list - 尝试创建一个函数，该函数接受一个列表并返回与其第一个元素相同的所有元素的列表，但我的函数不起作用

Question

专门用于字符串的一种。所以需要["so", "what", "and", "so", "for" "so"]并返回 ["so", "so", "so"] .我的问题是我的函数返回一个与输入的列表相同的列表。

这是所有相关代码:

lookAhead :: [String] -> String
lookAhead [] = []
lookAhead (c:cs) = c

groupFirst :: [String] -> [String]
groupFirst [] = []
groupFirst (x:xs) 
    | lookAhead xs == x  = x : (groupFirst ((lookAhead xs):(tail xs)))
    | lookAhead xs /= x  = x : (groupFirst xs)
    | lookAhead xs == [] = x : []

Answer 1

trail作为头元素相等谓词 filter :

trail :: Eq a => [a] -> [a]
trail []       = []
trail (x : xs) = x : filter (== x) xs

trailBy使用 Schwartzian transform和 memoization :

trailBy :: Eq b => (a -> b) -> [a] -> [a]
trailBy _ []        = []
trailBy f (x1 : xs) = let x1f = f x1
                      in  x1 : filter (\ x -> f x == x1f) xs

trail = trailBy id ，其中 id x = x , Haskell 的恒等函数。