置换haskell
我有一个Haskell工作，要求我使用之前写过的其他两个功能编写置换函数，这两个功能正常工作，但我不知道如何将这些2连接到PERM功能，我该如何执行这？
prepend :: Num a => a -> [[a]] -> [[a]]
prepend a l = map (a :) l
delete :: Eq a => a -> [a] -> [a]
delete x [] = []
delete x (y:ys)
| x == y = delete x ys
| otherwise = y : delete x ys
perm的声明应为：perm :: num ad⇒[a]→[[a]]
我尝试的内容：
perm [] = [[]]
perm p = [prepend x xs | x <- p, xs <- perm (delete x p)]
I have a Haskell work which ask me to write a permutation function using 2 other functions prepend and delete I wrote before, those two functions works fine but I somehow don't know how to connect those 2 into the perm function, how can i do this?
prepend :: Num a => a -> [[a]] -> [[a]]
prepend a l = map (a :) l
delete :: Eq a => a -> [a] -> [a]
delete x [] = []
delete x (y:ys)
| x == y = delete x ys
| otherwise = y : delete x ys
The declaration of perm should be : perm :: Num a ⇒ [a] → [[a]]
What I tried:
perm [] = [[]]
perm p = [prepend x xs | x <- p, xs <- perm (delete x p)]
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只需使用
：而不是prepend。Just use
:instead ofprepend.