From 1554095368e65bf6f1c4d0f0106ae4516117964a Mon Sep 17 00:00:00 2001
From: Gregor Kleen <gkleen@yggdrasil.li>
Date: Tue, 1 Mar 2016 05:15:01 +0100
Subject: Support for reading formula from environment

---
 rpn.hs | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/rpn.hs b/rpn.hs
index 64fdb89..b441db4 100755
--- a/rpn.hs
+++ b/rpn.hs
@@ -1,7 +1,8 @@
 #!/usr/bin/env runghc
 
 import Control.Monad (join, sequence, forever)
-import System.Environment (getArgs)
+import Control.Applicative
+import System.Environment
 import Data.Maybe (listToMaybe, isJust)
 import Data.Char (isSpace)
 import Data.List (nub)
@@ -49,8 +50,7 @@ main = do
   hSetBuffering stdin LineBuffering
   hSetBuffering stdout NoBuffering
   forever $ do
-    putStr "Formula: "
-    line <- getLine
+    line <- maybe (putStr "Formula: " >> getLine) (<$ unsetEnv "FORMULA") =<< lookupEnv "FORMULA"
     let --symbols = join . join $ map (map words) . map lines $ args
       symbols = words line
       unknownSymbols = nub $ filter (not . isKnownSymbol) symbols
@@ -62,10 +62,12 @@ main = do
     putStrLn . pPrint $ evalExpVec symbols''
 
 clarifySymbol :: String -> IO [String]
-clarifySymbol s = do
-  putStr $ "Define " ++ s ++ ": "
-  line <- getLine
-  return $ words line
+clarifySymbol s = maybe clarify' (return . words) =<< lookupEnv s
+  where
+    clarify' = do
+      putStr $ "Define " ++ s ++ ": "
+      line <- getLine
+      return $ words line
 
 isKnownSymbol :: String -> Bool
 isKnownSymbol s = or [
-- 
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