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Diffstat (limited to 'doc/guix.texi')
| -rw-r--r-- | doc/guix.texi | 31 |
1 files changed, 22 insertions, 9 deletions
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}: |