Haskell substring testing -

When I use the sub_string ("abberr", "habberyry") It is, obviously, it should be wrong. The point of the function is to search for the first argument within the second one. What is a wrong idea?

  sub-string :: (string, string) - & gt; Boole sub-string (_, []) = incorrect sub-string ([], _) = true sub string (a: x, b: y). A / = b = sub_string (a: x, y). Otherwise, you must first switch to the = sub-string (x, y)  

_ match [] and when you are matching, say, substring "abc" "abc" second, this Is idiomatic Haskell, which is to write a function with two arguments instead of one with one argument. So your code should start:

  substring :: string -> String - & gt; Bool substring [] _ ​​= true substring _ [] = wrong substring needle (h: aystack) | ...  

Now we get to the toughest case, where both of these lists are not empty, there is a problem with the recurring substring as bs The result of the substrings of code> "abc" such as "axbxcx" will be returned (because "ABC" will be the first match 'a' first, then "bc" will be seen in the string; substrings algorithm will then be left to see "bxcx" in "bc" Code> 'x' passed by 'B' and search for "xcx" in "c" , which will return true .

Instead your position is more complete. If you are ready to use the function from Data.List then it is:

  | IsPrefixOf needle (h: aystack) = true | otherwise = substring needle Aystack  

Otherwise you will have to write your own isPrefixOf , for example:

  isPrefixOf needle greenstack = needle == take (length needle) pile of grass