guix/records.scm

   1 ;;; GNU Guix --- Functional package management for GNU
   2 ;;; Copyright © 2012, 2013, 2014, 2015, 2016, 2017 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 record-error
  38   (syntax-rules ()
  39     "Report a syntactic error in use of CONSTRUCTOR."
  40     ((_ constructor form fmt args ...)
  41      (syntax-violation constructor
  42                        (format #f fmt args ...)
  43                        form))))
  44
  45 (define (report-invalid-field-specifier name bindings)
  46   "Report the first invalid binding among BINDINGS."
  47   (let loop ((bindings bindings))
  48     (syntax-case bindings ()
  49       (((field value) rest ...)                   ;good
  50        (loop #'(rest ...)))
  51       ((weird _ ...)                              ;weird!
  52        (syntax-violation name "invalid field specifier" #'weird)))))
  53
  54 (define-syntax make-syntactic-constructor
  55   (syntax-rules ()
  56     "Make the syntactic constructor NAME for TYPE, that calls CTOR, and
  57 expects all of EXPECTED fields to be initialized.  DEFAULTS is the list of
  58 FIELD/DEFAULT-VALUE tuples, THUNKED is the list of identifiers of thunked
  59 fields, and DELAYED is the list of identifiers of delayed fields."
  60     ((_ type name ctor (expected ...)
  61         #:thunked thunked
  62         #:delayed delayed
  63         #:innate innate
  64         #:defaults defaults)
  65      (define-syntax name
  66        (lambda (s)
  67          (define (record-inheritance orig-record field+value)
  68            ;; Produce code that returns a record identical to ORIG-RECORD,
  69            ;; except that values for the FIELD+VALUE alist prevail.
  70            (define (field-inherited-value f)
  71              (and=> (find (lambda (x)
  72                             (eq? f (car (syntax->datum x))))
  73                           field+value)
  74                     car))
  75
  76            ;; Make sure there are no unknown field names.
  77            (let* ((fields     (map (compose car syntax->datum) field+value))
  78                   (unexpected (lset-difference eq? fields '(expected ...))))
  79              (when (pair? unexpected)
  80                (record-error 'name s "extraneous field initializers ~a"
  81                              unexpected)))
  82
  83            #`(make-struct type 0
  84                           #,@(map (lambda (field index)
  85                                     (or (field-inherited-value field)
  86                                         (if (innate-field? field)
  87                                             (wrap-field-value
  88                                              field (field-default-value field))
  89                                             #`(struct-ref #,orig-record
  90                                                           #,index))))
  91                                   '(expected ...)
  92                                   (iota (length '(expected ...))))))
  93
  94          (define (thunked-field? f)
  95            (memq (syntax->datum f) 'thunked))
  96
  97          (define (delayed-field? f)
  98            (memq (syntax->datum f) 'delayed))
  99
 100          (define (innate-field? f)
 101            (memq (syntax->datum f) 'innate))
 102
 103          (define (wrap-field-value f value)
 104            (cond ((thunked-field? f)
 105                   #`(lambda () #,value))
 106                  ((delayed-field? f)
 107                   #`(delay #,value))
 108                  (else value)))
 109
 110          (define default-values
 111            ;; List of symbol/value tuples.
 112            (map (match-lambda
 113                   ((f v)
 114                    (list (syntax->datum f) v)))
 115                 #'defaults))
 116
 117          (define (field-default-value f)
 118            (car (assoc-ref default-values (syntax->datum f))))
 119
 120          (define (field-bindings field+value)
 121            ;; Return field to value bindings, for use in 'let*' below.
 122            (map (lambda (field+value)
 123                   (syntax-case field+value ()
 124                     ((field value)
 125                      #`(field
 126                         #,(wrap-field-value #'field #'value)))))
 127                 field+value))
 128
 129          (syntax-case s (inherit expected ...)
 130            ((_ (inherit orig-record) (field value) (... ...))
 131             #`(let* #,(field-bindings #'((field value) (... ...)))
 132                 #,(record-inheritance #'orig-record
 133                                       #'((field value) (... ...)))))
 134            ((_ (field value) (... ...))
 135             (let ((fields (map syntax->datum #'(field (... ...)))))
 136               (define (field-value f)
 137                 (or (find (lambda (x)
 138                             (eq? f (syntax->datum x)))
 139                           #'(field (... ...)))
 140                     (wrap-field-value f (field-default-value f))))
 141
 142               (let ((fields (append fields (map car default-values))))
 143                 (cond ((lset= eq? fields '(expected ...))
 144                        #`(let* #,(field-bindings
 145                                   #'((field value) (... ...)))
 146                            (ctor #,@(map field-value '(expected ...)))))
 147                       ((pair? (lset-difference eq? fields
 148                                                '(expected ...)))
 149                        (record-error 'name s
 150                                      "extraneous field initializers ~a"
 151                                      (lset-difference eq? fields
 152                                                       '(expected ...))))
 153                       (else
 154                        (record-error 'name s
 155                                      "missing field initializers ~a"
 156                                      (lset-difference eq?
 157                                                       '(expected ...)
 158                                                       fields)))))))
 159            ((_ bindings (... ...))
 160             ;; One of BINDINGS doesn't match the (field value) pattern.
 161             ;; Report precisely which one is faulty, instead of letting the
 162             ;; "source expression failed to match any pattern" error.
 163             (report-invalid-field-specifier 'name
 164                                             #'(bindings (... ...))))))))))
 165
 166 (define-syntax-rule (define-field-property-predicate predicate property)
 167   "Define PREDICATE as a procedure that takes a syntax object and, when passed
 168 a field specification, returns the field name if it has the given PROPERTY."
 169   (define (predicate s)
 170     (syntax-case s (property)
 171       ((field (property values (... ...)) _ (... ...))
 172        #'field)
 173       ((field _ properties (... ...))
 174        (predicate #'(field properties (... ...))))
 175       (_ #f))))
 176
 177 (define-syntax define-record-type*
 178   (lambda (s)
 179     "Define the given record type such that an additional \"syntactic
 180 constructor\" is defined, which allows instances to be constructed with named
 181 field initializers, à la SRFI-35, as well as default values.  An example use
 182 may look like this:
 183
 184   (define-record-type* <thing> thing make-thing
 185     thing?
 186     (name  thing-name (default \"chbouib\"))
 187     (port  thing-port
 188            (default (current-output-port)) (thunked))
 189     (loc   thing-location (innate) (default (current-source-location))))
 190
 191 This example defines a macro 'thing' that can be used to instantiate records
 192 of this type:
 193
 194   (thing
 195     (name \"foo\")
 196     (port (current-error-port)))
 197
 198 The value of 'name' or 'port' could as well be omitted, in which case the
 199 default value specified in the 'define-record-type*' form is used:
 200
 201   (thing)
 202
 203 The 'port' field is \"thunked\", meaning that calls like '(thing-port x)' will
 204 actually compute the field's value in the current dynamic extent, which is
 205 useful when referring to fluids in a field's value.
 206
 207 A field can also be marked as \"delayed\" instead of \"thunked\", in which
 208 case its value is effectively wrapped in a (delay …) form.
 209
 210 It is possible to copy an object 'x' created with 'thing' like this:
 211
 212   (thing (inherit x) (name \"bar\"))
 213
 214 This expression returns a new object equal to 'x' except for its 'name'
 215 field and its 'loc' field---the latter is marked as \"innate\", so it is not
 216 inherited."
 217
 218     (define (field-default-value s)
 219       (syntax-case s (default)
 220         ((field (default val) _ ...)
 221          (list #'field #'val))
 222         ((field _ properties ...)
 223          (field-default-value #'(field properties ...)))
 224         (_ #f)))
 225
 226     (define-field-property-predicate delayed-field? delayed)
 227     (define-field-property-predicate thunked-field? thunked)
 228     (define-field-property-predicate innate-field? innate)
 229
 230     (define (wrapped-field? s)
 231       (or (thunked-field? s) (delayed-field? s)))
 232
 233     (define (wrapped-field-accessor-name field)
 234       ;; Return the name (an unhygienic syntax object) of the "real"
 235       ;; getter for field, which is assumed to be a wrapped field.
 236       (syntax-case field ()
 237         ((field get properties ...)
 238          (let* ((getter      (syntax->datum #'get))
 239                 (real-getter (symbol-append '% getter '-real)))
 240            (datum->syntax #'get real-getter)))))
 241
 242     (define (field-spec->srfi-9 field)
 243       ;; Convert a field spec of our style to a SRFI-9 field spec of the
 244       ;; form (field get).
 245       (syntax-case field ()
 246         ((name get properties ...)
 247          #`(name
 248             #,(if (wrapped-field? field)
 249                   (wrapped-field-accessor-name field)
 250                   #'get)))))
 251
 252     (define (thunked-field-accessor-definition field)
 253       ;; Return the real accessor for FIELD, which is assumed to be a
 254       ;; thunked field.
 255       (syntax-case field ()
 256         ((name get _ ...)
 257          (with-syntax ((real-get (wrapped-field-accessor-name field)))
 258            #'(define-inlinable (get x)
 259                ;; The real value of that field is a thunk, so call it.
 260                ((real-get x)))))))
 261
 262     (define (delayed-field-accessor-definition field)
 263       ;; Return the real accessor for FIELD, which is assumed to be a
 264       ;; delayed field.
 265       (syntax-case field ()
 266         ((name get _ ...)
 267          (with-syntax ((real-get (wrapped-field-accessor-name field)))
 268            #'(define-inlinable (get x)
 269                ;; The real value of that field is a promise, so force it.
 270                (force (real-get x)))))))
 271
 272     (syntax-case s ()
 273       ((_ type syntactic-ctor ctor pred
 274           (field get properties ...) ...)
 275        (let* ((field-spec #'((field get properties ...) ...))
 276               (thunked    (filter-map thunked-field? field-spec))
 277               (delayed    (filter-map delayed-field? field-spec))
 278               (innate     (filter-map innate-field? field-spec))
 279               (defaults   (filter-map field-default-value
 280                                       #'((field properties ...) ...))))
 281          (with-syntax (((field-spec* ...)
 282                         (map field-spec->srfi-9 field-spec))
 283                        ((thunked-field-accessor ...)
 284                         (filter-map (lambda (field)
 285                                       (and (thunked-field? field)
 286                                            (thunked-field-accessor-definition
 287                                             field)))
 288                                     field-spec))
 289                        ((delayed-field-accessor ...)
 290                         (filter-map (lambda (field)
 291                                       (and (delayed-field? field)
 292                                            (delayed-field-accessor-definition
 293                                             field)))
 294                                     field-spec)))
 295            #`(begin
 296                (define-record-type type
 297                  (ctor field ...)
 298                  pred
 299                  field-spec* ...)
 300                thunked-field-accessor ...
 301                delayed-field-accessor ...
 302                (make-syntactic-constructor type syntactic-ctor ctor
 303                                            (field ...)
 304                                            #:thunked #,thunked
 305                                            #:delayed #,delayed
 306                                            #:innate #,innate
 307                                            #:defaults #,defaults))))))))
 308
 309 (define* (alist->record alist make keys
 310                         #:optional (multiple-value-keys '()))
 311   "Apply MAKE to the values associated with KEYS in ALIST.  Items in KEYS that
 312 are also in MULTIPLE-VALUE-KEYS are considered to occur possibly multiple
 313 times in ALIST, and thus their value is a list."
 314   (let ((args (map (lambda (key)
 315                      (if (member key multiple-value-keys)
 316                          (filter-map (match-lambda
 317                                       ((k . v)
 318                                        (and (equal? k key) v)))
 319                                      alist)
 320                          (assoc-ref alist key)))
 321                    keys)))
 322     (apply make args)))
 323
 324 (define (object->fields object fields port)
 325   "Write OBJECT (typically a record) as a series of recutils-style fields to
 326 PORT, according to FIELDS.  FIELDS must be a list of field name/getter pairs."
 327   (let loop ((fields fields))
 328     (match fields
 329       (()
 330        object)
 331       (((field . get) rest ...)
 332        (format port "~a: ~a~%" field (get object))
 333        (loop rest)))))
 334
 335 (define %recutils-field-charset
 336   ;; Valid characters starting a recutils field.
 337   ;; info "(recutils) Fields"
 338   (char-set-union char-set:upper-case
 339                   char-set:lower-case
 340                   (char-set #\%)))
 341
 342 (define (recutils->alist port)
 343   "Read a recutils-style record from PORT and return it as a list of key/value
 344 pairs.  Stop upon an empty line (after consuming it) or EOF."
 345   (let loop ((line   (read-line port))
 346              (result '()))
 347     (cond ((eof-object? line)
 348            (reverse result))
 349           ((string-null? line)
 350            (if (null? result)
 351                (loop (read-line port) result)     ; leading space: ignore it
 352                (reverse result)))                 ; end-of-record marker
 353           (else
 354            ;; Now check the first character of LINE, since that's what the
 355            ;; recutils manual says is enough.
 356            (let ((first (string-ref line 0)))
 357              (cond
 358               ((char-set-contains? %recutils-field-charset first)
 359                (let* ((colon (string-index line #\:))
 360                       (field (string-take line colon))
 361                       (value (string-trim (string-drop line (+ 1 colon)))))
 362                  (loop (read-line port)
 363                        (alist-cons field value result))))
 364               ((eqv? first #\#)                   ;info "(recutils) Comments"
 365                (loop (read-line port) result))
 366               ((eqv? first #\+)                   ;info "(recutils) Fields"
 367                (let ((new-line (if (string-prefix? "+ " line)
 368                                    (string-drop line 2)
 369                                    (string-drop line 1))))
 370                 (match result
 371                   (((field . value) rest ...)
 372                    (loop (read-line port)
 373                          `((,field . ,(string-append value "\n" new-line))
 374                            ,@rest))))))
 375               (else
 376                (error "unmatched line" line))))))))
 377
 378 ;;; records.scm ends here