Haskell无法与列表匹配

Question

I want to try out some haskell and started with 99 haskell problems, currently I'm stuck at 13: Run-length encoding of a list (though my question is more general)

--datatype
data En a = Multiple (Int, a) | Single a | Empty
    deriving (Show)

--helper
getnum :: (Eq a9) => [a9] -> Int
getnum [] = 0
getnum (x:xs)
    | x == head xs = 1 + getnum xs
    | otherwise = 1

encodeDirect :: [a] -> [En a]
endoceDirect xxs@(x:xs)
    | getnum xxs == 1 = Single x : encodeDirect xs
    | otherwise = Multiple ((getnum xxs), x) : encodeDirect (drop (getnum xxs) xxs)
encodeDirect _ = [Empty]

This should give me the solution with an [Empty] at the end, but when I call encodeDirect with any kind of list/string from ghci it just falls straight through, and all I get is [Empty].

Why does xxs@(x:xs) not match any list?

Answer 1

It should give you at least a warning about function endoceDirect , which, of course, doesn't have anything to do with encodeDirect .

As a side note, the catch-all case is usually a bad idea,