diff --git a/interpreter.rkt b/interpreter.rkt
index 8f7dab5..3c32607 100644
--- a/interpreter.rkt
+++ b/interpreter.rkt
@@ -112,7 +112,8 @@
   (prog-state (cons 0 '()) 0 1 "" p 0))
 
 ; Tests for program->prog-state
-(check-expect (program->prog-state "[->+<]") (prog-state (list 0) 0 1 "" "[->+<]" 0))
+(check-expect
+ (program->prog-state "[->+<]") (prog-state (list 0) 0 1 "" "[->+<]" 0))
 
 ; tape-help: Tape DP (Byte -> Byte) -> Tape
 ; Given a tape and a data pointer, returns the same tape with the data in the
@@ -177,15 +178,16 @@
 (check-error (exec-tape-left (prog-state (list 1 2 3) 0 3 "" "<" 0)))
 (check-expect (exec-tape-left (prog-state (list 1 2 3) 2 3 "" "<" 0))
               (prog-state (list 1 2 3) 1 3 "" "<" 1))
-     
+
 ; exec-tape-right: ProgState -> ProgState
 ; Given a ProgState, returns a new ProgState with the > instruction executed
 (define (exec-tape-right w)
-  (local [(define end-of-tape (= (prog-state-dp w) (sub1 (prog-state-tape-len w))))]
+  (local [(define end-of-tape
+            (= (prog-state-dp w) (sub1 (prog-state-tape-len w))))]
     (prog-state
      (if end-of-tape
          (append (prog-state-tape w) (list 0))
-         (prog-state-tape w)) 
+         (prog-state-tape w))
      (add1 (prog-state-dp w))
      (if end-of-tape
          (add1 (prog-state-tape-len w))
@@ -243,7 +245,7 @@
   (define brkt (if (symbol=? wd 'forward) #\] #\[))
   (define oppos-brkt (if (symbol=? wd 'forward) #\[ #\]))
   (define upd-start (if (symbol=? wd 'forward) add1 sub1))
-  
+
   ; fm-helper Nat Nat -> Nat
   ; Given a starting position and a nest accumulator, returns the position of
   ; the matching bracket waking in the direction provided by updating the
@@ -272,7 +274,8 @@
 ; exec-loop-start: ProgState -> ProgState
 ; Given a ProgState, returns a new ProgState with the [ instruction executed
 (define (exec-loop-start w)
-  (local [(define jump (zero? (list-ref (prog-state-tape w) (prog-state-dp w))))]
+  (local [(define jump
+            (zero? (list-ref (prog-state-tape w) (prog-state-dp w))))]
     (prog-state
      (prog-state-tape w)
      (prog-state-dp w)
@@ -280,7 +283,7 @@
      (prog-state-output w)
      (prog-state-program w)
      (add1 (if jump
-         (find-matching (prog-state-program w) (prog-state-ip w) 'forward) 
+         (find-matching (prog-state-program w) (prog-state-ip w) 'forward)
          (prog-state-ip w))))))
 
 ; Tests for exec-loop-start
@@ -294,7 +297,8 @@
 ; exec-loop-end: ProgState -> ProgState
 ; Given a ProgState, returns a new ProgState with the ] instruction executed
 (define (exec-loop-end w)
-  (local [(define jump (not (zero? (list-ref (prog-state-tape w) (prog-state-dp w)))))]
+  (local [(define jump
+            (not (zero? (list-ref (prog-state-tape w) (prog-state-dp w)))))]
     (prog-state
      (prog-state-tape w)
      (prog-state-dp w)
@@ -302,7 +306,7 @@
      (prog-state-output w)
      (prog-state-program w)
      (add1 (if jump
-         (find-matching (prog-state-program w) (prog-state-ip w) 'backward) 
+         (find-matching (prog-state-program w) (prog-state-ip w) 'backward)
          (prog-state-ip w))))))
 
 ; Tests for exec-loop-end
@@ -313,9 +317,25 @@
                (prog-state '(1) 0 1 "" "[++--]++--+-[]" 5))
               (prog-state '(1) 0 1 "" "[++--]++--+-[]" 1))
 
+; exec-in: ProgState ((Byte -> _) -> _) (ProgState -> _) -> _
+; Given a ProgState, a function that takes a callback function requiring a Byte
+; and a function which takes the new ProgState, calls done with the input
+; provided by get-input (provided by the call to the callback given in
+; get-input).
+(define (exec-in w get-input done)
+  (define (got-input byte)
+    (done (prog-state
+           (insert-in-tape byte (prog-state-dp w))
+           (prog-state-dp w)
+           (prog-state-tape-len w)
+           (prog-state-output w)
+           (prog-state-program w)
+           (add1 (prog-state-ip w)))))
+  (get-input got-input))
+
 ; execute: ProgState -> ProgState
-; Given an initial ProgState state, returns the final ProgState state executing the
-; program.
+; Given an initial ProgState state,
+; returns the final ProgState state executing the program.
 (define (execute w)
   (local [(define program-len (string-length (prog-state-program w)))]
     (cond [(>= (prog-state-ip w) program-len) w]