Begin parsing addition

diff --git a/AST.hs b/AST.hs
new file mode 100644 (file)
index 0000000..6528215
--- /dev/null
+++ b/AST.hs
@@ -0,0 +1,20 @@
+module AST where
+
+import Text.Read
+import Text.ParserCombinators.ReadP hiding ((+++))
+
+data Expr = Num Float
+          | Add Expr Expr
+  deriving Show
+
+instance Read Expr where
+  readPrec = parseNum +++ parseAdd
+    where parseNum = Num <$> readPrec
+          parseAdd = step $ do
+            a <- prec 11 readPrec
+            lift $ do
+              skipSpaces
+              char '+'
+              skipSpaces
+            b <- readPrec
+            return (Add a b)