diff options
Diffstat (limited to 'guix')
| -rw-r--r-- | guix/monads.scm | 27 |
1 files changed, 25 insertions, 2 deletions
diff --git a/guix/monads.scm b/guix/monads.scm index 4248525433..2196a9c991 100644 --- a/guix/monads.scm +++ b/guix/monads.scm @@ -112,6 +112,29 @@ (lambda (s) (syntax-violation 'return "return used outside of 'with-monad'" s))) +(define-syntax-rule (bind-syntax bind) + "Return a macro transformer that handles the expansion of '>>=' expressions +using BIND as the binary bind operator. + +This macro exists to allow the expansion of n-ary '>>=' expressions, even +though BIND is simply binary, as in: + + (with-monad %state-monad + (>>= (return 1) + (lift 1+ %state-monad) + (lift 1+ %state-monad))) +" + (lambda (stx) + (define (expand body) + (syntax-case body () + ((_ mval mproc) + #'(bind mval mproc)) + ((x mval mproc0 mprocs (... ...)) + (expand #'(>>= (>>= mval mproc0) + mprocs (... ...)))))) + + (expand stx))) + (define-syntax with-monad (lambda (s) "Evaluate BODY in the context of MONAD, and return its result." @@ -120,13 +143,13 @@ (eq? 'macro (syntax-local-binding #'monad)) ;; MONAD is a syntax transformer, so we can obtain the bind and return ;; methods by directly querying it. - #'(syntax-parameterize ((>>= (identifier-syntax (monad %bind))) + #'(syntax-parameterize ((>>= (bind-syntax (monad %bind))) (return (identifier-syntax (monad %return)))) body ...)) ((_ monad body ...) ;; MONAD refers to the <monad> record that represents the monad at run ;; time, so use the slow method. - #'(syntax-parameterize ((>>= (identifier-syntax + #'(syntax-parameterize ((>>= (bind-syntax (monad-bind monad))) (return (identifier-syntax (monad-return monad)))) |