Add coding exercises for Facebook interviews

Add attempts at solving coding problems to Briefcase.

scratch/facebook/evaluator.hs

@@ -0,0 +1,39 @@
+module Evaluator where
+
+data Token
+  = TokenInt Integer
+  | TokenAdd
+  | TokenMultiply
+  deriving (Eq, Show)
+
+newtype AST = AST [Token]
+  deriving (Eq, Show)
+
+tokens :: [Token]
+tokens =
+  [ TokenInt 13
+  , TokenAdd
+  , TokenInt 2
+  , TokenMultiply
+  , TokenInt 4
+  , TokenAdd
+  , TokenInt 7
+  , TokenAdd
+  , TokenInt 3
+  , TokenMultiply
+  , TokenInt 8
+  ]
+
+-- expression     -> addition ;
+-- addition       -> multiplication ( "+" multiplication )* ;
+-- multiplication -> terminal ( "*" terminal )* ;
+-- terminal       -> NUMBER ;
+
+parseExpression :: [Token] -> ([Token], AST)
+parseExpression tokens = do
+  lhs, rest = parseMultiplication tokens
+
+parseMulitplication :: [Token] -> ([Token], AST)
+
+main :: IO ()
+main = print $ parse tokens