Add input/output section for Racket

racket.html.markdown

@@ -6,6 +6,7 @@ contributors:
   - ["th3rac25", "https://github.com/voila"]
   - ["Eli Barzilay", "https://github.com/elibarzilay"]
   - ["Gustavo Schmidt", "https://github.com/gustavoschmidt"]
+  - ["Duong H. Nguyen", "https://github.com/cmpitg"]
 ---
 
 Racket is a general purpose, multi-paradigm programming language in the Lisp/Scheme family.
@@ -600,6 +601,45 @@ vec ; => #(1 2 3 4)
 ;; expected: positive?
 ;; given: -5
 ;; more details....
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; 11. Input & output
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; Racket has this concept of "port", which is very similar to file descriptor
+;; in other languages
+
+;; Open "/tmp/tmp.txt" and write "Hello World"
+;; This would trigger an error if the file's already existed
+(define out-port (open-output-file "/tmp/tmp.txt"))
+(displayln "Hello World" out-port)
+(close-output-port out-port)
+
+;; Append to "/tmp/tmp.txt"
+(define out-port (open-output-file "/tmp/tmp.txt"
+                                     #:exists 'append))
+(displayln "Hola mundo" out-port)
+(close-output-port out-port)
+
+;; Read from the file again
+(define in-port (open-input-file "/tmp/tmp.txt"))
+(displayln (read-line in-port))
+; => "Hello World"
+(displayln (read-line in-port))
+; => "Hola mundo"
+(close-input-port in-port)
+
+;; Alternatively, with call-with-output-file you don't need to explicitly
+;; close the file
+(call-with-output-file "/tmp/tmp.txt"
+  #:exists 'update ; Rewrite the content
+  (λ (out-port)
+    (displayln "World Hello!" out-port)))
+
+;; And call-with-input-file does the same thing for input
+(call-with-input-file "/tmp/tmp.txt"
+  (λ (in-port)
+    (displayln (read-line in-port))))
 ```
 
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