Non-exhaustive patterns in function

Question

I've got a problem with this code, it should count the longest substring of the same letter in a string, but there is an error:

*** Exception: test.hs:(15,0)-(21,17): 
Non-exhaustive patterns in function countLongest'

I know that is the wrong types problem, but i dont know where is the error, or how to find or debug it

countLongest :: (Eq a) => [a] -> Int
countLongest' :: (Eq a) => Int -> Int -> [a] -> Int

countLongest a = countLongest' 0 0 a
countLongest' n max (y:x:ys)
        | y == x = countLongest' (n+1) max (x:ys)
        | n > max = countLongest' 0 (n) (x:ys)
        | otherwise = countLongest' 0 (max) (x:ys)
countLongest' n max []
        | n > max = n
        | otherwise = max

Answer 1

It looks like you're missing the case where there's a one element list:

countLongest' n max (y:ys)
    | ... etc. ...
    | otherwise = ....

---

Here's a contrived example similar to yours:

f [] = 3         -- matches an empty list
f (a:b:bs) = 4   -- matches a list with at least two elements

Examples:

Prelude> :load myfile.hs 
[1 of 1] Compiling Main             ( myfile.hs, interpreted )
Ok, modules loaded: Main.
*Main> f [3]
*** Exception: myfile.hs:(3,0)-(4,13): Non-exhaustive patterns in function f

*Main> f []
3
*Main> f [1,2,3,4,5]
4
*Main> 

So it succeeds with 0 and 2 elements in the list, but fails when there's exactly one element.

---

Note that this behavior is not unique to lists. Here's an example using Maybe:

g :: Maybe x -> x
g (Just x) = x

Examples:

*Main> g (Just 4)
4
*Main> g Nothing 
*** Exception: myfile.hs:6:0-13: Non-exhaustive patterns in function g

This happened because there's two constructors for Maybe, Just <something> and Nothing. We didn't provide a case for Nothing, so when we passed that into g, it didn't work!

---

Check out this question and its answers for information on getting a little help from the compiler. I followed the advice of the first answer, and when I loaded my examples, this is what happened:

prompt$ ghci -fwarn-incomplete-patterns

Prelude> :load myfile.hs 
[1 of 1] Compiling Main             ( myfile.hs, interpreted )

myfile.hs:3:0:
    Warning: Pattern match(es) are non-exhaustive
             In the definition of `f': Patterns not matched: [_]

myfile.hs:6:0:
    Warning: Pattern match(es) are non-exhaustive
             In the definition of `g': Patterns not matched: Nothing
Ok, modules loaded: Main.

Cool! The compiler is pretty smart!