" and gets "spong", then passes it to
    \uppercase. So we end up with "SPONG".
    """

    opening = parser.next(
            level = 'deep',
            on_eof = 'raise',
            )

    # Right, let's read our argument.

    argument = [token for token in parser.another(
        level = 'executing',
        bounded ='single',
        on_eof = 'exhaust',
        )]

    rerun_parser = [t for t in parser.another(
            source = argument,
            on_eof = 'exhaust',
            )]

    parser.push(rerun_parser)
    parser.push(opening)