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-rw-r--r--.dir-locals.el2
-rw-r--r--guix/monads.scm30
2 files changed, 31 insertions, 1 deletions
diff --git a/.dir-locals.el b/.dir-locals.el
index 106c35bce6..b099068dac 100644
--- a/.dir-locals.el
+++ b/.dir-locals.el
@@ -46,6 +46,8 @@
(eval . (put 'syntax-parameterize 'scheme-indent-function 1))
(eval . (put 'with-monad 'scheme-indent-function 1))
(eval . (put 'mbegin 'scheme-indent-function 1))
+ (eval . (put 'mwhen 'scheme-indent-function 1))
+ (eval . (put 'munless 'scheme-indent-function 1))
(eval . (put 'mlet* 'scheme-indent-function 2))
(eval . (put 'mlet 'scheme-indent-function 2))
(eval . (put 'run-with-store 'scheme-indent-function 1))
diff --git a/guix/monads.scm b/guix/monads.scm
index b419ba066a..52cb3f5ed0 100644
--- a/guix/monads.scm
+++ b/guix/monads.scm
@@ -39,6 +39,8 @@
mlet
mlet*
mbegin
+ mwhen
+ munless
lift1 lift2 lift3 lift4 lift5 lift6 lift7 lift
listm
foldm
@@ -173,9 +175,15 @@ form is (VAR -> VAL), bind VAR to the non-monadic value VAL in the same way as
body ...)))))))
(define-syntax mbegin
- (syntax-rules ()
+ (syntax-rules (%current-monad)
"Bind the given monadic expressions in sequence, returning the result of
the last one."
+ ((_ %current-monad mexp)
+ mexp)
+ ((_ %current-monad mexp rest ...)
+ (>>= mexp
+ (lambda (unused-value)
+ (mbegin %current-monad rest ...))))
((_ monad mexp)
(with-monad monad
mexp))
@@ -185,6 +193,26 @@ the last one."
(lambda (unused-value)
(mbegin monad rest ...)))))))
+(define-syntax mwhen
+ (syntax-rules ()
+ "When CONDITION is true, evaluate EXP0..EXP* as in an 'mbegin'. When
+CONDITION is false, return *unspecified* in the current monad."
+ ((_ condition exp0 exp* ...)
+ (if condition
+ (mbegin %current-monad
+ exp0 exp* ...)
+ (return *unspecified*)))))
+
+(define-syntax munless
+ (syntax-rules ()
+ "When CONDITION is false, evaluate EXP0..EXP* as in an 'mbegin'. When
+CONDITION is true, return *unspecified* in the current monad."
+ ((_ condition exp0 exp* ...)
+ (if condition
+ (return *unspecified*)
+ (mbegin %current-monad
+ exp0 exp* ...)))))
+
(define-syntax define-lift
(syntax-rules ()
((_ liftn (args ...))