Haskell List of tuples to list?

Your question is not very certain about how the tuples should be converted into a list. I assume that you want to have them flattend - for instance, [(1,2),(3,4)] should become [1,2,3,4].

This translation is only possible, if the two elements of your tuple are of the same type. In this case you can do something like this:

tupleToList :: [(a,a)] -> [a]
tupleToList ((a,b):xs) = a : b : tupleToList xs
tupleToList _          = []

In the general case, such a translation is impossible. One thing I could imagine to make the impossible possible is to use Either to wrap up the two different types:

tupleToList :: [(a,b)] -> [Either a b]
tupleToList ((a,b):xs) = Left a : Right b : tupleToList xs

You could also use a fold and avoid explicit recursion:

tupleToList = foldr (\(f,s) a -> f : s : a) []

Or:

tupleToList = foldl (\a (f,s) -> a ++ [f,s]) []

(For elements of the same type)

The lens library handles this and similar cases consistently.

> import Control.Lens
> toListOf (traverse . both) [(1,2),(3,4)]
            ^          ^
            |          |> Traversal of the tuple (a, a)
            |> Traversal of a list [b]
[1,2,3,4]

To convert from a list of lists:

> toListOf (traverse . traverse) [[1,2],[3,4],[5,6,7]]
[1,2,3,4,5,6,7]

addition edit:

traverse works with Traversable

traverse will work with any datatype that has a Traversable instance, for example trees.

> import Data.Tree
> let t = Node 1 [Node 2 [Node 3 [], Node 4 []], Node 5 []]
> let prettyTree = drawTree . fmap show
> prettyTree t
1
|
+- 2
|  |
|  +- 3
|  |
|  `- 4
|
`- 5
> toListOf (traverse . traverse) [t, t]
[1,2,3,4,5,1,2,3,4,5]

This can be also achieved by the homogeneous tuples library (disclaimer: which I'm the author of). It defines wrappers for tuples that make them instances of Traversable (and others such as Applicative and Monad). So a tuple can be converted to a list by toList . Tuple2 (where toList is from Data.Foldable) and

f :: [(a, a)] -> [a]
f = concatMap (toList . Tuple2)

You can also use it for other tuples, for example concatMap (toList . Tuple5) etc.