.

copy-attribute.rkt

@@ -0,0 +1,73 @@
+#lang racket
+
+(provide copy-raw-syntax-attribute
+         attribute-val/c)
+
+(require stxparse-info/current-pvars
+         phc-toolkit/untyped
+         stxparse-info/parse
+         (for-syntax racket/contract
+                     racket/syntax
+                     phc-toolkit/untyped
+                     racket/function
+                     stxparse-info/parse))
+
+(begin-for-syntax
+  (define/contract (nest-map f last n)
+    (-> (-> syntax? syntax?) syntax? exact-nonnegative-integer? syntax?)
+    (if (= n 0)
+        last
+        (f (nest-map f last (sub1 n))))))
+
+(define/contract (attribute-val/c depth [bottom-predicate any/c])
+  (->* {exact-nonnegative-integer?} {flat-contract?} flat-contract?)
+  (flat-named-contract
+   (build-compound-type-name 'attribute-val/c depth bottom-predicate)
+   (λ (l)
+     (if (= depth 0)
+         (or (false? l) (bottom-predicate l))
+         (or (false? l)
+             (and (list? l)
+                  (andmap (attribute-val/c (sub1 depth)) l)))))))
+
+(struct wrapped (value))
+
+(define (attribute-wrap val depth)
+  (if (= depth 0)
+      (wrapped val)
+      (if val
+          (map (λ (v) (attribute-wrap v (sub1 depth)))
+               val)
+          #f)))
+
+;; manually creating the attribute with (make-attribute-mapping …)
+;; works, but the attribute behaves in a bogus way when put inside
+;; an (?@ yᵢ ...). I must be missing some step in the construction
+;; of the attribute
+(define-syntax/parse (copy-raw-syntax-attribute name:id
+                                                attr-value:expr
+                                                ellipsis-depth:nat
+                                                syntax?:boolean
+                                                props:expr)
+  ;; the ~and is important, to prevent the nested ~or from being treated as
+  ;; an ellipsis-head pattern.
+  #:with nested (nest-map (λ (pat) #`{~or #f ({~and #,pat} (... ...))})
+                          (if (syntax-e #'syntax?)
+                              #'{~or #f name}
+                              ;; variable with empty name, so that the attribute
+                              ;; gets exported without a prefix.
+                              #`{~or #f {~var #,(datum->syntax #'name '||)
+                                              extract-non-syntax}})
+                          (syntax-e #'ellipsis-depth))
+  (if (syntax-e #'syntax?)
+      #'(begin
+          (define/syntax-parse nested attr-value)
+          (define-pvars name))
+      #'(begin
+          (define-syntax-class extract-non-syntax
+            #:attributes (name)
+            (pattern v
+                     #:attr name (wrapped-value (syntax-e #'v))))
+          (define/syntax-parse nested (attribute-wrap attr-value
+                                                      ellipsis-depth))
+          (define-pvars name))))

ddd.rkt

@@ -0,0 +1,212 @@
+#lang racket
+
+(provide ddd)
+
+(require stxparse-info/current-pvars
+         phc-toolkit/untyped
+         subtemplate/copy-attribute
+         (prefix-in - syntax/parse/private/residual)
+         (for-syntax "derived-valvar.rkt"
+                     racket/contract
+                     racket/syntax
+                     phc-toolkit/untyped
+                     racket/function
+                     racket/struct
+                     racket/list
+                     syntax/id-set
+                     racket/private/sc
+                     scope-operations
+                     racket/string))
+
+(define-for-syntax x-pvar-scope (make-syntax-introducer))
+(define-for-syntax x-pvar-present-marker (make-syntax-introducer))
+
+(begin-for-syntax
+  (define/contract (attribute-real-valvar attr)
+    (-> identifier? (or/c #f identifier?))
+    (define valvar1
+      (let ([slv (syntax-local-value attr (λ () #f))])
+        (if (syntax-pattern-variable? slv)
+            (let* ([valvar (syntax-mapping-valvar slv)]
+                   [valvar-slv (syntax-local-value valvar (λ () #f))])
+              (if (-attribute-mapping? valvar-slv)
+                  (-attribute-mapping-var valvar-slv)
+                  valvar))
+            (raise-syntax-error
+             'attribute*
+             "not bound as an attribute or pattern variable"
+             attr))))
+    ;; Try to extract the actual variable from a subtemplate derived valvar.
+    (define valvar2
+      (let ([valvar1-slv (syntax-local-value valvar1 (λ () #f))])
+        (if (derived-valvar? valvar1-slv)
+            (derived-valvar-valvar valvar1-slv)
+            valvar1)))
+    (if (syntax-local-value valvar2 (λ () #f)) ;; is it a macro-ish thing?
+        (begin
+          (log-warning
+           (string-append "Could not extract the plain variable corresponding to"
+                          " the pattern variable or attribute ~a"
+                          (syntax-e attr)))
+          #f)
+        valvar2)))
+
+;; free-identifier=? seems to stop working on the valvars once we are outside of
+;; the local-expand containing the let which introduced these valvars, therefore
+;; we find which pvars were present within that let.
+(define-syntax/case (detect-present-pvars (pvar …) body) ()
+  (define/with-syntax (pvar-real-valvar …)
+    (map syntax-local-introduce
+         (stx-map attribute-real-valvar #'(pvar …))))
+
+  (define/with-syntax expanded-body
+    (local-expand #`(let-values ()
+                      (quote-syntax #,(stx-map x-pvar-scope #'(pvar-real-valvar …)) #:local)
+                      body)
+                  'expression
+                  '()))
+
+  ;; Separate the valvars marked with x-pvar-scope, so that we know which valvar
+  ;; to look for.
+  (define-values (marked-real-valvar expanded-ids)
+    (partition (λ (id) (all-scopes-in? x-pvar-scope id))
+               (extract-ids #'expanded-body)))
+  (define/with-syntax (real-valvar …)
+    (map (λ (x-vv) (x-pvar-scope x-vv 'remove))
+         marked-real-valvar))
+  (define expanded-ids-set (immutable-free-id-set expanded-ids))
+
+  ;; grep for valvars in expanded-body
+  (define/with-syntax present-variables
+    (for/vector ([x-vv (in-syntax #'(real-valvar …))]
+                 [pv (in-syntax #'(pvar …))])
+      (if (free-id-set-member? expanded-ids-set x-vv)
+          #t
+          #f)))
+  
+  #`(let-values ()
+      (quote-syntax #,(x-pvar-present-marker #'present-variables))
+      body))
+
+(define-syntax/case (ddd body) ()
+  (define/with-syntax (pvar …)
+    (map syntax-local-introduce
+         (filter (conjoin identifier?
+                          (λ~> (syntax-local-value _ (thunk #f))
+                               syntax-pattern-variable?)
+                          attribute-real-valvar)
+                 (current-pvars))))
+  (define-temp-ids "~aᵢ" (pvar …))
+  (define/with-syntax f
+    #`(#%plain-lambda (pvarᵢ …)
+                      (shadow pvar pvarᵢ) …
+                      (let-values ()
+                        (detect-present-pvars (pvar …)
+                                              body))))
+
+  ;; extract all the variable ids present in f
+  (define/with-syntax expanded-f (local-expand #'f 'expression '()))
+
+  (begin
+    (define present-variables** (find-present-variables-vector #'expanded-f))
+    (define present-variables*
+      (and (vector? present-variables**)
+           (vector->list present-variables**)))
+    (unless ((listof (syntax/c boolean?)) present-variables*)
+      (raise-syntax-error 'ddd
+                          (string-append
+                           "internal error: could not extract the vector of"
+                           " pattern variables present in the body.")
+                          stx))
+    (define present-variables (map syntax-e present-variables*)))
+
+  (unless (ormap identity present-variables)
+    (raise-syntax-error 'ddd
+                        "no pattern variables were found in the body"
+                        stx))
+
+  (begin
+    (define present?+pvars
+      (for/list ([present? (in-list present-variables)]
+                 [pv (in-syntax #'(pvar …))]
+                 [pvᵢ (in-syntax #'(pvarᵢ …))])
+        (if present?
+            (match (attribute-info pv)
+              [(list* _ _valvar depth _)
+               (if (> depth 0)
+                   (list #t pv pvᵢ #t depth)
+                   (list #f pv pvᵢ #t depth))]) ;; TODO: detect shadowed bindings, if the pvar was already iterated on, raise an error (we went too deep).
+            (list #f pv pvᵢ #f))))
+    ;; Pvars which are iterated over
+    (define/with-syntax ((_ iterated-pvar iterated-pvarᵢ _ _) …)
+      (filter car present?+pvars))
+
+    (when (stx-null? #'(iterated-pvar …))
+      (no-pvar-to-iterate-error present?+pvars))
+    
+    ;; If the pvar is iterated, use the iterated pvarᵢ 
+    ;; otherwise use the original (attribute* pvar)
+    (define/with-syntax (filling-pvar …)
+      (map (match-λ [(list #t pv pvᵢ _ _) pvᵢ]
+                    [(list #f pv pvᵢ _ _) #`(attribute* #,pv)])
+           present?+pvars)))
+  
+  #'(map (λ (iterated-pvarᵢ …)
+           (expanded-f filling-pvar …))
+         (attribute* iterated-pvar)
+         …))
+
+(define-syntax/case (shadow pvar new-value) ()
+  (match (attribute-info #'pvar '(pvar attr))
+    [`(attr ,valvar ,depth ,_name ,syntax?)
+     #`(copy-raw-syntax-attribute pvar
+                                  new-value
+                                  #,(max 0 (sub1 depth))
+                                  #,syntax?)]
+    [`(pvar ,valvar ,depth)
+     #`(define-raw-syntax-mapping pvar
+         tmp-valvar
+         new-value
+         #,(sub1 depth))]))
+
+(define-for-syntax (extract-ids/tree e)
+  (cond
+    [(identifier? e) e]
+    [(syntax? e) (extract-ids/tree (syntax-e e))]
+    [(pair? e) (cons (extract-ids/tree (car e)) (extract-ids/tree (cdr e)))]
+    [(vector? e) (extract-ids/tree (vector->list e))]
+    [(hash? e) (extract-ids/tree (hash->list e))]
+    [(prefab-struct-key e) (extract-ids/tree (struct->list e))]
+    [else null]))
+
+(define-for-syntax (extract-ids e)
+  (flatten (extract-ids/tree e)))
+
+(define-for-syntax (find-present-variables-vector e)
+  (cond
+    [(and (syntax? e)
+          (vector? (syntax-e e))
+          (all-scopes-in? x-pvar-present-marker e))
+     (syntax-e e)]
+    [(syntax? e) (find-present-variables-vector (syntax-e e))]
+    [(pair? e) (or (find-present-variables-vector (car e))
+                   (find-present-variables-vector (cdr e)))]
+    [(vector? e) (find-present-variables-vector (vector->list e))]
+    [(hash? e) (find-present-variables-vector (hash->list e))]
+    [(prefab-struct-key e) (find-present-variables-vector (struct->list e))]
+    [else #f]))
+
+(define-for-syntax (no-pvar-to-iterate-error present?+pvars)
+  (raise-syntax-error
+   'ddd
+   (string-append
+    "no pattern variables with depth > 0 were found in the body\n"
+    "  pattern varialbes present in the body:\n"
+    "   "
+    (string-join
+     (map (λ (present?+pvar)
+            (format "~a at depth ~a"
+                    (syntax-e (second present?+pvar))
+                    (fifth present?+pvar)))
+          (filter fourth present?+pvars))
+     "\n   "))))

derived-valvar.rkt

@@ -0,0 +1,48 @@
+#lang racket/base
+
+(provide valvar+props
+         valvar+props-valvar
+         valvar+props-properties
+         pvar->valvar+props
+         pvar-property)
+
+(require racket/function
+         racket/contract
+         racket/private/sc
+         (for-template (prefix-in - stxparse-info/parse/private/residual)))
+
+;; Act like a syntax transformer, but which is recognizable via the
+;; derived-pattern-variable? predicate.
+(struct valvar+props (valvar properties)
+  #:property prop:procedure
+  (λ (self stx)
+    #`(#%expression #,(valvar+props-valvar self))))
+
+(define (pvar->valvar+props id)
+  (define mapping (syntax-local-value id (thunk #f)))
+  (and mapping ;; … defined as syntax
+       (syntax-pattern-variable? mapping) ; and is a syntax pattern variable
+       (let ()
+         (define mapping-slv
+           (syntax-local-value (syntax-mapping-valvar mapping) (thunk #f)))
+         ;; either a mapping → attribute → derived,
+         ;; or directly mapping → derived
+         (or (and (-attribute-mapping? mapping-slv) ;; is an attribute
+                  (let ([attribute-slv (syntax-local-value
+                                        (-attribute-mapping-var mapping-slv)
+                                        (thunk #f))])
+                    ;; and the pvar's valvar is a derived
+                    (and (valvar+props? attribute-slv)
+                         attribute-slv))
+             ;; or the pvar's valvar is derived
+             (and (valvar+props? mapping-slv)
+                  mapping-slv))))))
+
+(define/contract (pvar-property id property)
+  (-> identifier? symbol? any/c)
+  (let ([valvar+props (valvar+props-properties id)])
+    (and valvar+props
+         (let ([properties (valvar+props-properties valvar+props)])
+         (hash? properties)
+         (immutable? properties)
+         (hash-ref properties property #f)))))

info.rkt

@@ -6,7 +6,8 @@
                "phc-toolkit"
                "srfi-lite-lib"
                "stxparse-info"
-               "alexis-util"))
+               "alexis-util"
+               "scope-operations"))
 (define build-deps '("scribble-lib"
                      "racket-doc"))
 (define scribblings '(("scribblings/subtemplate.scrbl" () (parsing-library))))

main.rkt

@@ -12,7 +12,9 @@
          (prefix-in dbg: stxparse-info/parse/private/runtime)
          syntax/id-table
          (subtract-in racket/syntax stxparse-info/case)
+         "copy-attribute.rkt"
          (for-syntax "patch-arrows.rkt"
+                     "derived-valvar.rkt"
                      racket/format
                      stxparse-info/parse
                      racket/private/sc
@@ -32,29 +34,6 @@
 (define derived-valvar-cache (make-weak-hash))
 
 (begin-for-syntax
-  ;; Act like a syntax transformer, but which is recognizable via the
-  ;; derived-pattern-variable? predicate.
-  (struct derived-valvar (valvar)
-    #:property prop:procedure
-    (λ (self stx)
-      #`(#%expression #,(derived-valvar-valvar self))))
-
-  (define (id-is-derived-valvar? id)
-    (define mapping (syntax-local-value id (thunk #f)))
-    (and mapping ;; … defined as syntax
-         (syntax-pattern-variable? mapping) ; and is a syntax pattern variable
-         (let ()
-           (define mapping-slv
-             (syntax-local-value (syntax-mapping-valvar mapping) (thunk #f)))
-           ;; either a mapping → attribute → derived,
-           ;; or directly mapping → derived
-           (or (and (-attribute-mapping? mapping-slv) ;; is an attribute
-                    (derived-valvar? ;; and the pvar's valvar is a derived
-                     (syntax-local-value (-attribute-mapping-var mapping-slv)
-                                         (thunk #f))))
-               ;; or the pvar's valvar is derived
-               (derived-valvar? mapping-slv)))))
-
   (define/contract (string-suffix a b)
     (-> string? string? string?)
     (define suffix-length (string-suffix-length a b))
@@ -100,12 +79,6 @@
         #`(#,(nest-ellipses stx (sub1 n))
            (… …))))
 
-  (define/contract (nest-map f last n)
-    (-> (-> syntax? syntax?) syntax? exact-nonnegative-integer? syntax?)
-    (if (= n 0)
-        last
-        (f (nest-map f last (sub1 n)))))
-
   (define/contract (find-subscript-binder bound)
     (-> identifier?
         (or/c #f (list/c identifier?                     ; bound
@@ -121,7 +94,9 @@
       
       (define/with-syntax ([binder . unique-at-runtime-id] …)
         (filter (compose (conjoin identifier?
-                                  (negate id-is-derived-valvar?)
+                                  (λ (pv)
+                                    (not
+                                     (pvar-property pv 'subtemplate-derived)))
                                   (λ~> (syntax-local-value _ (thunk #f))
                                        syntax-pattern-variable?)
                                   ;; force call syntax-local-value to prevent
@@ -141,9 +116,9 @@
       ;; Or write it as:
 
       #;(define/with-syntax ([binder . unique-at-runtime] …)
-          (for/list ([binder (current-pvars)]
+          (for/list ([binder (current-pvars+unique)]
                       #:when (identifier? (car binder))
-                      #:unless (id-is-derived-pvar? (car binder))
+                      #:unless (pvar-property (car binder) 'subtemplate-derived)
                       #:when (syntax-pattern-variable?
                               (syntax-local-value (car binder) (thunk #f)))
                       ;; force call syntax-local-value to prevent ambiguous
@@ -183,17 +158,6 @@
                     #'(unique-at-runtime-id …)
                     (car depths))))))
 
-(define/contract (attribute-val/c depth [bottom-predicate any/c])
-  (->* {exact-nonnegative-integer?} {flat-contract?} flat-contract?)
-  (flat-named-contract
-   (build-compound-type-name 'attribute-val/c depth bottom-predicate)
-   (λ (l)
-     (if (= depth 0)
-         (or (false? l) (bottom-predicate l))
-         (or (false? l)
-             (and (list? l)
-                  (andmap (attribute-val/c (sub1 depth)) l)))))))
-
 ;; Checks that all the given attribute values have the same structure.
 ;; 
 ;; ellipsis-count/c works with the value of pattern variables and of attributes
@@ -441,14 +405,6 @@
    (define/with-syntax temp-cached (generate-temporary #'bound))
    (define/with-syntax temp-generated (generate-temporary #'bound))
    (define/with-syntax temp-id-table (generate-temporary #'bound))
-   ;; works only for syntax patterns, luckily that's all we need since we
-   ;; produce a tree of (possibly missing) identifiers.
-   (define/with-syntax copy-attribute-pattern
-     ;; the ~and is important, to prevent the nested ~or from being treated as
-     ;; an ellipsis-head pattern.
-     (nest-map (λ (pat) #`{~or #f ({~and #,pat} (... ...))})
-               #'{~or #f {~var bound id}}
-               (syntax-e #'ellipsis-depth)))
    ;; HERE: cache the define-temp-ids in the free-id-table, and make sure
    ;; that we retrieve the cached ones, so that two subtemplate within the same
    ;; syntax-case or syntax-parse clause use the same derived ids.
@@ -539,9 +495,4 @@
           (quote-syntax bound)
           (free-id-table-map temp-id-table (λ (k v) k))))
        
-       ;; manually creating the attribute with (make-attribute-mapping …)
-       ;; works, but the attribute behaves in a bogus way when put inside
-       ;; an (?@ yᵢ ...). I must be missing some step in the construction
-       ;; of the attribute
-       (define/syntax-parse copy-attribute-pattern temp-cached)
-       (define-pvars bound))))
+       (copy-raw-syntax-attribute bound temp-cached ellipsis-depth #t))))

override.rkt

@@ -15,4 +15,5 @@
                        stxparse-info/parse
                        stxparse-info/parse/experimental/template
                        stxparse-info/case
-                       racket/syntax))
+                       racket/syntax)
+         (rename-out [... …]))

scribblings/subtemplate.scrbl

@@ -84,7 +84,7 @@ compile-time and run-time performance will not be as good as with
 Despite the rather extensive test suite, there are probably a few bugs lurking,
 please report them to @url{https://github.com/jsmaniac/subtemplate/issues}.
 
-@subsection{Omitted elements in attributes (via @racket[~optional]}
+@subsection{Omitted elements in attributes (via @racket[~optional])}
 
 When some values are missing in the ellipses of a template variable, e.g. via
 @racket[~optional], @racket[subtemplate] combines all the existing bound

test/test-copy-attribute.rkt

@@ -0,0 +1,323 @@
+#lang racket
+
+(require subtemplate/copy-attribute
+         stxparse-info/parse
+         stxparse-info/parse/experimental/template
+         phc-toolkit/untyped
+         rackunit)
+
+(define (to-datum x) (syntax->datum (datum->syntax #f x)))
+
+;; Depth 2, no missing values
+(begin
+  ;; with just x in the pattern
+  (check-equal? (syntax->datum
+                 (syntax-parse #'([1 2 3] [4 5])
+                   [((x …) …)
+                    (copy-raw-syntax-attribute y (attribute* x) 2 #t)
+                    (template [(?@ y …) … ((y …) …)])]))
+                '(1 2 3 4 5 ((1 2 3) (4 5))))
+
+  ;; shadowing the y in the pattern
+  (check-equal? (syntax->datum
+                 (syntax-parse #'([1 2 3] [4 5])
+                   [((x …) … y)
+                    (copy-raw-syntax-attribute y (attribute* x) 2 #t)
+                    (template [(?@ y …) … ((y …) …)])]))
+                '(1 2 3 ((1 2 3))))
+
+  ;; syntax? is #f (the leaves are still syntax though)
+  (check-equal? (to-datum
+                 (syntax-parse #'([1 2 3] [4 5])
+                   [((x …) …)
+                    (copy-raw-syntax-attribute y (attribute* x) 2 #f)
+                    (attribute* y)]))
+                '([1 2 3] [4 5]))
+
+  ;; same as above, check that we have syntax at the leaves
+  (check-match (syntax-parse #'([1 2 3] [4 5])
+                 [((x …) …)
+                  (copy-raw-syntax-attribute y (attribute* x) 2 #f)
+                  (attribute* y)])
+               (list (list (? syntax?) ...) ...))
+
+  ;; syntax? is #f (the leaves are still syntax though), use it in a template
+  (check-equal? (to-datum
+                 (syntax-parse #'([1 2 3] [4 5])
+                   [((x …) …)
+                    (copy-raw-syntax-attribute y (attribute* x) 2 #f)
+                    (template [(?@ y …) … ((y …) …)])]))
+                '(1 2 3 4 5 ((1 2 3) (4 5))))
+
+  ;; syntax? is #f, the leaves are NOT syntax.
+  ;; Checks that (attribute* y) is not syntax either.
+  (check-equal? (let ()
+                  (copy-raw-syntax-attribute y `((1 2 3) (4 5)) 2 #f)
+                  (attribute* y))
+                '([1 2 3] [4 5])))
+
+;; Depth 2, missing values at depth 1
+(begin
+  ;; with just x in the pattern
+  (check-equal? (syntax->datum
+                 (syntax-parse #'([1 2 3] #:kw [4 5])
+                   [({~and {~or #:kw (x …)}} …)
+                    (copy-raw-syntax-attribute y (attribute* x) 2 #t)
+                    (template [(?? (?@ y …) empty) … ((?? (y …) empty) …)])]))
+                '(1 2 3 empty 4 5 ((1 2 3) empty (4 5))))
+
+  ;; shadowing the y in the pattern
+  (check-equal? (syntax->datum
+                 (syntax-parse #'([1 2 3] #:kw [4 5])
+                   [({~and {~or #:kw (x …)}} … y)
+                    (copy-raw-syntax-attribute y (attribute* x) 2 #t)
+                    (template [(?? (?@ y …) empty) … ((?? (y …) empty) …)])]))
+                '(1 2 3 empty ((1 2 3) empty)))
+
+  ;; syntax? is #f (the leaves are still syntax though)
+  (check-equal? (to-datum
+                 (syntax-parse #'([1 2 3] #:kw [4 5])
+                   [({~and {~or #:kw (x …)}} …)
+                    (copy-raw-syntax-attribute y (attribute* x) 2 #f)
+                    (attribute* y)]))
+                '([1 2 3] #f [4 5]))
+
+  ;; same as above, check that we have syntax at the leaves
+  (check-match (syntax-parse #'([1 2 3] #:kw [4 5])
+                 [({~and {~or #:kw (x …)}} …)
+                  (copy-raw-syntax-attribute y (attribute* x) 2 #f)
+                  (attribute* y)])
+               (list (list (? syntax?) ...) #f (list (? syntax?) ...)))
+
+  ;; syntax? is #f (the leaves are still syntax though), use it in a template
+  (check-equal? (to-datum
+                 (syntax-parse #'([1 2 3] #:kw [4 5])
+                   [({~and {~or #:kw (x …)}} …)
+                    (copy-raw-syntax-attribute y (attribute* x) 2 #f)
+                    (template [(?? (?@ y …) empty) … ((?? (y …) empty) …)])]))
+                '(1 2 3 empty 4 5 ((1 2 3) empty (4 5))))
+
+  ;; syntax? is #f, the leaves are NOT syntax.
+  ;; Checks that (attribute* y) is not syntax either.
+  (check-equal? (let ()
+                  (copy-raw-syntax-attribute y '((1 2 3) #f (4 5)) 2 #f)
+                  (attribute* y))
+                '([1 2 3] #f [4 5])))
+
+;; Depth 2, missing values at depth 2
+(begin
+  ;; with just x in the pattern
+  (check-equal? (syntax->datum
+                 (syntax-parse #'([1 #:kw 3] [4 5])
+                   [(({~and {~or #:kw x}} …) …)
+                    (copy-raw-syntax-attribute y (attribute* x) 2 #t)
+                    (template [(?@ (?? y empty) …) … (((?? y empty) …) …)])]))
+                '(1 empty 3 4 5 ((1 empty 3) (4 5))))
+
+  ;; shadowing the y in the pattern
+  (check-equal? (syntax->datum
+                 (syntax-parse #'([1 #:kw 3] [4 5])
+                   [(({~and {~or #:kw x}} …) … y)
+                    (copy-raw-syntax-attribute y (attribute* x) 2 #t)
+                    (template [(?@ (?? y empty) …) … (((?? y empty) …) …)])]))
+                '(1 empty 3 ((1 empty 3))))
+
+  ;; syntax? is #f (the leaves are still syntax though)
+  (check-equal? (to-datum
+                 (syntax-parse #'([1 #:kw 3] [4 5])
+                   [(({~and {~or #:kw x}} …) …)
+                    (copy-raw-syntax-attribute y (attribute* x) 2 #f)
+                    (attribute* y)]))
+                '([1 #f 3] [4 5]))
+
+  ;; same as above, check that we have syntax at the leaves
+  (check-match (syntax-parse #'([1 #:kw 3] [4 5])
+                 [(({~and {~or #:kw x}} …) …)
+                  (copy-raw-syntax-attribute y (attribute* x) 2 #f)
+                  (attribute* y)])
+               (list (list (or #f (? syntax?)) ...) ...))
+
+  ;; syntax? is #f (the leaves are still syntax though), use it in a template
+  (check-equal? (to-datum
+                 (syntax-parse #'([1 #:kw 3] [4 5])
+                   [(({~and {~or #:kw x}} …) …)
+                    (copy-raw-syntax-attribute y (attribute* x) 2 #f)
+                    (template [(?@ (?? y empty) …) … (((?? y empty) …) …)])]))
+                '(1 empty 3 4 5 ((1 empty 3) (4 5))))
+
+  ;; syntax? is #f, the leaves are NOT syntax.
+  ;; Checks that (attribute* y) is not syntax either.
+  (check-equal? (let ()
+                  (copy-raw-syntax-attribute y '((1 #f 3) (4 5)) 2 #f)
+                  (attribute* y))
+                '([1 #f 3] [4 5])))
+
+;; Depth 1, missing values at depth 1
+(begin
+  ;; with just x in the pattern
+  (check-equal? (syntax->datum
+                 (syntax-parse #'(1 #:kw 3)
+                   [({~and {~or #:kw x}} …)
+                    (copy-raw-syntax-attribute y (attribute* x) 1 #t)
+                    (template ({?? y empty} …))]))
+                '(1 empty 3))
+
+  ;; shadowing the y in the pattern
+  (check-equal? (syntax->datum
+                 (syntax-parse #'(1 #:kw 3 4)
+                   [({~and {~or #:kw x}} … y)
+                    (copy-raw-syntax-attribute y (attribute* x) 1 #t)
+                    (template ({?? y empty} …))]))
+                '(1 empty 3))
+
+  ;; syntax? is #f (the leaves are still syntax though)
+  (check-equal? (to-datum
+                 (syntax-parse #'(1 #:kw 3)
+                   [({~and {~or #:kw x}} …)
+                    (copy-raw-syntax-attribute y (attribute* x) 1 #f)
+                    (attribute* y)]))
+                '(1 #f 3))
+
+  ;; same as above, check that we have syntax at the leaves
+  (check-match (syntax-parse #'(1 #:kw 3)
+                 [({~and {~or #:kw x}} …)
+                  (copy-raw-syntax-attribute y (attribute* x) 1 #f)
+                  (attribute* y)])
+               (list (or #f (? syntax?)) ...))
+
+  ;; syntax? is #f (the leaves are still syntax though), use it in a template
+  (check-equal? (to-datum
+                 (syntax-parse #'(1 #:kw 3)
+                   [({~and {~or #:kw x}} …)
+                    (copy-raw-syntax-attribute y (attribute* x) 1 #f)
+                    (template ({?? y empty} …))]))
+                '(1 empty 3))
+
+  ;; syntax? is #f, the leaves are NOT syntax.
+  ;; Checks that (attribute* y) is not syntax either.
+  (check-equal? (let ()
+                  (copy-raw-syntax-attribute y '(1 #f 3) 1 #f)
+                  (attribute* y))
+                '(1 #f 3))
+
+  ;; syntax? is #f, compound values
+  (check-equal? (let ()
+                  (copy-raw-syntax-attribute y '((1 1 1) #f (3 (#t) #f)) 1 #f)
+                  (attribute* y))
+                '((1 1 1) #f (3 (#t) #f))))
+
+;; Depth 1, no missing values
+(begin
+  ;; with just x in the pattern
+  (check-equal? (syntax->datum
+                 (syntax-parse #'(1 2 3)
+                   [(x …)
+                    (copy-raw-syntax-attribute y (attribute* x) 1 #t)
+                    (template ({?? y empty} …))]))
+                '(1 2 3))
+
+  ;; shadowing the y in the pattern
+  (check-equal? (syntax->datum
+                 (syntax-parse #'(1 2 3 4)
+                   [(x … y)
+                    (copy-raw-syntax-attribute y (attribute* x) 1 #t)
+                    (template ({?? y empty} …))]))
+                '(1 2 3))
+
+  ;; syntax? is #f (the leaves are still syntax though)
+  (check-equal? (to-datum
+                 (syntax-parse #'(1 2 3)
+                   [(x …)
+                    (copy-raw-syntax-attribute y (attribute* x) 1 #f)
+                    (attribute* y)]))
+                '(1 2 3))
+
+  ;; same as above, check that we have syntax at the leaves
+  (check-match (syntax-parse #'(1 2 3)
+                 [(x …)
+                  (copy-raw-syntax-attribute y (attribute* x) 1 #f)
+                  (attribute* y)])
+               (list (? syntax?) ...))
+
+  ;; syntax? is #f (the leaves are still syntax though), use it in a template
+  (check-equal? (to-datum
+                 (syntax-parse #'(1 2 3)
+                   [(x …)
+                    (copy-raw-syntax-attribute y (attribute* x) 1 #f)
+                    (template ({?? y empty} …))]))
+                '(1 2 3))
+
+  ;; syntax? is #f, the leaves are NOT syntax.
+  ;; Checks that (attribute* y) is not syntax either.
+  (check-equal? (let ()
+                  (copy-raw-syntax-attribute y '(1 2 3) 1 #f)
+                  (attribute* y))
+                '(1 2 3))
+
+  ;; syntax? is #f, compound values
+  (check-equal? (let ()
+                  (copy-raw-syntax-attribute y '((1 1 1) 2 (3 (#t) #f)) 1 #f)
+                  (attribute* y))
+                '((1 1 1) 2 (3 (#t) #f))))
+
+;; Depth 1, missing value at depth 0
+(begin
+  ;; with just x in the pattern
+  (check-equal? (syntax->datum
+                 (syntax-parse #'(#:kw)
+                   [({~optional (x …)} #:kw)
+                    (copy-raw-syntax-attribute y (attribute* x) 1 #t)
+                    (template {?? (y …) empty})]))
+                'empty)
+
+  ;; syntax? is #f, use it in a template
+  (check-equal? (to-datum
+                 (syntax-parse #'(#:kw)
+                   [({~optional (x …)} #:kw)
+                    (copy-raw-syntax-attribute y (attribute* x) 1 #f)
+                    (template {?? (y …) empty})]))
+                'empty)
+
+  ;; syntax? is #f, check with a raw attribute explicitly
+  (check-equal? (let ()
+                  (copy-raw-syntax-attribute y #f 1 #f)
+                  (attribute* y))
+                #f)
+
+  ;; syntax? is #f, check (in a template) with a raw attribute explicitly
+  (check-equal? (syntax->datum
+                 (let ()
+                   (copy-raw-syntax-attribute y #f 1 #f)
+                   (template {?? (y …) empty})))
+                'empty))
+
+;; Depth 2, missing value at depth 0
+(begin
+  ;; with just x in the pattern
+  (check-equal? (syntax->datum
+                 (syntax-parse #'(#:kw)
+                   [({~optional ((x …) …)} #:kw)
+                    (copy-raw-syntax-attribute y (attribute* x) 2 #t)
+                    (template {?? ((y …) …) empty})]))
+                'empty)
+
+  ;; syntax? is #f, use it in a template
+  (check-equal? (to-datum
+                 (syntax-parse #'(#:kw)
+                   [({~optional ((x …) …)} #:kw)
+                    (copy-raw-syntax-attribute y (attribute* x) 2 #f)
+                    (template {?? ((y …) …) empty})]))
+                'empty)
+
+  ;; syntax? is #f, check with a raw attribute explicitly
+  (check-equal? (let ()
+                  (copy-raw-syntax-attribute y #f 2 #f)
+                  (attribute* y))
+                #f)
+
+  ;; syntax? is #f, check (in a template) with a raw attribute explicitly
+  (check-equal? (syntax->datum
+                 (let ()
+                   (copy-raw-syntax-attribute y #f 2 #f)
+                   (template {?? ((y …) …) empty})))
+                'empty))

test/test-ddd.rkt

@@ -0,0 +1,30 @@
+#lang racket
+(require subtemplate/ddd
+         stxparse-info/case
+         stxparse-info/parse
+         (only-in racket/base [... …])
+         rackunit
+         syntax/macro-testing)
+
+(check-equal? (syntax-case #'(1 2 3) ()
+                [(x …)
+                 (ddd (+ (syntax-e #'x) 3))])
+              '(4 5 6))
+
+(check-equal? (syntax-parse #'(1 2 3)
+                [(x …)
+                 (ddd (+ (syntax-e #'x) 3))])
+              '(4 5 6))
+
+(check-exn
+ #rx"no pattern variables with depth > 0 were found in the body"
+ (λ ()
+   (convert-compile-time-error
+    (syntax-parse #'(1 2 3)
+      [(x y z)
+       (ddd (+ (syntax-e #'x) 3))]))))
+
+(check-equal? (syntax-parse #'(1 2 3 4)
+                [(x … y)
+                 (ddd (+ (syntax-e #'x) (syntax-e #'y)))])
+              '(5 6 7))