"N. Y." <ningyuan...@gmail.com> writes:
> Hi all,
>
> I was wondering what is the meaning of symbols prefixed with "#$", for
> example "#$version" in Guix package definitions.
G-Expressions in Guix are created using #~<exp ...>, similar to how
S-Expressions are created using '<exp ...> or `<exp ...> in Scheme (and
other Lisps). The single quote in Scheme is actually a reader macro that
expands to the following:
'exp => (quote exp)
'(exps ...) => (quote (exps ...))
Similarly, the backtick is a reader macro that expands to the following:
`exp => (quasiquote exp)
`(exps ...) => (quasiquote (exps ...))
The latter is interesting because it allows us to /unquote/ data in the
S-expression, like so:
(let ((x 5)
(y 6))
(quasiquote (x (unquote x) y (unquote y)))) => (list 'x 5 'y 6)
I used the long form to avoid ambiguity, but that is the same as:
(let ((x 5)
(y 6))
`(x ,x y ,y)) => (list 'x 5 'y 6)
Where the comma is used as the unquote reader macro.
I apologize if this is review for you, but it's important to say because
this is exactly what is happening with #~ and #$. They are both reader
macros, that behave similarly to the quasiquote and unquote reader
macros: ` and ,.
(let ((x 5)
(y 6))
#~(x #$x y #$y))
Is the same as:
(let ((x 5)
(y 6))
(gexp (x (ungexp x) y (ungexp y))))
After expanding the reader macros.
I'm not sure about your package error, but I hope this helps point you
in the right direction :-)
Robby
