From 8a66ac4f81c3d1eab3b13a9e58cee0098573eff1 Mon Sep 17 00:00:00 2001
From: Luke Lau <luke_lau@icloud.com>
Date: Sat, 18 May 2019 17:54:06 +0100
Subject: [PATCH] Parse defs and externs

They both share a common prototype so we can split that out into a
separate definition. We also want to be able to parse top level
expressions, so we add constructor for that in AST.
Hopefully now you can see the beauty of monadic parsing:
Function <$> readPrec <*> readPrec
---
 AST.hs | 28 +++++++++++++++++++++++++---
 1 file changed, 25 insertions(+), 3 deletions(-)

diff --git a/AST.hs b/AST.hs
index c766afc..7dad3a1 100644
--- a/AST.hs
+++ b/AST.hs
@@ -1,5 +1,6 @@
 module AST where
 
+import Data.Char
 import Text.Read 
 import Text.ParserCombinators.ReadP hiding ((+++), choice)
 
@@ -21,9 +22,6 @@ instance Read Expr where
                     ]
     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
           parseBinOp s prc op = prec prc $ do
             a <- step readPrec
             lift $ do
@@ -33,3 +31,27 @@ instance Read Expr where
             b <- readPrec
             return (BinOp op a b)
             
+data Prototype = Prototype String [String]
+  deriving Show
+
+instance Read Prototype where
+  readPrec = lift $ do
+    name <- munch1 isAlpha
+    params <- between (char '(') (char ')') $
+                sepBy (munch1 isAlpha) skipSpaces
+    return (Prototype name params)
+
+data AST = Function Prototype Expr
+         | Extern Prototype
+         | TopLevelExpr Expr
+  deriving Show
+
+instance Read AST where
+  readPrec = parseFunction +++ parseExtern +++ parseTopLevel
+    where parseFunction = do
+            lift $ string "def" >> skipSpaces
+            Function <$> readPrec <*> readPrec
+          parseExtern = do
+            lift $ string "extern" >> skipSpaces
+            Extern <$> readPrec
+          parseTopLevel = TopLevelExpr <$> readPrec
-- 
2.30.2