在Haskell字符串列表中获取长度为奇数的元素

Question

I have a list of strings in Haskell and I need to get those elements with odd length in another list. How can this be done using higher order functions like foldr, foldl, foldr1, foldl1, filter, map, and so on? I will very much appreciate your help. Can list comprehension be used in this case?

Answer 1

It seems that you are aware that filter exists (since you've mentioned), but perhaps are uncertain how it works. If you're trying to extract a specific subset of a list, this seems to be the right path. If you look at its type-signature, you'll find it's pretty straight-forward:

(a -> Bool) -> [a] -> [a]

That is, it takes a function that returns True or False (ie true to contain in the new set, false otherwise) and produces a new list. Similarly, Haskell provides a function called odd in Prelude. It's signature looks as follows:

Integral a => a -> Bool

That is, it can take any Integral type and returns True if it is odd, false otherwise.

Now, let's consider a solution:

filter odd [1..10]

This will extract all the odd numbers between [1,10].

I noticed you mentioned list comprehensions. You probably do not want to use this if you are already given a list and you are simply filtering it. However, a list comprehension would be a perfectly acceptable solution:

[x | x <- [1..10], odd x]

In general, list comprehensions are used to express the generation of lists with more complicated constraints.

Now, to actually answer your question. Since we know we can filter numbers, and if we're using Hoogle searching for the following type (notice that String is simply [Char] ):

[a] -> Int

You will see a length function. With some function composition, we can quickly see how to create a function which filters odd length. In summary, we have odd which is type Int -> Bool (in this case) and we have length which is [a] -> Int or-- specifically-- String -> Int . Our solution now looks like this:

filter (odd . length) ["abc","def","eh","123","hm","even"]

Answer 2

Here ya go.

getOddOnes = filter . flip (foldr (const (. not)) id) $ False

Note: if you turn this in for your homework, you'd best be prepared to explain it!