Add tokens to Lexer

app/Lexer.hs

@@ -0,0 +1,10 @@
+module Lexer where
+
+data Token = Plus
+           | Minus
+           | Multiply
+           | Divide
+           | Integer Int
+           | LBrace
+           | RBrace
+           deriving Show

simple-arithmetic-compiler.cabal

@@ -52,7 +52,7 @@ executable simple-arithmetic-compiler
     main-is:          Main.hs
 
     -- Modules included in this executable, other than Main.
-    -- other-modules:
+    other-modules:    Lexer
 
     -- LANGUAGE extensions used by modules in this package.
     -- other-extensions: