guix/records.scm

   1 ;;; GNU Guix --- Functional package management for GNU
   2 ;;; Copyright © 2012, 2013 Ludovic Courtès <ludo@gnu.org>
   3 ;;;
   4 ;;; This file is part of GNU Guix.
   5 ;;;
   6 ;;; GNU Guix is free software; you can redistribute it and/or modify it
   7 ;;; under the terms of the GNU General Public License as published by
   8 ;;; the Free Software Foundation; either version 3 of the License, or (at
   9 ;;; your option) any later version.
  10 ;;;
  11 ;;; GNU Guix is distributed in the hope that it will be useful, but
  12 ;;; WITHOUT ANY WARRANTY; without even the implied warranty of
  13 ;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14 ;;; GNU General Public License for more details.
  15 ;;;
  16 ;;; You should have received a copy of the GNU General Public License
  17 ;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
  18
  19 (define-module (guix records)
  20   #:use-module (srfi srfi-1)
  21   #:use-module (srfi srfi-9)
  22   #:use-module (srfi srfi-26)
  23   #:use-module (ice-9 match)
  24   #:use-module (ice-9 regex)
  25   #:use-module (ice-9 rdelim)
  26   #:export (define-record-type*
  27             alist->record
  28             object->fields
  29             recutils->alist))
  30
  31 ;;; Commentary:
  32 ;;;
  33 ;;; Utilities for dealing with Scheme records.
  34 ;;;
  35 ;;; Code:
  36
  37 (define-syntax define-record-type*
  38   (lambda (s)
  39     "Define the given record type such that an additional \"syntactic
  40 constructor\" is defined, which allows instances to be constructed with named
  41 field initializers, à la SRFI-35, as well as default values."
  42     (define (make-syntactic-constructor type name ctor fields thunked defaults)
  43       "Make the syntactic constructor NAME for TYPE, that calls CTOR, and
  44 expects all of FIELDS to be initialized.  DEFAULTS is the list of
  45 FIELD/DEFAULT-VALUE tuples, and THUNKED is the list of identifiers of
  46 thunked fields."
  47       (with-syntax ((type     type)
  48                     (name     name)
  49                     (ctor     ctor)
  50                     (expected fields)
  51                     (defaults defaults))
  52         #`(define-syntax name
  53             (lambda (s)
  54               (define (record-inheritance orig-record field+value)
  55                 ;; Produce code that returns a record identical to
  56                 ;; ORIG-RECORD, except that values for the FIELD+VALUE alist
  57                 ;; prevail.
  58                 (define (field-inherited-value f)
  59                   (and=> (find (lambda (x)
  60                                  (eq? f (car (syntax->datum x))))
  61                                field+value)
  62                          car))
  63
  64                 #`(make-struct type 0
  65                                #,@(map (lambda (field index)
  66                                          (or (field-inherited-value field)
  67                                              #`(struct-ref #,orig-record
  68                                                            #,index)))
  69                                        'expected
  70                                        (iota (length 'expected)))))
  71
  72               (define (thunked-field? f)
  73                 (memq (syntax->datum f) '#,thunked))
  74
  75               (define (field-bindings field+value)
  76                 ;; Return field to value bindings, for use in `letrec*' below.
  77                 (map (lambda (field+value)
  78                        (syntax-case field+value ()
  79                          ((field value)
  80                           #`(field
  81                              #,(if (thunked-field? #'field)
  82                                    #'(lambda () value)
  83                                    #'value)))))
  84                      field+value))
  85
  86               (syntax-case s (inherit #,@fields)
  87                 ((_ (inherit orig-record) (field value) (... ...))
  88                  #`(letrec* #,(field-bindings #'((field value) (... ...)))
  89                      #,(record-inheritance #'orig-record
  90                                            #'((field value) (... ...)))))
  91                 ((_ (field value) (... ...))
  92                  (let ((fields (map syntax->datum #'(field (... ...))))
  93                        (dflt   (map (match-lambda
  94                                      ((f v)
  95                                       (list (syntax->datum f) v)))
  96                                     #'defaults)))
  97
  98                    (define (field-value f)
  99                      (or (and=> (find (lambda (x)
 100                                         (eq? f (car (syntax->datum x))))
 101                                       #'((field value) (... ...)))
 102                                 car)
 103                          (let ((value
 104                                 (car (assoc-ref dflt
 105                                                 (syntax->datum f)))))
 106                            (if (thunked-field? f)
 107                                #`(lambda () #,value)
 108                                value))))
 109
 110                    (let-syntax ((error*
 111                                  (syntax-rules ()
 112                                    ((_ fmt args (... ...))
 113                                     (syntax-violation 'name
 114                                                       (format #f fmt args
 115                                                               (... ...))
 116                                                       s)))))
 117                      (let ((fields (append fields (map car dflt))))
 118                        (cond ((lset= eq? fields 'expected)
 119                               #`(letrec* #,(field-bindings
 120                                             #'((field value) (... ...)))
 121                                   (ctor #,@(map field-value 'expected))))
 122                              ((pair? (lset-difference eq? fields 'expected))
 123                               (error* "extraneous field initializers ~a"
 124                                       (lset-difference eq? fields 'expected)))
 125                              (else
 126                               (error* "missing field initializers ~a"
 127                                       (lset-difference eq? 'expected
 128                                                        fields)))))))))))))
 129
 130     (define (field-default-value s)
 131       (syntax-case s (default)
 132         ((field (default val) _ ...)
 133          (list #'field #'val))
 134         ((field _ options ...)
 135          (field-default-value #'(field options ...)))
 136         (_ #f)))
 137
 138     (define (thunked-field? s)
 139       ;; Return the field name if the field defined by S is thunked.
 140       (syntax-case s (thunked)
 141         ((field (thunked) _ ...)
 142          #'field)
 143         ((field _ options ...)
 144          (thunked-field? #'(field options ...)))
 145         (_ #f)))
 146
 147     (define (thunked-field-accessor-name field)
 148       ;; Return the name (an unhygienic syntax object) of the "real"
 149       ;; getter for field, which is assumed to be a thunked field.
 150       (syntax-case field ()
 151         ((field get options ...)
 152          (let* ((getter      (syntax->datum #'get))
 153                 (real-getter (symbol-append '% getter '-real)))
 154            (datum->syntax #'get real-getter)))))
 155
 156     (define (field-spec->srfi-9 field)
 157       ;; Convert a field spec of our style to a SRFI-9 field spec of the
 158       ;; form (field get).
 159       (syntax-case field ()
 160         ((name get options ...)
 161          #`(name
 162             #,(if (thunked-field? field)
 163                   (thunked-field-accessor-name field)
 164                   #'get)))))
 165
 166     (define (thunked-field-accessor-definition field)
 167       ;; Return the real accessor for FIELD, which is assumed to be a
 168       ;; thunked field.
 169       (syntax-case field ()
 170         ((name get _ ...)
 171          (with-syntax ((real-get (thunked-field-accessor-name field)))
 172            #'(define-inlinable (get x)
 173                ;; The real value of that field is a thunk, so call it.
 174                ((real-get x)))))))
 175
 176     (syntax-case s ()
 177       ((_ type syntactic-ctor ctor pred
 178           (field get options ...) ...)
 179        (let* ((field-spec #'((field get options ...) ...)))
 180          (with-syntax (((field-spec* ...)
 181                         (map field-spec->srfi-9 field-spec))
 182                        ((thunked-field-accessor ...)
 183                         (filter-map (lambda (field)
 184                                       (and (thunked-field? field)
 185                                            (thunked-field-accessor-definition
 186                                             field)))
 187                                     field-spec)))
 188            #`(begin
 189                (define-record-type type
 190                  (ctor field ...)
 191                  pred
 192                  field-spec* ...)
 193                (begin thunked-field-accessor ...)
 194                #,(make-syntactic-constructor #'type #'syntactic-ctor #'ctor
 195                                              #'(field ...)
 196                                              (filter-map thunked-field? field-spec)
 197                                              (filter-map field-default-value
 198                                                          #'((field options ...)
 199                                                             ...))))))))))
 200
 201 (define (alist->record alist make keys)
 202   "Apply MAKE to the values associated with KEYS in ALIST."
 203   (let ((args (map (cut assoc-ref alist <>) keys)))
 204     (apply make args)))
 205
 206 (define (object->fields object fields port)
 207   "Write OBJECT (typically a record) as a series of recutils-style fields to
 208 PORT, according to FIELDS.  FIELDS must be a list of field name/getter pairs."
 209   (let loop ((fields fields))
 210     (match fields
 211       (()
 212        object)
 213       (((field . get) rest ...)
 214        (format port "~a: ~a~%" field (get object))
 215        (loop rest)))))
 216
 217 (define %recutils-field-rx
 218   (make-regexp "^([[:graph:]]+): (.*)$"))
 219
 220 (define %recutils-comment-rx
 221   ;; info "(recutils) Comments"
 222   (make-regexp "^#"))
 223
 224 (define (recutils->alist port)
 225   "Read a recutils-style record from PORT and return it as a list of key/value
 226 pairs.  Stop upon an empty line (after consuming it) or EOF."
 227   (let loop ((line   (read-line port))
 228              (result '()))
 229     (cond ((eof-object? line)
 230            (reverse result))
 231           ((string-null? line)
 232            (if (null? result)
 233                (loop (read-line port) result)     ; leading space: ignore it
 234                (reverse result)))                 ; end-of-record marker
 235           ((regexp-exec %recutils-comment-rx line)
 236            (loop (read-line port) result))
 237           ((regexp-exec %recutils-field-rx line)
 238            =>
 239            (lambda (match)
 240              (loop (read-line port)
 241                    (alist-cons (match:substring match 1)
 242                                (match:substring match 2)
 243                                result))))
 244           (else
 245            (error "unmatched line" line)))))
 246
 247 ;;; records.scm ends here