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doc: Fix dangling references in the 'sh-symlink' monad example.

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Reported by Christopher A. Webber <cwebber@dustycloud.org>.

* doc/guix.texi (The Store Monad): Reintroduce 'mlet' in 'sh-symlink'
  example.  Move the simplified version below.  Explain that the 'store'
  parameter is threaded.
This commit is contained in:
Ludovic Courtès 2015-05-04 15:05:13 +02:00
parent eabebb8143
commit c6f30b8185
1 changed files with 22 additions and 9 deletions
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doc/guix.texi
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@ -2346,21 +2346,34 @@ Consider this ``normal'' procedure:
`(symlink ,sh %output))))
@end example
Using @code{(guix monads)}, it may be rewritten as a monadic function:
Using @code{(guix monads)} and @code{(guix gexp)}, it may be rewritten
as a monadic function:
@c FIXME: Find a better example, one that uses 'mlet'.
@example
(define (sh-symlink)
;; Same, but return a monadic value.
(mlet %store-monad ((drv (package->derivation bash)))
(gexp->derivation "sh"
#~(symlink (string-append #$bash "/bin/bash") #$output)))
#~(symlink (string-append #$drv "/bin/bash")
#$output))))
@end example
There are two things to note in the second version: the @code{store}
parameter is now implicit, and the monadic value returned by
@code{package-file}---a wrapper around @code{package-derivation} and
@code{derivation->output-path}---is @dfn{bound} using @code{mlet}
instead of plain @code{let}.
There several things to note in the second version: the @code{store}
parameter is now implicit and is ``threaded'' in the calls to the
@code{package->derivation} and @code{gexp->derivation} monadic
procedures, and the monadic value returned by @code{package->derivation}
is @dfn{bound} using @code{mlet} instead of plain @code{let}.
As it turns out, the call to @code{package->derivation} can even be
omitted since it will take place implicitly, as we will see later
(@pxref{G-Expressions}):
@example
(define (sh-symlink)
(gexp->derivation "sh"
#~(symlink (string-append #$bash "/bin/bash")
#$output)))
@end example
Calling the monadic @code{sh-symlink} has no effect. To get the desired
effect, one must use @code{run-with-store}: