diff --git a/xenomorph/xenomorph/scribblings/xenomorph.scrbl b/xenomorph/xenomorph/scribblings/xenomorph.scrbl
index be546522..28e954ac 100644
--- a/xenomorph/xenomorph/scribblings/xenomorph.scrbl
+++ b/xenomorph/xenomorph/scribblings/xenomorph.scrbl
@@ -1651,7 +1651,7 @@ Generate an instance of @racket[x:reserved%] (or a subclass of @racket[x:reserve
 boolean?]{
 Whether @racket[x] is something that can be used as a length argument with @racket[xenomorphic?] objects that have length. For instance, an @racket[x:list] or @racket[x:stream].
 
-The following values are deemed to be resolvable: any @racket[exact-nonnegative-integer?], any @racket[procedure?] that returns a @racket[exact-nonnegative-integer?], or an @racket[x:int?]. 
+The following values are deemed to be resolvable: any @racket[exact-nonnegative-integer?], an @racket[x:int?], or any @racket[procedure?] that takes one argument (= the parent object) returns a @racket[exact-nonnegative-integer?].
 }
 
 @section{License & source code}
diff --git a/xenomorph/xenomorph/test/bytes-test.rkt b/xenomorph/xenomorph/test/bytes-test.rkt
index 4f99cee7..4cc59ff3 100644
--- a/xenomorph/xenomorph/test/bytes-test.rkt
+++ b/xenomorph/xenomorph/test/bytes-test.rkt
@@ -31,7 +31,7 @@ https://github.com/mbutterick/restructure/blob/master/test/Buffer.coffee
 (test-case
  "bytes: should decode with parent key length"
  (parameterize ([current-input-port (open-input-bytes (bytes #xab #xff #x1f #xb6))])
-   (define buf (x:bytes #:length 'len))
+   (define buf (x:bytes #:length (λ (p) (hash-ref p 'len))))
    (check-equal? (decode buf #:parent (hash 'len 3)) (bytes #xab #xff #x1f))
    (check-equal? (decode buf #:parent (hash 'len 1)) (bytes #xb6))))
 
diff --git a/xenomorph/xenomorph/test/list-test.rkt b/xenomorph/xenomorph/test/list-test.rkt
index 8a5eecab..2e0d5546 100644
--- a/xenomorph/xenomorph/test/list-test.rkt
+++ b/xenomorph/xenomorph/test/list-test.rkt
@@ -41,12 +41,12 @@ https://github.com/mbutterick/restructure/blob/master/test/Array.coffee
 (test-case 
  "list: decode length from parent key"
  (parameterize ([current-input-port (open-input-bytes (bytes 1 2 3 4 5))])
-   (check-equal? (decode (x:list #:type uint8 #:length 'len) (current-input-port) #:parent (mhash 'len 4)) '(1 2 3 4))))
+   (check-equal? (decode (x:list #:type uint8 #:length (λ (p) (hash-ref p 'len))) (current-input-port) #:parent (mhash 'len 4)) '(1 2 3 4))))
 
 (test-case 
  "list: decode byte count from parent key"
  (parameterize ([current-input-port (open-input-bytes (bytes 1 2 3 4 5))])
-   (check-equal? (decode (x:list #:type uint16be #:length 'len #:count-bytes #t) (current-input-port) #:parent (mhash 'len 4)) '(258 772))))
+   (check-equal? (decode (x:list #:type uint16be #:length (λ (p) (hash-ref p 'len)) #:count-bytes #t) (current-input-port) #:parent (mhash 'len 4)) '(258 772))))
 
 (test-case 
  "list: decode length as number before array"
diff --git a/xenomorph/xenomorph/test/string-test.rkt b/xenomorph/xenomorph/test/string-test.rkt
index fc18e332..0faca60b 100644
--- a/xenomorph/xenomorph/test/string-test.rkt
+++ b/xenomorph/xenomorph/test/string-test.rkt
@@ -26,7 +26,7 @@ https://github.com/mbutterick/restructure/blob/master/test/String.coffee
 (test-case
  "string: decode length from parent key"
  (parameterize ([current-input-port (open-input-bytes #"testing")])
-   (check-equal? (decode (x:string 'len) (current-input-port) #:parent (mhash 'len 7)) "testing")))
+   (check-equal? (decode (x:string (λ (p) (hash-ref p 'len))) (current-input-port) #:parent (mhash 'len 7)) "testing")))
 
 (test-case
  "string: decode length as number before string"
diff --git a/xenomorph/xenomorph/test/symbol-test.rkt b/xenomorph/xenomorph/test/symbol-test.rkt
index fcc55823..327b2624 100644
--- a/xenomorph/xenomorph/test/symbol-test.rkt
+++ b/xenomorph/xenomorph/test/symbol-test.rkt
@@ -22,7 +22,7 @@
 (test-case
  "symbol: decode length from parent key"
  (parameterize ([current-input-port (open-input-bytes #"testing")])
-   (check-equal? (decode (x:symbol 'len) (current-input-port) #:parent (mhash 'len 7)) 'testing)))
+   (check-equal? (decode (x:symbol (λ (p) (hash-ref p 'len))) (current-input-port) #:parent (mhash 'len 7)) 'testing)))
 
 (test-case
  "symbol: decode length as number before symbol"
diff --git a/xenomorph/xenomorph/test/vector-test.rkt b/xenomorph/xenomorph/test/vector-test.rkt
index 8612f2ce..0d638f72 100644
--- a/xenomorph/xenomorph/test/vector-test.rkt
+++ b/xenomorph/xenomorph/test/vector-test.rkt
@@ -42,12 +42,12 @@ https://github.com/mbutterick/restructure/blob/master/test/Array.coffee
 (test-case 
  "vector: decode length from parent key"
  (parameterize ([current-input-port (open-input-bytes (bytes 1 2 3 4 5))])
-   (check-equal? (decode (x:vector #:type uint8 #:length 'len) (current-input-port) #:parent (mhash 'len 4)) '#(1 2 3 4))))
+   (check-equal? (decode (x:vector #:type uint8 #:length (λ (p) (hash-ref p 'len))) (current-input-port) #:parent (mhash 'len 4)) '#(1 2 3 4))))
 
 (test-case 
  "vector: decode byte count from parent key"
  (parameterize ([current-input-port (open-input-bytes (bytes 1 2 3 4 5))])
-   (check-equal? (decode (x:vector #:type uint16be #:length 'len #:count-bytes #t) (current-input-port) #:parent (mhash 'len 4)) '#(258 772))))
+   (check-equal? (decode (x:vector #:type uint16be #:length (λ (p) (hash-ref p 'len)) #:count-bytes #t) (current-input-port) #:parent (mhash 'len 4)) '#(258 772))))
 
 (test-case 
  "vector: decode length as number before array"
diff --git a/xenomorph/xenomorph/util.rkt b/xenomorph/xenomorph/util.rkt
index 9807a20a..23414a62 100644
--- a/xenomorph/xenomorph/util.rkt
+++ b/xenomorph/xenomorph/util.rkt
@@ -14,7 +14,6 @@
     [#false #false]
     [(? exact-nonnegative-integer?) x]
     [(? procedure? proc) (proc parent)]
-    [(? symbol? key) #:when parent (dict-ref parent key)]
     [(? x:int?) #:when input-port (decode x input-port)]
     [_ (raise-argument-error 'resolve-length "fixed-size argument" x)]))