Final Racket parser

Author: mohanr

_posts/2023-04-20-Lexer.md

@@ -92,17 +92,19 @@ The result is this. The error condition is not tested.
 Tokens: [Literal '1',Plus,InvalidChar 'x',Literal '2']
 {% endhighlight %}
 
-# Racket code
+# Typed Racket code
+
+I read that Racket has many levels and types of languages and here I am using Typed Racket.
 
 I have to note that this is my first ever Racket code and the intention is to port my haskell to Typed Racket.
-So I have explained what I learn using pieces of code. Neither the Racket code or Haskell code implementes
+So I have explained what I learnt using pieces of code. Neither the Racket code or Haskell code implementes
 the entire interpreter or even the lexer. The git repo. will have the entire code if I manage to learn
 Racket sufficiently to code the whole interpreter in the book.
 
 ## Step 1
 
 This compiles without errors. But the code is not tested. I will add more test code while I refactor.
-In this case I have a feeling the the code is more verbose than the equivalent Haskell code.
+In this case I have a feeling that the code is more verbose than the equivalent Haskell code.
 
 {% highlight racket %}
 #lang typed/racket
@@ -183,3 +185,29 @@ But this prints {% highlight racket %}#<Literal>{% endhighlight %} without the v
 This is the Racket way of printing the _struct_ with the value.
 
 {% highlight racket %}(Literal 2){% endhighlight %}
+ 
+## Step 2
+ 
+This took longer than I expected as Racket seems to be very different than even Haskell. Many iterations later with help
+from Racket experts I was able to understand how to code the function.
+ 
+{% highlight racket %}
+(: parse-exp (-> (U (Listof Char)  )
+                 (Listof(U (ErrorType Integer Char) (Operator Char) ))))
+(define (parse-exp lst )
+  (match lst 
+  ['() (list(EndOfInput))]
+  [(list x xs ...)
+  (match x 
+    [(? char-numeric?)  (cons (Literal x) (parse-exp xs))]
+    [#\+ (cons (Plus) (parse-exp xs)) ]
+    [_ (list (Unexpected 0)) ])])) 
+
+ 
+ {% endhighlight %}
+ 
+The type of this function is only slightly different than the Haskell code and the error conditions
+are not tested fully. The _Offset_ of each charater is not tracked but that should not be difficult.
+ 
+The output is {% highlight racket %}(list (Literal #\1) #<Plus> (Literal #\2) #<EndOfInput>){% endhighlight %}
+