[英]How can I fix my replace haskell function?
It eill work when : replace :: Eq a => a -> a -> [a] -> [a] will be.它会在以下情况下工作:replace :: Eq a => a -> a -> [a] -> [a] 将是。 How can I convert az a to an [a] in my code ?如何在我的代码中将 az a 转换为 [a] ?
replace :: Eq a => a -> [a] -> [a] -> [a]
replace _ _ [] = []
replace a x (y:ys)
| a == y = x : replace a x ys
| otherwise = y : replace a x ys
Example:
replace '?' "a" "" == ""
replace 'a' "e" "alma" == "elme"
replace 'a' "e" "nincsbenne" == "nincsbenne"
You are using wrong operator for the first guard ( a == y
) - :
is used to prepend a head element to a list but x
is a list not a single element, so you need to use ++
which concatenates two lists ( x
and one returned by recursive call):您为第一个守卫( a == y
)使用了错误的运算符 - :
用于将头元素添加到列表中,但x
是列表而不是单个元素,因此您需要使用++
连接两个列表( x
一个由递归调用返回):
replace :: Eq a => a -> [a] -> [a] -> [a]
replace _ _ [] = []
replace a x (y:ys)
| a == y = x ++ replace a x ys -- ++ instead of :
| otherwise = y : replace a x ys
Related - Haskell (:) and (++) differences相关 - Haskell (:) 和 (++) 差异
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