From 6e280c2c5b2903ae38f4da15a41ea94793907407 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=C3=96mer=20Sinan=20A=C4=9Facan?= <omeragacan@gmail.com>
Date: Thu, 7 Jul 2016 20:01:47 +0000
Subject: Utils: Fix `lengthIs` and `lengthExceeds` for negative args

Credits goes to SPJ for finding this.
---
 compiler/utils/Util.hs | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

(limited to 'compiler/utils/Util.hs')

diff --git a/compiler/utils/Util.hs b/compiler/utils/Util.hs
index ff0f45f725..d20a604519 100644
--- a/compiler/utils/Util.hs
+++ b/compiler/utils/Util.hs
@@ -443,9 +443,9 @@ atLength :: ([a] -> b)   -- Called when length ls >= n, passed (drop n ls)
          -> [a]
          -> Int
          -> b
-atLength atLenPred atEnd ls n
-  | n < 0     = atLenPred ls
-  | otherwise = go n ls
+atLength atLenPred atEnd ls0 n0
+  | n0 < 0    = atLenPred ls0
+  | otherwise = go n0 ls0
   where
     -- go's first arg n >= 0
     go 0 ls     = atLenPred ls
@@ -454,15 +454,24 @@ atLength atLenPred atEnd ls n
 
 -- Some special cases of atLength:
 
+-- | @(lengthExceeds xs n) = (length xs > n)@
 lengthExceeds :: [a] -> Int -> Bool
--- ^ > (lengthExceeds xs n) = (length xs > n)
-lengthExceeds = atLength notNull False
+lengthExceeds lst n
+  | n < 0
+  = True
+  | otherwise
+  = atLength notNull False lst n
 
 lengthAtLeast :: [a] -> Int -> Bool
 lengthAtLeast = atLength (const True) False
 
+-- | @(lengthIs xs n) = (length xs == n)@
 lengthIs :: [a] -> Int -> Bool
-lengthIs = atLength null False
+lengthIs lst n
+  | n < 0
+  = False
+  | otherwise
+  = atLength null False lst n
 
 listLengthCmp :: [a] -> Int -> Ordering
 listLengthCmp = atLength atLen atEnd
-- 
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