diff options
Diffstat (limited to 'guix/monads.scm')
| -rw-r--r-- | guix/monads.scm | 211 |
1 files changed, 191 insertions, 20 deletions
diff --git a/guix/monads.scm b/guix/monads.scm index 317f85d079..6ae616aca9 100644 --- a/guix/monads.scm +++ b/guix/monads.scm @@ -1,5 +1,5 @@ ;;; GNU Guix --- Functional package management for GNU -;;; Copyright © 2013, 2014, 2015 Ludovic Courtès <ludo@gnu.org> +;;; Copyright © 2013, 2014, 2015, 2017 Ludovic Courtès <ludo@gnu.org> ;;; ;;; This file is part of GNU Guix. ;;; @@ -29,6 +29,8 @@ monad-bind monad-return + template-directory + ;; Syntax. >>= return @@ -92,6 +94,9 @@ ;; The record type, for use at run time. (make-monad b r)) + ;; Instantiate all the templates, specialized for this monad. + (template-directory instantiations name) + (define-syntax name ;; An "inlined record", for use at expansion time. The goal is ;; to allow 'bind' and 'return' to be resolved at expansion @@ -103,6 +108,172 @@ ((_ %return) #'r) (_ #'rtd)))))))))) +;; Expansion- and run-time state of the template directory. This needs to be +;; available at run time (and not just at expansion time) so we can +;; instantiate templates defined in other modules, or use instances defined +;; elsewhere. +(eval-when (load expand eval) + ;; Mapping of syntax objects denoting the template to a pair containing (1) + ;; the syntax object of the parameter over which it is templated, and (2) + ;; the syntax of its body. + (define-once %templates (make-hash-table)) + + (define (register-template! name param body) + (hash-set! %templates name (cons param body))) + + ;; List of template instances, where each entry is a triplet containing the + ;; syntax of the name, the actual parameter for which the template is + ;; specialized, and the syntax object referring to this specialization (the + ;; procedure's identifier.) + (define-once %template-instances '()) + + (define (register-template-instance! name actual instance) + (set! %template-instances + (cons (list name actual instance) %template-instances)))) + +(define-syntax template-directory + (lambda (s) + "This is a \"stateful macro\" to register and lookup templates and +template instances." + (define location + (syntax-source s)) + + (define current-info-port + ;; Port for debugging info. + (const (%make-void-port "w"))) + + (define location-string + (format #f "~a:~a:~a" + (assq-ref location 'filename) + (and=> (assq-ref location 'line) 1+) + (assq-ref location 'column))) + + (define (matching-instance? name actual) + (match-lambda + ((name* instance-param proc) + (and (free-identifier=? name name*) + (or (equal? actual instance-param) + (and (identifier? actual) + (identifier? instance-param) + (free-identifier=? instance-param + actual))) + proc)))) + + (define (instance-identifier name actual) + (define stem + (string-append + " " + (symbol->string (syntax->datum name)) + (if (identifier? actual) + (string-append " " (symbol->string (syntax->datum actual))) + "") + " instance")) + (datum->syntax actual (string->symbol stem))) + + (define (instance-definition name template actual) + (match template + ((formal . body) + (let ((instance (instance-identifier name actual))) + (format (current-info-port) + "~a: info: specializing '~a' for '~a' as '~a'~%" + location-string + (syntax->datum name) (syntax->datum actual) + (syntax->datum instance)) + + (register-template-instance! name actual instance) + + #`(begin + (define #,instance + (let-syntax ((#,formal (identifier-syntax #,actual))) + #,body)) + + ;; Generate code to register the thing at run time. + (register-template-instance! #'#,name #'#,actual + #'#,instance)))))) + + (syntax-case s (register! lookup exists? instantiations) + ((_ register! name param body) + ;; Register NAME as a template on PARAM with the given BODY. + (begin + (register-template! #'name #'param #'body) + + ;; Generate code to register the template at run time. XXX: Because + ;; of this, BODY must not contain ellipses. + #'(register-template! #'name #'param #'body))) + ((_ lookup name actual) + ;; Search for an instance of template NAME for this ACTUAL parameter. + ;; On success, expand to the identifier of the instance; otherwise + ;; expand to #f. + (any (matching-instance? #'name #'actual) %template-instances)) + ((_ exists? name actual) + ;; Likewise, but return a Boolean. + (let ((result (->bool + (any (matching-instance? #'name #'actual) + %template-instances)))) + (unless result + (format (current-warning-port) + "~a: warning: no specialization of template '~a' for '~a'~%" + location-string + (syntax->datum #'name) (syntax->datum #'actual))) + result)) + ((_ instantiations actual) + ;; Expand to the definitions of all the existing templates + ;; specialized for ACTUAL. + #`(begin + #,@(hash-map->list (cut instance-definition <> <> #'actual) + %templates)))))) + +(define-syntax define-template + (lambda (s) + "Define a template, which is a procedure that can be specialized over its +first argument. In our case, the first argument is typically the identifier +of a monad. + +Defining templates for procedures like 'mapm' allows us to make have a +specialized version of those procedures for each monad that we define, such +that calls to: + + (mapm %state-monad proc lst) + +automatically expand to: + + (#{ mapm %state-monad instance}# proc lst) + +Here, #{ mapm %state-monad instance}# is specialized for %state-monad, and +thus it contains inline calls to %state-bind and %state-return. This avoids +repeated calls to 'struct-ref' to get the 'bind' and 'return' procedure of the +monad, and allows 'bind' and 'return' to be inlined, which in turn allows for +more optimizations." + (syntax-case s () + ((_ (name arg0 args ...) body ...) + (with-syntax ((generic-name (datum->syntax + #'name + (symbol-append '#{ %}# + (syntax->datum #'name) + '-generic))) + (original-name #'name)) + #`(begin + (template-directory register! name arg0 + (lambda (args ...) + body ...)) + (define (generic-name arg0 args ...) + ;; The generic instance of NAME, for when no specialization was + ;; found. + body ...) + + (define-syntax name + (lambda (s) + (syntax-case s () + ((_ arg0* args ...) + ;; Expand to either the specialized instance or the + ;; generic instance of template ORIGINAL-NAME. + #'(if (template-directory exists? original-name arg0*) + ((template-directory lookup original-name arg0*) + args ...) + (generic-name arg0* args ...))) + (_ + #'generic-name)))))))))) + (define-syntax-parameter >>= ;; The name 'bind' is already taken, so we choose this (obscure) symbol. (lambda (s) @@ -265,7 +436,7 @@ MONAD---i.e., return a monadic function in MONAD." (with-monad monad (return (apply proc args))))) -(define (foldm monad mproc init lst) +(define-template (foldm monad mproc init lst) "Fold MPROC over LST and return a monadic value seeded by INIT. (foldm %state-monad (lift2 cons %state-monad) '() '(a b c)) @@ -277,33 +448,33 @@ MONAD---i.e., return a monadic function in MONAD." (match lst (() (return result)) - ((head tail ...) + ((head . tail) (>>= (mproc head result) (lambda (result) (loop tail result)))))))) -(define (mapm monad mproc lst) +(define-template (mapm monad mproc lst) "Map MPROC over LST and return a monadic list. (mapm %state-monad (lift1 1+ %state-monad) '(0 1 2)) => (1 2 3) ;monadic " - (mlet monad ((result (foldm monad - (lambda (item result) - (>>= (mproc item) - (lambda (item) - (return (cons item result))))) - '() - lst))) - (return (reverse result)))) - -(define-syntax-rule (sequence monad lst) + ;; XXX: We don't use 'foldm' because template specialization wouldn't work + ;; in this context. + (with-monad monad + (let mapm ((lst lst) + (result '())) + (match lst + (() + (return (reverse result))) + ((head . tail) + (>>= (mproc head) + (lambda (head) + (mapm tail (cons head result))))))))) + +(define-template (sequence monad lst) "Turn the list of monadic values LST into a monadic list of values, by evaluating each item of LST in sequence." - ;; XXX: Making it a macro is a bit brutal as it leads to a lot of code - ;; duplication. However, it allows >>= and return to be open-coded, which - ;; avoids struct-ref's to MONAD and a few closure allocations when using - ;; %STATE-MONAD. (with-monad monad (let seq ((lstx lst) (result '())) @@ -315,7 +486,7 @@ evaluating each item of LST in sequence." (lambda (item) (seq tail (cons item result))))))))) -(define (anym monad mproc lst) +(define-template (anym monad mproc lst) "Apply MPROC to the list of values LST; return as a monadic value the first value for which MPROC returns a true monadic value or #f. For example: @@ -327,7 +498,7 @@ value for which MPROC returns a true monadic value or #f. For example: (match lst (() (return #f)) - ((head tail ...) + ((head . tail) (>>= (mproc head) (lambda (result) (if result |