如何用 Haskell 中的守卫替换大小写或匹配模式？

Question

I am trying (as a part of learning) replace case-of expression with expression with guards. This is a function with case-of

myList :: [a] -> String
myList xs = case xs of []  -> "empty"
                       [x] -> "one"
                       xs  -> "more"

I've managed to write a function with where and matching pattern

myList' :: [a] -> String
myList' xs = someF xs 
where someF []  = "empty"
      someF [x] = "one"
      someF xs  = "more"

However, I could not make another function myList'' which is using guards (|). I am new in Haskell, and would much appreciate if somebody can help me with this problem. Thank you in advance. Al

Answer 1

You can make use of pattern guards , for example:

myList'' :: [a] -> String
myList'' xs
    | [] <- xs = "empty"
    | [_] <- xs = "one"
    | otherwise = "more"

Or you can make use of null:: Foldable f => fa -> Bool and drop:: Int -> [a] -> [a] to check if the list is empty:

myList'' :: [a] -> String
myList'' xs
    | null xs = "empty"
    | null (drop 1 xs) = "one"
    | otherwise = "more"