Fixed the isSubsequence function

Author: BoeingX

src/AlgebraicDataTypes/ChapterExercises/AsPatterns.hs

@@ -1,14 +1,14 @@
 module AlgebraicDataTypes.ChapterExercises.AsPatterns where
-    
+
 import Data.Char
+import Data.List (elemIndex)
 
 isSubsequenceOf :: (Eq a) => [a] -> [a] -> Bool
 isSubsequenceOf [] _ = True
 isSubsequenceOf _ [] = False
-isSubsequenceOf (x:xs) ys
-  | null ys'' = False
-  | otherwise = isSubsequenceOf xs (ys' ++ tail ys'')
-    where (ys', ys'') = span (/= x) ys
+isSubsequenceOf xs@(x:xs') (y:ys')
+    | x == y = isSubsequenceOf xs' ys'
+    | otherwise = isSubsequenceOf xs ys'
 
 capitalizeWords :: String -> [(String, String)]
 capitalizeWords = map f . words

test/AlgebraicDataTypes/ChapterExercises/AsPatternsSpec.hs

@@ -14,8 +14,10 @@ spec = do
             isSubsequenceOf "blah" "wboloath" `shouldBe` True
         it "isSubsequenceOf \"blah\" \"wootbla\"" $ do
             isSubsequenceOf "blah" "wootbla" `shouldBe` False
+        it "isSubsequenceOf \"blah\" \"halbwoot\"" $ do
+            isSubsequenceOf "blah" "halbwoot" `shouldBe` False
         it "isSubsequenceOf \"abc\" \"cba\"" $ do
-            isSubsequenceOf "abc" "cba" `shouldBe` True
+            isSubsequenceOf "abc" "cba" `shouldBe` False
     describe "Test capitalizeWords" $ do
         it "capitalizeWords \"hello world\"" $ do
             capitalizeWords "hello world" `shouldBe` [("hello", "Hello"), ("world", "World")]