How do I convert a list to a tuple in Haskell?

Question

How can I best convert a list to a tuple in Haskell:

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

Answer 1

In a general way, you can't. Each size of tuple is a distinct type, whereas lists of any length are a single type. Thus, there's no good way to write a function that takes a list and returns a tuple of the same length--it wouldn't have a well-defined return type.

For instance, you could have functions like:

tuplify2 :: [a] -> (a,a)
tuplify2 [x,y] = (x,y)

tuplify3 :: [a] -> (a,a,a)
tuplify3 [x,y,z] = (x,y,z)

...but not one that does the job of both.

You can write a generic version using various kinds of meta-programming, but you'd rarely want to.

Note that the same problem applies to other things, such as writing class instances for various tuples--take a look at the source code for Data.Tuple from the standard libraries!