如何在haskell中显示给定范围的列表

Question

How do I write a function that displays a given range in a list in Haskell? Say that I have the function:

dispRange l x y

that when given values:

dispRange [1,2,4,5,6,7] 0 3 

displays all the elements from position 0 to 3, thus the list returned would be:

[1,2,4,5]

Answer 1

We can use a combination of drop :: Int -> [a] -> [a] and take :: Int -> [a] -> [a] for this:

For a range i to j , we first drop i elements, and then take j-i+1 elements (since both indices are inclusive , we thus need to add one).

For example:

dispRange :: [a] -> Int -> Int -> [a]
dispRange l i j = take (j-i+1) (drop i l)

We can guard against negative numbers and j being less than i with:

dispRange :: [a] -> Int -> Int -> Maybe [a]
dispRange l i j | i < 0 || j < i = Nothing
                | otherwise = Just (take (j-i+1) (drop i l))

Answer 2

dispRange consumes a list: l , the start: x and the end: y of a range inclusively. It returns the elements within that range if the range is not negative.

import Data.List ((\\))

dispRange :: Eq a => [a] -> Int -> Int -> [a]
dispRange l x y  
  | x <= y = (\\) <$> take (y + 1) <*> take x $ l
  | otherwise = []

Answer 3

If you want to do it in a way that doesn't use built-in functions (for beginner's learning), you can break it into two stages:

1. Chop off the first x elements, recursively calling your function and changing the arguments as appropriate.
2. Return the next y elements, recursively calling your function and changing the arguments as appropriate.

Your code will look like this:

dispRange :: …
dispRange … … … = []
dispRange … … … = dispRange … … …
⋮  ⋮  ⋮  ⋮  ⋮  ⋮  ⋮  ⋮  ⋮  ⋮
dispRange … … … = dispRange … … …

So, the questions are:

• What are the types of the arguments and the value returned?
• What do the inputs of the base and recursive cases look like for stage one above?
• What do the inputs of the base and recursive cases look like for stage two above?

Now, you can write the type signature and the patterns on the left of the equal signs (remember that patterns above take priority over patterns below, so order them properly). Then, you can write the implementation of each case on the right of the equal sign.

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If you have trouble, another way is to try making a function that does just stage one. Then, try making a function that does just stage two. Then, use those functions in your dispRange function.