[PATCH RFC 1/4] records: Support field sanitizers.

["Ludovic Courtès" <ludo@gnu.org>]

* guix/records.scm (make-syntactic-constructor): Add #:sanitizers.
[field-sanitizer]: New procedure.
[wrap-field-value]: Honor F's sanitizer.
(define-record-type*)[field-sanitizer]: New procedure.
Pass #:sanitizer to 'make-syntactic-constructor'.
* tests/records.scm ("define-record-type* & sanitize")
("define-record-type* & sanitize & thunked"): New tests.
---
 guix/records.scm  | 65 +++++++++++++++++++++++++++++++++++++----------
 tests/records.scm | 38 +++++++++++++++++++++++++++
 2 files changed, 89 insertions(+), 14 deletions(-)

diff --git a/guix/records.scm b/guix/records.scm
index 3d54a51956..ed94c83dac 100644
--- a/guix/records.scm
+++ b/guix/records.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2018 Mark H Weaver <mhw@netris.org>
 ;;;
 ;;; This file is part of GNU Guix.
@@ -120,7 +120,8 @@ context of the definition of a thunked field."
     "Make the syntactic constructor NAME for TYPE, that calls CTOR, and
 expects all of EXPECTED fields to be initialized.  DEFAULTS is the list of
 FIELD/DEFAULT-VALUE tuples, THUNKED is the list of identifiers of thunked
-fields, and DELAYED is the list of identifiers of delayed fields.
+fields, DELAYED is the list of identifiers of delayed fields, and SANITIZERS
+is the list of FIELD/SANITIZER tuples.
 
 ABI-COOKIE is the cookie (an integer) against which to check the run-time ABI
 of TYPE matches the expansion-time ABI."
@@ -130,6 +131,7 @@ of TYPE matches the expansion-time ABI."
         #:this-identifier this-identifier
         #:delayed delayed
         #:innate innate
+        #:sanitizers sanitizers
         #:defaults defaults)
      (define-syntax name
        (lambda (s)
@@ -169,19 +171,30 @@ of TYPE matches the expansion-time ABI."
          (define (innate-field? f)
            (memq (syntax->datum f) 'innate))
 
+         (define field-sanitizer
+           (let ((lst (map (match-lambda
+                             ((f p)
+                              (list (syntax->datum f) p)))
+                           #'sanitizers)))
+             (lambda (f)
+               (or (and=> (assoc-ref lst (syntax->datum f)) car)
+                   #'(lambda (x) x)))))
+
          (define (wrap-field-value f value)
-           (cond ((thunked-field? f)
-                  #`(lambda (x)
-                      (syntax-parameterize ((#,this-identifier
-                                             (lambda (s)
-                                               (syntax-case s ()
-                                                 (id
-                                                  (identifier? #'id)
-                                                  #'x)))))
-                        #,value)))
-                 ((delayed-field? f)
-                  #`(delay #,value))
-                 (else value)))
+           (let* ((sanitizer (field-sanitizer f))
+                  (value     #`(#,sanitizer #,value)))
+             (cond ((thunked-field? f)
+                    #`(lambda (x)
+                        (syntax-parameterize ((#,this-identifier
+                                               (lambda (s)
+                                                 (syntax-case s ()
+                                                   (id
+                                                    (identifier? #'id)
+                                                    #'x)))))
+                          #,value)))
+                   ((delayed-field? f)
+                    #`(delay #,value))
+                   (else value))))
 
          (define default-values
            ;; List of symbol/value tuples.
@@ -291,6 +304,19 @@ can access the record it belongs to via the 'this-thing' identifier.
 A field can also be marked as \"delayed\" instead of \"thunked\", in which
 case its value is effectively wrapped in a (delay …) form.
 
+A field can also have an associated \"sanitizer\", which is a procedure that
+takes a user-supplied field value and returns a \"sanitized\" value for the
+field:
+
+  (define-record-type* <thing> thing make-thing
+    thing?
+    this-thing
+    (name  thing-name
+           (sanitize (lambda (value)
+                       (cond ((string? value) value)
+                             ((symbol? value) (symbol->string value))
+                             (else (throw 'bad! value)))))))
+
 It is possible to copy an object 'x' created with 'thing' like this:
 
   (thing (inherit x) (name \"bar\"))
@@ -307,6 +333,14 @@ inherited."
          (field-default-value #'(field properties ...)))
         (_ #f)))
 
+    (define (field-sanitizer s)
+      (syntax-case s (sanitize)
+        ((field (sanitize proc) _ ...)
+         (list #'field #'proc))
+        ((field _ properties ...)
+         (field-sanitizer #'(field properties ...)))
+        (_ #f)))
+
     (define-field-property-predicate delayed-field? delayed)
     (define-field-property-predicate thunked-field? thunked)
     (define-field-property-predicate innate-field? innate)
@@ -376,6 +410,8 @@ inherited."
               (innate     (filter-map innate-field? field-spec))
               (defaults   (filter-map field-default-value
                                       #'((field properties ...) ...)))
+              (sanitizers (filter-map field-sanitizer
+                                        #'((field properties ...) ...)))
               (cookie     (compute-abi-cookie field-spec)))
          (with-syntax (((field-spec* ...)
                         (map field-spec->srfi-9 field-spec))
@@ -421,6 +457,7 @@ of a record instantiation"
                                            #:this-identifier #'this-identifier
                                            #:delayed #,delayed
                                            #:innate #,innate
+                                           #:sanitizers #,sanitizers
                                            #:defaults #,defaults)))))
       ((_ type syntactic-ctor ctor pred
           (field get properties ...) ...)
diff --git a/tests/records.scm b/tests/records.scm
index 706bb3dbfd..d014e7a995 100644
--- a/tests/records.scm
+++ b/tests/records.scm
@@ -283,6 +283,44 @@
              (equal? (foo-bar y) 1))              ;promise was already forced
            (eq? (foo-baz y) 'b)))))
 
+(test-assert "define-record-type* & sanitize"
+  (begin
+    (define-record-type* <foo> foo make-foo
+      foo?
+      (bar foo-bar
+           (default "bar")
+           (sanitize (lambda (x) (string-append x "!")))))
+
+    (let* ((p (foo))
+           (q (foo (inherit p)))
+           (r (foo (inherit p) (bar "baz")))
+           (s (foo (bar "baz"))))
+      (and (string=? (foo-bar p) "bar!")
+           (equal? q p)
+           (string=? (foo-bar r) "baz!")
+           (equal? s r)))))
+
+(test-assert "define-record-type* & sanitize & thunked"
+  (let ((sanitized 0))
+    (define-record-type* <foo> foo make-foo
+      foo?
+      (bar foo-bar
+           (default "bar")
+           (sanitize (lambda (x)
+                       (set! sanitized (+ 1 sanitized))
+                       (string-append x "!")))))
+
+    (let ((p (foo)))
+      (and (string=? (foo-bar p) "bar!")
+           (string=? (foo-bar p) "bar!")          ;twice
+           (= sanitized 1)             ;sanitizer was called at init time only
+           (let ((q (foo (bar "baz"))))
+             (and (string=? (foo-bar q) "baz!")
+                  (string=? (foo-bar q) "baz!")   ;twice
+                  (= sanitized 2)
+                  (let ((r (foo (inherit q))))
+                    (and (string=? (foo-bar r) "baz!")
+                         (= sanitized 2)))))))))  ;no re-sanitization
 (test-assert "define-record-type* & wrong field specifier"
   (let ((exp '(begin
                 (define-record-type* <foo> foo make-foo
-- 
2.31.1