From c6f30b81851fde4be31aae7b9d36356c8f72963a Mon Sep 17 00:00:00 2001
From: Ludovic Courtès <ludo@gnu.org>
Date: Mon, 4 May 2015 15:05:13 +0200
Subject: doc: Fix dangling references in the 'sh-symlink' monad example.
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.
---
doc/guix.texi | 31 ++++++++++++++++++++++---------
1 file changed, 22 insertions(+), 9 deletions(-)
(limited to 'doc')
diff --git a/doc/guix.texi b/doc/guix.texi
index dea1415616..d9db408e57 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -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.
- (gexp->derivation "sh"
- #~(symlink (string-append #$bash "/bin/bash") #$output)))
+ (mlet %store-monad ((drv (package->derivation bash)))
+ (gexp->derivation "sh"
+ #~(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}:
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