如何在haskell中展平列表列表

Question

All I want to do is what I ask. The type signature of the function should be this:

flatten::[[[Int]]] -> [[Int]]

I tried to search some flatten code but they define new types and that confuses me. Any help? Thanks in advance.

Answer 1

There are (at least) two ways to write

flatten::[[[Int]]] -> [[Int]]

One is

flatten1 = concat
-- Example: flatten [[[1], [2]], [[3]]] = [[1], [2], [3]] :: [[Int]]

Another one is

flatten2 = map concat
-- Example: flatten [[[1], [2]], [[3]]] = [[1,2], [3]] :: [[Int]]

Basically, flatten1 flattens the "middle" level of brackets, while flatten2 flattens the "innermost" level of brackets.

As an exercise, you might want to convince yourself that

concat . flatten1 = concat . flatten2 :: [[[Int]]] -> [Int]

Indeed, both would produce [1,2,3] on the example above.

A more advanced remark

The law above is actually a very famous one, since it is a special case of the monad law

join . fmap join = join . join :: Monad m => m (m (m a)) -> m a

where m = [] (ie, in the list monad), and a=Int

Answer 2

You are looking for

concat :: [[a]] -> [a]

In your use case, the element type happens to be [Int] .

Answer 3

One of the best ways to find answers to questions like yours is to use hoogle. For example see http://www.haskell.org/hoogle/?hoogle=%5B%5B%5Ba%5D%5D%5D+-%3E+%5B%5Ba%5D%5D .

The concat function is second in the resulting list.

There is also hayoo: http://holumbus.fh-wedel.de/hayoo/hayoo.html which searches all of hackage.