diff --git a/xenomorph/xenomorph/redo/array.rkt b/xenomorph/xenomorph/redo/array.rkt
index 19666ba7..0d7195c7 100644
--- a/xenomorph/xenomorph/redo/array.rkt
+++ b/xenomorph/xenomorph/redo/array.rkt
@@ -32,12 +32,13 @@ https://github.com/mbutterick/restructure/blob/master/src/Array.coffee
     [else (for/list ([i (in-range decoded-len)])
             (decode (xarray-type xa) port #:parent ctx))]))
 
-(define (xarray-encode xa array [port-arg #f] #:parent [parent #f])
+(define (xarray-encode xa array [port-arg (current-output-port)] #:parent [parent #f])
   (unless (sequence? array)
     (raise-argument-error 'xarray-encode "sequence" array))
   (define port (if (output-port? port-arg) port-arg (open-output-bytes)))
   (define (encode-items ctx)
     ;; todo: should array with fixed length stop encoding after it reaches max?
+    ;; cf. xstring, which rejects input that is too big for fixed length.
     (let* (#;[items (sequence->list array)]
            #;[item-count (length items)]
            #;[max-items (if (number? (xarray-len xa)) (xarray-len xa) item-count)])
@@ -89,6 +90,6 @@ https://github.com/mbutterick/restructure/blob/master/src/Array.coffee
 (module+ test
   (require rackunit)
   (check-equal? (decode (+xarray uint16be 3) #"ABCDEF") '(16706 17220 17734))
-  (check-equal? (encode (+xarray uint16be 3) '(16706 17220 17734)) #"ABCDEF")
+  (check-equal? (encode (+xarray uint16be 3) '(16706 17220 17734) #f) #"ABCDEF")
   (check-equal? (size (+xarray uint16be) '(1 2 3)) 6)
   (check-equal? (size (+xarray doublebe) '(1 2 3 4 5)) 40))
diff --git a/xenomorph/xenomorph/redo/number.rkt b/xenomorph/xenomorph/redo/number.rkt
index de8766d8..07945c3d 100644
--- a/xenomorph/xenomorph/redo/number.rkt
+++ b/xenomorph/xenomorph/redo/number.rkt
@@ -12,7 +12,7 @@ https://github.com/mbutterick/restructure/blob/master/src/Number.coffee
 
 (define system-endian (if (system-big-endian?) 'be 'le))
 
-(define (xint-encode i val [port #f] #:parent [parent #f])
+(define (xint-encode i val [port (current-output-port)] #:parent [parent #f])
   (unless (xint? i)
     (raise-argument-error 'encode "xint instance" i))
   (define-values (bound-min bound-max) (bounds i))
@@ -102,15 +102,15 @@ https://github.com/mbutterick/restructure/blob/master/src/Number.coffee
 (module+ test
   (require rackunit)
   (check-exn exn:fail:contract? (λ () (+xint 'not-a-valid-type)))
-  (check-exn exn:fail:contract? (λ () (encode uint8 256)))
-  (check-not-exn (λ () (encode uint8 255)))
-  (check-exn exn:fail:contract? (λ () (encode int8 256)))
-  (check-exn exn:fail:contract? (λ () (encode int8 255)))
-  (check-not-exn (λ () (encode int8 127)))
-  (check-not-exn (λ () (encode int8 -128)))
-  (check-exn exn:fail:contract? (λ () (encode int8 -129)))
-  (check-exn exn:fail:contract? (λ () (encode uint16 (add1 #xffff))))
-  (check-not-exn (λ () (encode uint16 #xffff)))
+  (check-exn exn:fail:contract? (λ () (encode uint8 256 #f)))
+  (check-not-exn (λ () (encode uint8 255 #f)))
+  (check-exn exn:fail:contract? (λ () (encode int8 256 #f)))
+  (check-exn exn:fail:contract? (λ () (encode int8 255 #f)))
+  (check-not-exn (λ () (encode int8 127 #f)))
+  (check-not-exn (λ () (encode int8 -128 #f)))
+  (check-exn exn:fail:contract? (λ () (encode int8 -129 #f)))
+  (check-exn exn:fail:contract? (λ () (encode uint16 (add1 #xffff)  #f)))
+  (check-not-exn (λ () (encode uint16 #xffff #f)))
 
   (let ([i (+xint 2 #:signed #f #:endian 'le)]
         [ip (open-input-bytes (bytes 1 2 3 4))]
@@ -139,8 +139,8 @@ https://github.com/mbutterick/restructure/blob/master/src/Number.coffee
 
   (check-equal? (decode int8 (bytes 127)) 127)
   (check-equal? (decode int8 (bytes 255)) -1)
-  (check-equal? (encode int8 -1) (bytes 255))
-  (check-equal? (encode int8 127) (bytes 127)))
+  (check-equal? (encode int8 -1 #f) (bytes 255))
+  (check-equal? (encode int8 127 #f) (bytes 127)))
 
 (define (xfloat-decode xf [port-arg (current-input-port)] #:parent [parent #f])
   (unless (xfloat? xf)
@@ -148,7 +148,7 @@ https://github.com/mbutterick/restructure/blob/master/src/Number.coffee
   (define bs (read-bytes (xfloat-size xf) (->input-port port-arg)))
   (floating-point-bytes->real bs (eq? (xfloat-endian xf) 'be)))
 
-(define (xfloat-encode xf val [port #f] #:parent [parent #f])
+(define (xfloat-encode xf val [port (current-output-port)] #:parent [parent #f])
   (unless (xfloat? xf)
     (raise-argument-error 'encode "xfloat instance" xf))
   (unless (or (not port) (output-port? port))
@@ -183,7 +183,7 @@ https://github.com/mbutterick/restructure/blob/master/src/Number.coffee
   (define int (xint-decode xf port-arg))
   (exact-if-possible (/ int (fixed-shift xf) 1.0)))
 
-(define (xfixed-encode xf val [port #f] #:parent [parent #f])
+(define (xfixed-encode xf val [port (current-output-port)] #:parent [parent #f])
   (unless (xfixed? xf)
     (raise-argument-error 'encode "xfixed instance" xf))
   (define int (exact-if-possible (floor (* val (fixed-shift xf)))))
@@ -215,6 +215,6 @@ https://github.com/mbutterick/restructure/blob/master/src/Number.coffee
 (define fixed32le (+xfixed 4 #:endian 'le))
 
 (module+ test
-  (define bs (encode fixed16be 123.45))
+  (define bs (encode fixed16be 123.45 #f))
   (check-equal? bs #"{s")
   (check-equal? (ceiling (* (decode fixed16be bs) 100)) 12345.0))
diff --git a/xenomorph/xenomorph/redo/string.rkt b/xenomorph/xenomorph/redo/string.rkt
new file mode 100644
index 00000000..d1f51d03
--- /dev/null
+++ b/xenomorph/xenomorph/redo/string.rkt
@@ -0,0 +1,125 @@
+#lang debug racket/base
+(require racket/dict "base.rkt" "util.rkt" "number.rkt")
+(provide (all-defined-out))
+
+#|
+approximates
+https://github.com/mbutterick/restructure/blob/master/src/String.coffee
+|#
+
+(define (read-encoded-string port len [encoding 'ascii])
+  (define proc (case encoding
+                 [(utf16le) (error 'bah)]
+                 [(ucs2) (error 'bleh)]
+                 [(utf8) bytes->string/utf-8]
+                 [(ascii) bytes->string/latin-1]
+                 [else values]))
+  (proc (read-bytes len port)))
+
+(define (write-encoded-string port string [encoding 'ascii])
+  ;; todo: handle encodings correctly.
+  ;; right now just utf8 and ascii are correct
+  (define proc (case encoding
+                 [(ucs2 utf8 ascii) string->bytes/utf-8]
+                 [(utf16le) (error 'swap-bytes-unimplemented)]
+                 [else (error 'unsupported-string-encoding)]))
+  (write-bytes (proc string) port))
+
+(define (count-nonzero-chars port)
+  ;; helper function for String
+  ;; counts nonzero chars from current position
+  (bytes-length (car (regexp-match-peek "[^\u0]*" port))))
+
+(define (bytes-left-in-port? port)
+  (not (eof-object? (peek-byte port))))
+
+(define (byte-length val encoding)
+  (define encoder
+    (case encoding
+      [(ascii utf8) string->bytes/utf-8]))
+  (bytes-length (encoder (format "~a" val))))
+
+(define (xstring-decode xs [port-arg (current-input-port)] #:parent [parent #f])
+  (define port (->input-port port-arg))
+  (let ([len (or (resolve-length (xstring-len xs) port parent) (count-nonzero-chars port))]
+        [encoding (if (procedure? (xstring-encoding xs))
+                      (or ((xstring-encoding xs) parent) 'ascii)
+                      (xstring-encoding xs))]
+        [adjustment (if (and (not (xstring-len xs)) (bytes-left-in-port? port)) 1 0)])
+    (define string (read-encoded-string port len encoding))
+    (pos port (+ (pos port) adjustment))
+    string))
+
+(define (xstring-encode xs val [port-arg (current-output-port)] #:parent [parent #f])
+  (define port (if (output-port? port-arg) port-arg (open-output-bytes)))
+  (let* ([val (format "~a" val)]
+         [encoding (if (procedure? (xstring-encoding xs))
+                       (or ((xstring-encoding xs) (and parent (dict-ref parent val)) 'ascii))
+                       (xstring-encoding xs))])
+    (define encoded-length (byte-length val encoding))
+    (when (and (exact-nonnegative-integer? (xstring-len xs)) (> encoded-length (xstring-len xs)))
+      (raise-argument-error 'xstring-encode (format "string no longer than ~a" (xstring-len xs)) val)) 
+    (when (xint? (xstring-len xs))
+      (encode (xstring-len xs) encoded-length port))
+    (write-encoded-string port val encoding)
+    (when (not (xstring-len xs)) (write-byte #x00 port)) ; null terminated when no len
+    (unless port-arg (get-output-bytes port)))) 
+
+(define (xstring-size xs [val #f] [parent #f])
+  (if (not val)
+      (resolve-length (xstring-len xs) #f parent)
+      (let* ([encoding (if (procedure? (xstring-encoding xs))
+                           (or ((xstring-encoding xs) (and parent (dict-ref parent val)) 'ascii))
+                           (xstring-encoding xs))]
+             [encoding (if (eq? encoding 'utf16be) 'utf16le encoding)])
+        (+ (byte-length val encoding) (cond
+                                        [(not (xstring-len xs)) 1]
+                                        [(xint? (xstring-len xs)) (size (xstring-len xs))]
+                                        [else 0])))))
+
+(struct xstring (len encoding) #:transparent
+  #:methods gen:xenomorphic
+  [(define decode xstring-decode)
+   (define encode xstring-encode)
+   (define size xstring-size)])
+
+(define supported-encodings '(ascii utf8))
+(define (+xstring [len #f] [encoding 'ascii])
+  (unless (length-resolvable? len)
+    (raise-argument-error '+xarray "length-resolvable?" len))
+  (unless (or (procedure? encoding) (memq encoding supported-encodings))
+    (raise-argument-error '+xarray (format "procedure or member of ~v" supported-encodings) encoding))
+  (xstring len encoding))
+
+(define (xsymbol-decode xs [port-arg (current-input-port)] #:parent [parent #f])
+  (string->symbol (xstring-decode xs port-arg #:parent parent)))
+
+(define (xsymbol-encode xs val [port (current-output-port)] #:parent [parent #f])
+  (unless (xsymbol? xs)
+    (raise-argument-error 'encode "xsymbol instance" xs))
+  (unless (or (string? val) (symbol? val))
+    (raise-argument-error 'xsymbol-encode "symbol or string" val))
+  (xstring-encode xs (if (symbol? val) val (string->symbol val)) port #:parent parent))
+
+(struct xsymbol xstring () #:transparent
+  #:methods gen:xenomorphic
+  [(define decode xsymbol-decode)
+   (define encode xsymbol-encode)
+   (define size xstring-size)])
+
+(define (+xsymbol [len #f] [encoding 'ascii])
+  (xsymbol len encoding))
+
+(module+ test
+  (require rackunit)
+  (define S-fixed (+xstring 4 'utf8))
+  (check-equal? (encode S-fixed "Mike" #f) #"Mike")
+  (check-exn exn:fail? (λ () (encode S-fixed "Mikes" #f))) ; too long for fixed string 
+  (define S (+xstring uint8 'utf8))
+  (check-equal? (decode S #"\2BCDEF") "BC")
+  (check-equal? (encode S "Mike" #f) #"\4Mike")
+  (check-equal? (size (+xstring) "foobar") 7) ; null terminated when no len
+  (check-equal? (decode (+xsymbol 4) #"Mike") 'Mike)
+  (check-equal? (encode (+xsymbol 4) 'Mike #f) #"Mike")
+  (check-equal? (encode (+xsymbol 4) "Mike" #f) #"Mike")
+  (check-exn exn:fail:contract? (λ () (encode (+xsymbol 4) 42 #f))))
\ No newline at end of file
diff --git a/xenomorph/xenomorph/redo/test/array-test.rkt b/xenomorph/xenomorph/redo/test/array-test.rkt
index 1e153648..273a64b4 100644
--- a/xenomorph/xenomorph/redo/test/array-test.rkt
+++ b/xenomorph/xenomorph/redo/test/array-test.rkt
@@ -79,11 +79,11 @@ https://github.com/mbutterick/restructure/blob/master/test/Array.coffee
 
 (test-case 
  "encode using array length"
- (check-equal? (encode (+xarray uint8 10) '(1 2 3 4)) (bytes 1 2 3 4)))
+ (check-equal? (encode (+xarray uint8 10) '(1 2 3 4) #f) (bytes 1 2 3 4)))
 
 (test-case 
  "encode length as number before array"
- (check-equal? (encode (+xarray uint8 uint8) '(1 2 3 4)) (bytes 4 1 2 3 4)))
+ (check-equal? (encode (+xarray uint8 uint8) '(1 2 3 4) #f) (bytes 4 1 2 3 4)))
 
 ;; todo: reinstate pointer test
 #;(test-case 
diff --git a/xenomorph/xenomorph/redo/test/string-test.rkt b/xenomorph/xenomorph/redo/test/string-test.rkt
new file mode 100644
index 00000000..ba6d4cb9
--- /dev/null
+++ b/xenomorph/xenomorph/redo/test/string-test.rkt
@@ -0,0 +1,109 @@
+#lang racket/base
+(require rackunit
+         "../base.rkt"
+         "../string.rkt"
+         "../number.rkt"
+         sugar/unstable/dict)
+
+#|
+approximates
+https://github.com/mbutterick/restructure/blob/master/test/String.coffee
+|#
+
+(test-case
+ "decode fixed length"
+ (parameterize ([current-input-port (open-input-bytes #"testing")])
+   (check-equal? (decode (+xstring 7)) "testing")))
+
+(test-case
+ "decode length from parent key"
+ (parameterize ([current-input-port (open-input-bytes #"testing")])
+   (check-equal? (decode (+xstring 'len) #:parent (mhash 'len 7)) "testing")))
+
+(test-case
+ "decode length as number before string"
+ (parameterize ([current-input-port (open-input-bytes #"\x07testing")])
+   (check-equal? (decode (+xstring uint8) #:parent (mhash 'len 7)) "testing")))
+
+(test-case
+ "decode utf8"
+ (parameterize ([current-input-port (open-input-bytes (string->bytes/utf-8 "🍻"))])
+   (check-equal? (decode (+xstring 4 'utf8)) "🍻")))
+
+(test-case
+ "decode encoding computed from function"
+ (parameterize ([current-input-port (open-input-bytes (string->bytes/utf-8 "🍻"))])
+   (check-equal? (decode (+xstring 4 (λ _ 'utf8))) "🍻")))
+
+(test-case
+ "decode null-terminated string and read past terminator"
+ (parameterize ([current-input-port (open-input-bytes (string->bytes/utf-8 "🍻\x00"))])
+   (check-equal? (decode (+xstring #f 'utf8)) "🍻")
+   (check-equal? (pos (current-input-port)) 5)))
+
+(test-case
+ "decode remainder of buffer when null-byte missing"
+ (parameterize ([current-input-port (open-input-bytes (string->bytes/utf-8 "🍻"))])
+   (check-equal? (decode (+xstring #f 'utf8)) "🍻")))
+
+(test-case
+ "size should use string length"
+ (check-equal? (size (+xstring 7) "testing") 7))
+
+(test-case
+ "size should use correct encoding"
+ (check-equal? (size (+xstring 10 'utf8) "🍻") 4))
+
+(test-case
+ "size should use encoding from function"
+ (check-equal? (size (+xstring 10 (λ _ 'utf8)) "🍻") 4))
+
+(test-case
+ "should add size of length field before string"
+ (check-equal? (size (+xstring uint8 'utf8) "🍻") 5))
+
+; todo: it 'should work with utf16be encoding', ->
+
+(test-case
+ "size should take null-byte into account"
+ (check-equal? (size (+xstring #f 'utf8) "🍻") 5))
+
+(test-case
+ "size should use defined length if no value given"
+ (check-equal? (size (+xstring 10)) 10))
+
+(test-case
+ "encode using string length"
+ (parameterize ([current-output-port (open-output-bytes)])
+   (encode (+xstring 7) "testing")
+   (check-equal? (dump (current-output-port)) #"testing")))
+
+(test-case
+ "encode length as number before string"
+ (parameterize ([current-output-port (open-output-bytes)])
+   (encode (+xstring uint8) "testing")
+   (check-equal? (dump (current-output-port)) #"\x07testing")))
+
+(test-case
+ "encode length as number before string utf8"
+ (parameterize ([current-output-port (open-output-bytes)])
+   (encode (+xstring uint8 'utf8) "testing 😜")
+   (check-equal? (dump (current-output-port)) (string->bytes/utf-8 "\x0ctesting 😜"))))
+
+(test-case
+ "encode utf8"
+ (parameterize ([current-output-port (open-output-bytes)])
+   (encode (+xstring 4 'utf8) "🍻" )
+   (check-equal? (dump (current-output-port)) (string->bytes/utf-8 "🍻"))))
+
+(test-case
+ "encode encoding computed from function"
+ (parameterize ([current-output-port (open-output-bytes)])
+   (encode (+xstring 4 (λ _ 'utf8)) "🍻")
+   (check-equal? (dump (current-output-port)) (string->bytes/utf-8 "🍻"))))
+
+(test-case
+ "encode null-terminated string"
+ (parameterize ([current-output-port (open-output-bytes)])
+   (encode (+xstring #f 'utf8) "🍻"  )
+   (check-equal? (dump (current-output-port)) (string->bytes/utf-8 "🍻\x00"))))
\ No newline at end of file