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doc: Mention gexps in the "Scheme Crash Course".

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* doc/guix-cookbook.texi (A Scheme Crash Course): Add note on gexps.
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Ludovic Courtès 2023-07-14 16:15:38 +02:00
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@ -234,10 +234,11 @@ A list structure can be created with the @code{list} procedure:
@end lisp
@item
The @dfn{quote} disables evaluation of a parenthesized expression: the
first term is not called over the other terms (@pxref{Expression Syntax,
quote,, guile, GNU Guile Reference Manual}). Thus it effectively
returns a list of terms.
@cindex S-expression
The @dfn{quote} disables evaluation of a parenthesized expression, also
called an S-expression or ``s-exp'': the first term is not called over
the other terms (@pxref{Expression Syntax, quote,, guile, GNU Guile
Reference Manual}). Thus it effectively returns a list of terms.
@lisp
'(display (string-append "Hello " "Guix" "\n"))
@ -248,9 +249,10 @@ returns a list of terms.
@end lisp
@item
The @dfn{quasiquote} disables evaluation of a parenthesized expression
until @dfn{unquote} (a comma) re-enables it. Thus it provides us with
fine-grained control over what is evaluated and what is not.
The @code{quasiquote} (@code{`}, a backquote) disables evaluation of a
parenthesized expression until @code{unquote} (@code{,}, a comma)
re-enables it. Thus it provides us with fine-grained control over what
is evaluated and what is not.
@lisp
`(2 a 5 7 (2 ,a 5 ,(+ a 4)))
@ -260,6 +262,37 @@ fine-grained control over what is evaluated and what is not.
Note that the above result is a list of mixed elements: numbers, symbols (here
@code{a}) and the last element is a list itself.
@item
@cindex G-expressions, syntax
@cindex gexps, syntax
@findex #~
@findex #$
@findex gexp
@findex ungexp
Guix defines a variant of S-expressions on steroids called
@dfn{G-expressions} or ``gexps'', which come with a variant of
@code{quasiquote} and @code{unquote}: @code{#~} (or @code{gexp}) and
@code{#$} (or @code{ungexp}). They let you @emph{stage code for later
execution}.
For example, you'll encounter gexps in some package definitions where
they provide code to be executed during the package build process. They
look like this:
@lisp
;; Below is a G-expression representing staged code.
#~(begin
;; Invoke 'ls' from the package defined by the 'coreutils'
;; variable.
(system* #$(file-append coreutils "/bin/ls") "-l")
;; Create this package's output directory.
(mkdir #$output))
@end lisp
@xref{G-Expressions,,, guix, GNU Guix Reference Manual}, for more on
gexps.
@item
Multiple variables can be named locally with @code{let} (@pxref{Local
Bindings,,, guile, GNU Guile Reference Manual}):