From 9e7ad28c5ccf816bbcc7bd67009955bb46edbce6 Mon Sep 17 00:00:00 2001
From: VegOwOtenks <vegowotenks@jossco.de>
Date: Mon, 16 Sep 2024 11:38:22 +0200
Subject: [PATCH] Error out for undefined variables

---
 app/Main.hs |  7 ++++---
 src/Lib.hs  | 18 +++++++++++++-----
 2 files changed, 17 insertions(+), 8 deletions(-)

diff --git a/app/Main.hs b/app/Main.hs
index 0100982..167abc2 100644
--- a/app/Main.hs
+++ b/app/Main.hs
@@ -2,7 +2,7 @@ module Main (main) where
 
 import Text.Parsec
 
-import Lib (exprparser, evaluate, replaceVars)
+import Lib (exprparser, evaluate, replaceVars, Expr)
 
 import Data.Map (Map)
 import qualified Data.Map as Map
@@ -21,8 +21,9 @@ showRatio p r = (if (r < 0) then "-" else "") ++ prepoint_digits ++ (if (length
     	prepoint_digits  = init . show . round . abs $ (r * 10)
 	postpoint_digits = reverse .dropWhile (=='0') . reverse .(take p) . (drop (length prepoint_digits)) . show . round . abs $ (r * 10^p)
 
-useResult (Right e) = (showRatio precision) . evaluate $ replaceVars e vars
-useResult (Left  e) = show e
+useResult :: Either ParseError Expr -> String
+useResult (Right expr)  = either id ((showRatio precision) . evaluate) $ replaceVars expr vars
+useResult (Left  error) = show error
 
 vars :: Map String Rational
 vars = Map.fromList [("pi", 245850922 % 78256779), ("e", 271801 % 99990)]
diff --git a/src/Lib.hs b/src/Lib.hs
index 2b24d47..b9c05aa 100644
--- a/src/Lib.hs
+++ b/src/Lib.hs
@@ -1,5 +1,5 @@
 module Lib
-    ( exprparser, evaluate, replaceVars 
+    ( exprparser, evaluate, replaceVars, Expr
     ) where
 import Control.Applicative((<*))
 
@@ -92,10 +92,17 @@ term = m_parens exprparser
        <|> fmap Constant constantInteger
        <|> fmap Constant constantRational
 
-replaceVars :: Expr -> Map.Map String Rational -> Expr
-replaceVars (Variable name) vs = Constant . maybe (0 % 1) id $ Map.lookup name vs
-replaceVars (Binary op a b) vs = Binary op (replaceVars a vs) (replaceVars b vs) 
-replaceVars (Constant c)    vs = Constant c
+replaceVars :: Expr -> Map.Map String Rational -> Either String Expr
+replaceVars (Constant c)    vs = Right $ Constant c
+replaceVars (Variable name) vs = maybe (Left ("Usage of unknown variable: '" ++ name ++ "'\nSuggestion: define the variable\n++  " ++ name ++ "=42")) (Right . Constant . id) $ Map.lookup name vs
+replaceVars (Binary op l r) vs = case leftBranch of
+        Left  s -> Left  s
+        Right a -> case rightBranch of
+            Left s  -> Left s
+            Right b -> Right $ Binary op a b
+    where
+        leftBranch  = replaceVars l vs
+	rightBranch = replaceVars r vs
 
 evaluate :: Expr -> Rational
 evaluate (Constant c) = c
@@ -104,3 +111,4 @@ evaluate (Binary Minus    a b) = evaluate a - evaluate b
 evaluate (Binary Divide   a b) = evaluate a / evaluate b
 evaluate (Binary Multiply a b) = evaluate a * evaluate b
 evaluate (Binary Power    a b) = rationalPower (evaluate a) (evaluate b)
+evaluate (Variable _)          = error "Unreachable Code"