add simple Peggy expression parser

parser2.hs

@@ -0,0 +1,28 @@
+{-# Language TemplateHaskell, QuasiQuotes, FlexibleContexts #-}
+
+import Text.Peggy
+import AST
+
+[peggy|
+top :: AST = expr !.
+
+expr :: AST
+  = expr "+" fact { Add $1 $2 }
+  / expr "-" fact { Sub $1 $2 }
+  / fact
+
+fact :: AST
+  = fact "*" term { Mul $1 $2 }
+  / fact "/" term { Div $1 $2 }
+  / term
+
+term :: AST
+  = "(" expr ")"
+  / number
+
+number ::: AST
+  = [1-9] [0-9]* { IntConst $ read ($1 : $2) }
+|]
+
+main :: IO ()
+main = print . parseString top "<stdin>" =<< getContents