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[jackhill/guix/guix.git] / guix / monads.scm

;;; GNU Guix --- Functional package management for GNU
;;; Copyright © 2013, 2014 Ludovic Courtès <ludo@gnu.org>
;;;
;;; This file is part of GNU Guix.
;;;
;;; GNU Guix is free software; you can redistribute it and/or modify it
;;; under the terms of the GNU General Public License as published by
;;; the Free Software Foundation; either version 3 of the License, or (at
;;; your option) any later version.
;;;
;;; GNU Guix is distributed in the hope that it will be useful, but
;;; WITHOUT ANY WARRANTY; without even the implied warranty of
;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;;; GNU General Public License for more details.
;;;
;;; You should have received a copy of the GNU General Public License
;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.

(define-module (guix monads)
  #:use-module (guix store)
  #:use-module (guix derivations)
  #:use-module (guix packages)
  #:use-module ((system syntax)
                #:select (syntax-local-binding))
  #:use-module (ice-9 match)
  #:use-module (srfi srfi-9)
  #:use-module (srfi srfi-26)
  #:export (;; Monads.
            define-monad
            monad?
            monad-bind
            monad-return

            ;; Syntax.
            >>=
            return
            with-monad
            mlet
            mlet*
            lift1 lift2 lift3 lift4 lift5 lift6 lift7 lift
            listm
            foldm
            mapm
            sequence
            anym

            ;; Concrete monads.
            %identity-monad

            %store-monad
            store-bind
            store-return
            store-lift
            run-with-store
            text-file
            package-file
            package->derivation
            built-derivations
            derivation-expression
            lower-inputs)
  #:replace (imported-modules
             compiled-modules))

;;; Commentary:
;;;
;;; This module implements the general mechanism of monads, and provides in
;;; particular an instance of the "store" monad.  The API was inspired by that
;;; of Racket's "better-monads" module (see
;;; <http://planet.racket-lang.org/package-source/toups/functional.plt/1/1/planet-docs/better-monads-guide/index.html>).
;;; The implementation and use case were influenced by Oleg Kysielov's
;;; "Monadic Programming in Scheme" (see
;;; <http://okmij.org/ftp/Scheme/monad-in-Scheme.html>).
;;;
;;; The store monad allows us to (1) build sequences of operations in the
;;; store, and (2) make the store an implicit part of the execution context,
;;; rather than a parameter of every single function.
;;;
;;; Code:

;; Record type for monads manipulated at run time.
(define-record-type <monad>
  (make-monad bind return)
  monad?
  (bind   monad-bind)
  (return monad-return))                         ; TODO: Add 'plus' and 'zero'

(define-syntax define-monad
  (lambda (s)
    "Define the monad under NAME, with the given bind and return methods."
    (define prefix (string->symbol "% "))
    (define (make-rtd-name name)
      (datum->syntax name
                     (symbol-append prefix (syntax->datum name) '-rtd)))

    (syntax-case s (bind return)
      ((_ name (bind b) (return r))
       (with-syntax ((rtd (make-rtd-name #'name)))
         #`(begin
             (define rtd
               ;; The record type, for use at run time.
               (make-monad b r))

             (define-syntax name
               ;; An "inlined record", for use at expansion time.  The goal is
               ;; to allow 'bind' and 'return' to be resolved at expansion
               ;; time, in the common case where the monad is accessed
               ;; directly as NAME.
               (lambda (s)
                 (syntax-case s (%bind %return)
                   ((_ %bind)   #'b)
                   ((_ %return) #'r)
                   (_           #'rtd))))))))))

(define-syntax-parameter >>=
  ;; The name 'bind' is already taken, so we choose this (obscure) symbol.
  (lambda (s)
    (syntax-violation '>>= ">>= (bind) used outside of 'with-monad'" s)))

(define-syntax-parameter return
  (lambda (s)
    (syntax-violation 'return "return used outside of 'with-monad'" s)))

(define-syntax with-monad
  (lambda (s)
    "Evaluate BODY in the context of MONAD, and return its result."
    (syntax-case s ()
      ((_ monad body ...)
       (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)))
                               (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
                                        (monad-bind monad)))
                               (return (identifier-syntax
                                        (monad-return monad))))
           body ...)))))

(define-syntax mlet*
  (syntax-rules (->)
    "Bind the given monadic values MVAL to the given variables VAR.  When the
form is (VAR -> VAL), bind VAR to the non-monadic value VAL in the same way as
'let'."
    ;; Note: the '->' symbol corresponds to 'is:' in 'better-monads.rkt'.
    ((_ monad () body ...)
     (with-monad monad body ...))
    ((_ monad ((var mval) rest ...) body ...)
     (with-monad monad
       (>>= mval
            (lambda (var)
              (mlet* monad (rest ...)
                body ...)))))
    ((_ monad ((var -> val) rest ...) body ...)
     (let ((var val))
       (mlet* monad (rest ...)
         body ...)))))

(define-syntax mlet
  (lambda (s)
    (syntax-case s ()
      ((_ monad ((var mval ...) ...) body ...)
       (with-syntax (((temp ...) (generate-temporaries #'(var ...))))
         #'(mlet* monad ((temp mval ...) ...)
             (let ((var temp) ...)
               body ...)))))))

(define-syntax define-lift
  (syntax-rules ()
    ((_ liftn (args ...))
     (define (liftn proc monad)
       "Lift PROC to MONAD---i.e., return a monadic function in MONAD."
       (lambda (args ...)
         (with-monad monad
           (return (proc args ...))))))))

(define-lift lift1 (a))
(define-lift lift2 (a b))
(define-lift lift3 (a b c))
(define-lift lift4 (a b c d))
(define-lift lift5 (a b c d e))
(define-lift lift6 (a b c d e f))
(define-lift lift7 (a b c d e f g))

(define (lift nargs proc monad)
  "Lift PROC, a procedure that accepts NARGS arguments, to MONAD---i.e.,
return a monadic function in MONAD."
  (lambda args
    (with-monad monad
      (return (apply proc args)))))

(define (foldm monad mproc init lst)
  "Fold MPROC over LST, a list of monadic values in MONAD, and return a
monadic value seeded by INIT."
  (with-monad monad
    (let loop ((lst    lst)
               (result init))
      (match lst
        (()
         (return result))
        ((head tail ...)
         (mlet* monad ((item   head)
                       (result (mproc item result)))
           (loop tail result)))))))

(define (mapm monad mproc lst)
  "Map MPROC over LST, a list of monadic values in MONAD, and return a monadic
list."
  (foldm monad
         (lambda (item result)
           (mlet monad ((item (mproc item)))
             (return (cons item result))))
         '()
         (reverse lst)))

(define-inlinable (sequence monad lst)
  "Turn the list of monadic values LST into a monadic list of values, by
evaluating each item of LST in sequence."
  (with-monad monad
    (mapm monad return lst)))

(define (anym monad proc lst)
  "Apply PROC to the list of monadic values LST; return the first value,
lifted in MONAD, for which PROC returns true."
  (with-monad monad
    (let loop ((lst lst))
      (match lst
        (()
         (return #f))
        ((head tail ...)
         (mlet* monad ((value  head)
                       (result -> (proc value)))
           (if result
               (return result)
               (loop tail))))))))

(define-syntax listm
  (lambda (s)
    "Return a monadic list in MONAD from the monadic values MVAL."
    (syntax-case s ()
      ((_ monad mval ...)
       (with-syntax (((val ...) (generate-temporaries #'(mval ...))))
         #'(mlet monad ((val mval) ...)
             (return (list val ...))))))))


\f
;;;
;;; Identity monad.
;;;

(define-inlinable (identity-return value)
  value)

(define-inlinable (identity-bind mvalue mproc)
  (mproc mvalue))

(define-monad %identity-monad
  (bind   identity-bind)
  (return identity-return))

\f
;;;
;;; Store monad.
;;;

;; return:: a -> StoreM a
(define-inlinable (store-return value)
  "Return VALUE from a monadic function."
  ;; The monadic value is just this.
  (lambda (store)
    value))

;; >>=:: StoreM a -> (a -> StoreM b) -> StoreM b
(define-inlinable (store-bind mvalue mproc)
  "Bind MVALUE in MPROC."
  (lambda (store)
    (let* ((value   (mvalue store))
           (mresult (mproc value)))
      (mresult store))))

(define-monad %store-monad
  (bind   store-bind)
  (return store-return))


(define (store-lift proc)
  "Lift PROC, a procedure whose first argument is a connection to the store,
in the store monad."
  (define result
    (lambda args
      (lambda (store)
        (apply proc store args))))

  (set-object-property! result 'documentation
                        (procedure-property proc 'documentation))
  result)

;;;
;;; Store monad operators.
;;;

(define* (text-file name text)
  "Return as a monadic value the absolute file name in the store of the file
containing TEXT."
  (lambda (store)
    (add-text-to-store store name text '())))

(define* (package-file package
                       #:optional file
                       #:key (system (%current-system)) (output "out"))
  "Return as a monadic value the absolute file name of FILE within the
OUTPUT directory of PACKAGE.  When FILE is omitted, return the name of the
OUTPUT directory of PACKAGE."
  (lambda (store)
    (let* ((drv (package-derivation store package system))
           (out (derivation->output-path drv output)))
      (if file
          (string-append out "/" file)
          out))))

(define (lower-inputs inputs)
  "Turn any package from INPUTS into a derivation; return the corresponding
input list as a monadic value."
  ;; XXX: Should probably be in (guix packages).
  (with-monad %store-monad
    (sequence %store-monad
              (map (match-lambda
                    ((name (? package? package) sub-drv ...)
                     (mlet %store-monad ((drv (package->derivation package)))
                       (return `(,name ,drv ,@sub-drv))))
                    ((name (? string? file))
                     (return `(,name ,file)))
                    (tuple
                     (return tuple)))
                   inputs))))

(define derivation-expression
  (store-lift build-expression->derivation))

(define package->derivation
  (store-lift package-derivation))

(define imported-modules
  (store-lift (@ (guix derivations) imported-modules)))

(define compiled-modules
  (store-lift (@ (guix derivations) compiled-modules)))

(define built-derivations
  (store-lift build-derivations))

(define* (run-with-store store mval
                         #:key
                         (guile-for-build (%guile-for-build))
                         (system (%current-system)))
  "Run MVAL, a monadic value in the store monad, in STORE, an open store
connection."
  (parameterize ((%guile-for-build (or guile-for-build
                                       (package-derivation store
                                                           (@ (gnu packages base)
                                                              guile-final)
                                                           system)))
                 (%current-system system))
    (mval store)))

;;; monads.scm end here