Non-blocking REPL

Makeshift REPL so that one can evaluate expressions while the game is
running. I have a curses project which used lua to do something
similiar, I'd like to repurpose that at some point. For now, this is
satisfactory.

src/main.ss

@@ -88,6 +88,57 @@
 
      (end-render-frame))))
 
+(define read-without-blocking
+  (lambda (port)
+    (letrec
+	([iter!
+	  (lambda (read-stack expr)
+	    (let ([chr (peek-char port)])
+	      (cond
+	       
+	       [(or (char=? chr #\newline) (eof-object? chr))
+		(read-char port)
+		(if (and (null? read-stack) (pair? expr))
+		    (reverse expr)
+		    '())]
+	       
+	       [(or (char=? chr #\() (char=? chr #\[))
+		(read-char port)
+		(iter! (cons chr read-stack) (cons chr expr))]
+	       
+	       [(char=? chr #\))
+		(read-char port)
+		(if (and (pair? read-stack) (char=? (car read-stack) #\())
+		    (iter! (cdr read-stack) (cons chr expr))
+		    '())]
+	       
+	       [(char=? chr #\])
+		(read-char port)
+		(if (and (pair? read-stack) (char=? (car read-stack) #\[))
+		    (iter! (cdr read-stack) (cons chr expr))
+		    '())]
+	       
+	       [else
+		(read-char port)
+		(iter! read-stack (cons chr expr))])))])
+      
+      (if (char-ready? port)
+	  (let ([expr-list (iter! '() '())])
+	    (if (pair? expr-list)
+		(read (open-input-string [list->string expr-list]))))))))
+
+(define eval-print
+  (lambda (expr port)
+    (write (eval expr) port)
+    (write-char #\newline port)
+    (flush-output-port port)))
+
+(define read-eval-print-without-blocking
+  (lambda ()
+    (let ([expr (read-without-blocking (console-input-port))])
+      (if (pair? expr)
+	  (eval-print expr (console-output-port))))))
+
 (define run-game!
   (lambda ()
     (letrec* ([lag 0.0] [prev-time-utc (current-time)] [curr-time-utc prev-time-utc]
@@ -103,6 +154,9 @@
 	       (lambda ()
 		 (when (and game-running? (window-open?))
 
+		   ;; handle expressions without blocking
+		   (read-eval-print-without-blocking)
+
 		   ;; handle window events
 		   (process-window-input!)