From ae80d076cba678169dbddac4a53edd2cb1e091fb Mon Sep 17 00:00:00 2001
From: Luke Lau <luke_lau@icloud.com>
Date: Sat, 18 May 2019 16:53:40 +0100
Subject: [PATCH] Start parsing expressions

This starts off with defining not the AST, but just what expressions we
want to be able to parse.
So far we just handle numbers, and addition involving 2 numbers.
We use the built-in ReadPrec parser combinators, which allow us to
recursively parse the binary op of addition, and even allow us to reuse
the Read instance on Int!
prec and step are needed since without them, parseAdd will just get
stuck repeatedly trying to parse the left expression (a).
---
 AST.hs | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)
 create mode 100644 AST.hs

diff --git a/AST.hs b/AST.hs
new file mode 100644
index 0000000..5628e7c
--- /dev/null
+++ b/AST.hs
@@ -0,0 +1,23 @@
+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
+          -- use 'prec 1' and 'step' so that parsing 'a'
+          -- can only go one step deep, to prevent ininfite
+          -- recursion
+          parseAdd = prec 1 $ do
+            a <- step readPrec
+            lift $ do
+              skipSpaces
+              char '+'
+              skipSpaces
+            b <- readPrec
+            return (Add a b)
-- 
2.30.2