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monads: Add 'mwhen' and 'munless'.

* guix/monads.scm (mbegin): Add special '%current-monad' syntactic
  keyword.
  (mwhen, munless): New macros.
version-0.8.3
Ludovic Courtès 7 years ago
parent
commit
21caa6deeb
  1. 2
      .dir-locals.el
  2. 30
      guix/monads.scm

2
.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))

30
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 ...))

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