在Haskell列表中访问元组的元素

Question

I have a list of tuples, for example:

[(1,2), (3,4), (5,6)]

Now I have to write function which sum up the first an second element of each tuple and create a list of these values.

For the example above it should be:

[3, 7, 11]

This should be done with use of list comprehension. It's not allowed to use functions like map, filter and contact.

Any ideas how I could access the elements of the tuple in the list?

Answer 1

Try this:

[ x + y | (x,y) <- yourlist]

The trick is representing the two elements in a tuple from your input list as x and y and then dealing with them as needed.

Answer 2

Let's do it without list comprehensions, using functions from the Prelude:

map (uncurry (+)) [(1,2), (3,4), (5,6)]

-- Result: [3, 7, 11]

How does this work? Let's consider the types:

(+) :: Num a => a -> a -> a
uncurry :: (a -> b -> c) -> (a, b) -> c
map :: (a -> b) -> [a] -> [b]

As you may already know, in Haskell, the normal way of doing multi-argument functions is by **currying* them. The type of (+) reflects this: conceptually it takes one argument and produces a function that then takes the "second" argument to produce the final result.

uncurry takes such a curried two-argument function and adapts it to work on a pair. It's trivial to implement:

uncurry :: (a -> b -> c) -> (a, b) -> c
uncurry f (a, b) = f a b

Funnily enough, the uncurry function is curried, so its partial application uncurry (+) has type Num a => (a, a) -> a . This would then be a function that takes a pair of numbers and adds them.

And map simply applies a function to every element of a list, collecting the individual results into a list. Plug them all together and that's a solution.