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monads: Add a template and specialization mechanism for monadic procedures.

* guix/monads.scm (%templates, %template-instances): New variables.
(register-template!, register-template-instance!): New procedures.
(template-directory, define-template): New macro.
(foldm, sequence, anym): Define using 'define-template'.  Avoid replace
ellipses with dots.
(mapm): Likewise, but do not use 'foldm'.
* guix/store.scm: Add 'template-directory' invocation.
Ludovic Courtès 5 years ago
No known key found for this signature in database GPG Key ID: 90B11993D9AEBB5
  1. 211
  2. 4


@ -1,5 +1,5 @@
;;; GNU Guix --- Functional package management for GNU
;;; Copyright © 2013, 2014, 2015 Ludovic Courtès <>
;;; Copyright © 2013, 2014, 2015, 2017 Ludovic Courtès <>
;;; This file is part of GNU Guix.
@ -29,6 +29,8 @@
;; Syntax.
@ -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)
((name* instance-param proc)
(and (free-identifier=? name name*)
(or (equal? actual instance-param)
(and (identifier? actual)
(identifier? instance-param)
(free-identifier=? instance-param
(define (instance-identifier name actual)
(define stem
" "
(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'~%"
(syntax->datum name) (syntax->datum actual)
(syntax->datum instance))
(register-template-instance! name actual instance)
(define #,instance
(let-syntax ((#,formal (identifier-syntax #,actual)))
;; Generate code to register the thing at run time.
(register-template-instance! #'#,name #'#,actual
(syntax-case s (register! lookup exists? instantiations)
((_ register! name param body)
;; Register NAME as a template on PARAM with the given BODY.
(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)
(unless result
(format (current-warning-port)
"~a: warning: no specialization of template '~a' for '~a'~%"
(syntax->datum #'name) (syntax->datum #'actual)))
((_ instantiations actual)
;; Expand to the definitions of all the existing templates
;; specialized for ACTUAL.
#,@(hash-map->list (cut instance-definition <> <> #'actual)
(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
(symbol-append '#{ %}#
(syntax->datum #'name)
(original-name #'name))
(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 ...)))
(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)))))
(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
(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


@ -1237,6 +1237,10 @@ be used internally by the daemon's build hook."
(define-alias store-return state-return)
(define-alias store-bind state-bind)
;; Instantiate templates for %STORE-MONAD since it's syntactically different
;; from %STATE-MONAD.
(template-directory instantiations %store-monad)
(define (preserve-documentation original proc)
"Return PROC with documentation taken from ORIGINAL."
(set-object-property! proc 'documentation