Intro doc

Intro.md

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+# Beginning Typed Racket
+
+## What is Typed Racket?
+
+ * A type system for Racket
+ * That interoperates with untyped code
+
+## Basics
+
+    #lang typed/racket
+    "Hello RacketCon!"
+
+## Type Errors
+
+    #lang typed/racket
+    (+ "Hello" "RacketCon!")
+
+### Type Annotations
+
+    (define: ...)
+
+    (: ...)
+
+    (struct: ...)
+
+    (lambda: ...)
+
+## A Simple TR Program
+
+```
+#lang typed/racket
+
+(: f2c : Number -> Number)
+(define (f2c n)
+  (* (- n 32) 5/9))
+
+(module+ test
+  (require typed/rackunit)
+  (check-equal (f2c 32) 0)
+  (check-equal (f2c -40) -40)
+  (check-equal (f2c 98.6) 37.))
+```
+
+## Exercise 1
+
+Port this program to Typed Racket
+
+```
+#lang racket  ; An echo server
+(define listener (tcp-listen 12345))
+(define (echo-server)
+  (define-values (in out) (tcp-accept listener))
+  (thread (lambda () (copy-port in out)
+                     (close-output-port out)))
+  (echo-server))
+
+(echo-server)
+```