From 25142daea5d819e9881df2be57e541821e5c53fe Mon Sep 17 00:00:00 2001
From: Ryan Culpepper <ryanc@racket-lang.org>
Date: Thu, 21 Nov 2019 13:21:04 +0100
Subject: [PATCH] change `datum` to work with syntax variables too

---
 .../racket-test/tests/stxparse/test-datum.rkt | 94 +++++++++++++++++++
 racket/collects/racket/private/template.rkt   |  4 +-
 2 files changed, 96 insertions(+), 2 deletions(-)
 create mode 100644 pkgs/racket-test/tests/stxparse/test-datum.rkt

diff --git a/pkgs/racket-test/tests/stxparse/test-datum.rkt b/pkgs/racket-test/tests/stxparse/test-datum.rkt
new file mode 100644
index 0000000000..8f317e70fb
--- /dev/null
+++ b/pkgs/racket-test/tests/stxparse/test-datum.rkt
@@ -0,0 +1,94 @@
+#lang racket/base
+(require rackunit syntax/parse syntax/datum)
+
+(define (equalish? a b)
+  (cond [(and (syntax? a) (syntax? b))
+         (equal? (syntax->datum a) (syntax->datum b))]
+        [else (equal?/recur a b equalish?)]))
+
+(check equalish?
+       (syntax-case #'(a b c) ()
+         [(x ...) (datum (x ...))])
+       (list #'a #'b #'c))
+
+(struct ast:bind (var rhs) #:prefab)
+
+(define-syntax-class binding
+  (pattern [var:id rhs:expr]
+           #:attr ast (ast:bind #'var #'rhs)))
+
+(check equalish?
+       (syntax-parse #'([x 1] [y 2])
+         [(b:binding ...) (datum (b ...))])
+       (list #'[x 1] #'[y 2]))
+
+(check equalish?
+       (syntax-parse #'([x 1] [y 2])
+         [(b:binding ...) (datum ((b.var b.rhs) ...))])
+       (list (list #'x #'1) (list #'y #'2)))
+
+(check equalish?
+       (syntax-parse #'([x 1] [y 2])
+         [(b:binding ...) (datum (b.ast ...))])
+       (list (ast:bind #'x #'1) (ast:bind #'y #'2)))
+
+(check equalish?
+       (syntax-parse #'([x 1] [y 2])
+         [(b:binding ...) (datum ((~@ b.var b.rhs) ...))])
+       (list #'x #'1 #'y #'2))
+
+(check equalish?
+       (syntax-parse #'([x 1] [y 2])
+         [(b:binding ...) (datum ((~@ . b) ...))])
+       (list #'x #'1 #'y #'2))
+
+(define-syntax-class obinding
+  (pattern [var:id (~optional (rhs:expr ...))]
+           #:attr ast (ast:bind #'var (datum (~? (rhs ...) #f)))))
+
+(check equalish?
+       (syntax-parse #'([x (1)] [y])
+         [(b:obinding ...) (datum ([b.var (~? (b.rhs ...))] ...))])
+       (list (list #'x (list #'1)) (list #'y)))
+
+(check equalish?
+       (syntax-parse #'([x (1)] [y])
+         [(b:obinding ...) (datum (b.ast ...))])
+       (list (ast:bind #'x (list #'1)) (ast:bind #'y #f)))
+
+;; ------------------------------------------------------------
+;; The strange corner cases...
+
+(require racket/list racket/promise)
+
+;; The following are two consequences of the decision to make (datum a)
+;; equivalent to (attribute a), where a is an attribute. Thus if a is "absent"
+;; (has the value #f), (datum a) returns #f rather than signaling an
+;; error. Likewise, if the value of a is a promise, it just returns the promise.
+
+;; 1: ~? catches attempts to iterate over absent attrs, but not uses of absent
+;; attrs. Maybe add some sort of annotation to get other behavior?
+
+(check-equal? (syntax-parse #'(m)
+                [(_ (~optional x:id)) (datum (~? x default))])
+              #f)
+
+(check-equal? (syntax-parse #'(m)
+                [(_ (~optional (x:id ...))) (datum (~? (x ...) default))])
+              'default)
+
+;; 2: Promises are forced only when necessary for iterating over lists.
+
+(define-syntax-class nrange #:attributes ([r 0] [k 1])
+  (pattern n:nat
+           ;; Note: these attribute declarations are identical except for depth.
+           #:attr [r 0] (delay (range (syntax-e #'n)))
+           #:attr [k 1] (delay (range (syntax-e #'n)))))
+
+;; This returns a list of numbers:
+(check-equal? (syntax-parse #'(m 10) [(_ n:nrange) (datum (n.k ...))])
+              (range 10))
+
+;; But this returns a promise:
+(check-pred promise?
+            (syntax-parse #'(m 10) [(_ n:nrange) (datum n.r)]))
diff --git a/racket/collects/racket/private/template.rkt b/racket/collects/racket/private/template.rkt
index e3de8fb263..0c41958af9 100644
--- a/racket/collects/racket/private/template.rkt
+++ b/racket/collects/racket/private/template.rkt
@@ -364,7 +364,7 @@
     (define (lookup id depth0)
       (define (make-pvar var check pvar-depth)
         (define (make-ref var)
-          (cond [check `(t-check-var (,check ,var 0 #t (quote-syntax ,id)))]
+          (cond [check `(t-check-var (,check ,var 0 ,stx? (quote-syntax ,id)))]
                 [else `(t-var ,var)]))
         (define (make-src-ref var id)
           (cond [check `(#%expression (,check ,var 1 #f (quote-syntax ,id)))]
@@ -386,7 +386,7 @@
                         (dotsframe-add! (car depth) iter src (make-src-ref src id))
                         iter))]))))
       (let ([v (syntax-local-value id (lambda () #f))])
-        (cond [(and stx? (syntax-pattern-variable? v))
+        (cond [(syntax-pattern-variable? v)
                (define pvar-depth (syntax-mapping-depth v))
                (define attr
                  (let ([attr (syntax-local-value (syntax-mapping-valvar v) (lambda () #f))])