This package implements the macro ##, which provides compact
syntax for short lambda, without actually being new syntax,
which would be difficult to get merged into Emacs. Past attempts
to add syntax were met with determined pushback and the use of a
macro was suggested as an alternative.
The ## macro, whose signature is (## FN &rest BODY), expands
to a lambda expression, which wraps around these arguments.
This lambda expression calls the function FN with arguments
BODY and returns its value. Its own arguments are derived from
symbols found in BODY.
Each symbol from %1 through %9, which appears in BODY,
specifies an argument. Each symbol from &1 through &9, which
appears in BODY, specifies an optional argument. All arguments
following an optional argument have to be optional as well, thus
their names have to begin with &. Symbol &* specifies extra
(&rest) arguments.
Instead of %1, the shorthand % can be used; but that should
only be done if it is the only argument, and using both %1 and
% is not allowed. Likewise & can be substituted for &1.
Instead of:
(lambda (a _ &optional c &rest d)
(foo a (bar c) d))you can use this macro and write:
(##foo %1 (bar &3) &*)which expands to:
(lambda (%1 &optional _&2 &3 &rest &*)
(foo %1 (bar &3) &*))Unused trailing arguments and mandatory unused arguments at the border between mandatory and optional arguments are also supported:
(##list %1 _%3 &5 _&6)becomes:
(lambda (%1 _%2 _%3 &optional _&4 &5 _&6)
(list %1 &5))Note how _%3 and _&6 are removed from the body, because their
names begin with an underscore. Also note that _&4 is optional,
unlike the explicitly specified _%3.
The name ## was chosen because that allows (optionally)
omitting the whitespace between it and the following symbol.
It also looks similar to #'function.