From 45afeea77cd017d2ac37cf330c595c22c90a7f87 Mon Sep 17 00:00:00 2001
From: Luke Lau <luke_lau@icloud.com>
Date: Sat, 18 May 2019 17:51:22 +0100
Subject: [PATCH] Parse variables in expressions

---
 AST.hs | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/AST.hs b/AST.hs
index 5b22ea6..c766afc 100644
--- a/AST.hs
+++ b/AST.hs
@@ -4,6 +4,7 @@ import Text.Read
 import Text.ParserCombinators.ReadP hiding ((+++), choice)
 
 data Expr = Num Float
+          | Var String
           | BinOp BinOp Expr Expr
   deriving Show
 
@@ -12,12 +13,14 @@ data BinOp = Add | Sub | Mul | Cmp Ordering
 
 instance Read Expr where
   readPrec = choice [ parseNum
+                    , parseVar
                     , parseBinOp "<" 10 (Cmp LT)
                     , parseBinOp "+" 20 Add
                     , parseBinOp "-" 20 Sub
                     , parseBinOp "*" 40 Mul
                     ]
     where parseNum = Num <$> readPrec
+          parseVar = Var <$> lift (munch1 isAlpha)
           -- use 'prec 1' and 'step' so that parsing 'a'
           -- can only go one step deep, to prevent ininfite
           -- recursion
-- 
2.30.2