From c6f30b81851fde4be31aae7b9d36356c8f72963a Mon Sep 17 00:00:00 2001 From: Ludovic Courtès 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 . * 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}: -- cgit v1.2.3