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monads, doc: Improve mwhen and munless documentation.

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* doc/guix.texi (The Store Monad) <mwhen, munless>: Document them.
* guix/monads.scm (mwhen, munless): Clarify their intended use.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>
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Chris Marusich 2017-04-06 02:28:35 -07:00 • committed by Ludovic Courtès
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commit 60a9fcb137
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2 changed files with 22 additions and 4 deletions
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14
doc/guix.texi
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@ -4054,6 +4054,20 @@ monadic expressions are ignored. In that sense, it is analogous to
@code{begin}, but applied to monadic expressions.
@end deffn
@deffn {Scheme System} mwhen @var{condition} @var{mexp0} @var{mexp*} ...
When @var{condition} is true, evaluate the sequence of monadic
expressions @var{mexp0}..@var{mexp*} as in an @code{mbegin}. When
@var{condition} is false, return @code{*unspecified*} in the current
monad. Every expression in the sequence must be a monadic expression.
@end deffn
@deffn {Scheme System} munless @var{condition} @var{mexp0} @var{mexp*} ...
When @var{condition} is false, evaluate the sequence of monadic
expressions @var{mexp0}..@var{mexp*} as in an @code{mbegin}. When
@var{condition} is true, return @code{*unspecified*} in the current
monad. Every expression in the sequence must be a monadic expression.
@end deffn
@cindex state monad
The @code{(guix monads)} module provides the @dfn{state monad}, which
allows an additional value---the state---to be @emph{threaded} through

12
guix/monads.scm
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@ -204,8 +204,10 @@ the last one."
(define-syntax mwhen
(syntax-rules ()
"When CONDITION is true, evaluate MEXP0..MEXP* as in an 'mbegin'. When
CONDITION is false, return *unspecified* in the current monad."
"When CONDITION is true, evaluate the sequence of monadic expressions
MEXP0..MEXP* as in an 'mbegin'. When CONDITION is false, return *unspecified*
in the current monad. Every expression in the sequence must be a monadic
expression."
((_ condition mexp0 mexp* ...)
(if condition
(mbegin %current-monad
@ -214,8 +216,10 @@ CONDITION is false, return *unspecified* in the current monad."
(define-syntax munless
(syntax-rules ()
"When CONDITION is false, evaluate MEXP0..MEXP* as in an 'mbegin'. When
CONDITION is true, return *unspecified* in the current monad."
"When CONDITION is false, evaluate the sequence of monadic expressions
MEXP0..MEXP* as in an 'mbegin'. When CONDITION is true, return *unspecified*
in the current monad. Every expression in the sequence must be a monadic
expression."
((_ condition mexp0 mexp* ...)
(if condition
(return *unspecified*)