Accessing a Specific Element in a Tuple

Question

Haskell-newbie reporting in. Question is as follows: In Haskell, we have fst and snd that return the first and the second elements of a 2-tuple. Wh

Answer 1

Check out the tuple library on hackage. It has overloaded functions for various operations on tuples (up to a predefined size).

Answer 2

Why can't this be done easier? Or maybe there is some easy way?

It can be easier using a recent alternative the lens package. The Tuple module has selectors for up to 9 element tuples and it is straight forward to define more if needed.

> import Control.Lens
> data A = A deriving (Show)
> (1, '2', "3", A) ^. _1
1
> (1, '2', "3", A) ^. _2
'2'
> (1, '2', "3", A) ^. _3
"3"
> (1, '2', "3", A) ^. _4
A

You can also use the lens package to update elements polymorphically, change type on update.

With and without infix operators:

> (1, '2', "3", A) & _1 .~ "wow"
("wow",'2',"3",A)
 > set _1 "wow" (1, '2', "3", A)
("wow",'2',"3",A)

The github readme is a good place to start to find out more about the underlying theory as well as numerous examples.

Not just tuples

Similar syntax works for Traverables and Foldables, so Trees, Maps, Vectors, etc. For example if I had a list of tuples I can access the third tuple element at the 1 index by composing the element 1 to access the first index element with _3 to access the third tuple element.

[(1,2,3),(4,5,6),(7,8,9)] ^? element 1 . _3
Just 6

Answer 3

The question of an approach to doing this using template haskell was previously addressed here.

An example of its usage:

> $(sel 2 3) ('a','b','c')
'b'
> $(sel 3 4) ('a','b','c','d')
'c'

From here.

Answer 4

Nikita Volkov's new "record" library has a feature that appears to do what you want. Search for the heading "Tuples are records too!" on the linked page.

It looks like the library is still under development, so it may not be as easy to install and use right now as it will be in the future.

Answer 5

cabal update
cabal install tuple
ghci

λ> import Data.Tuple.Select
λ> sel3 (0, "1", 2) --select the third element

Answer 6

What prevents the language from having the special construct you want is its design. The designers just didn't put this in, because it would complicate the language definition, which is quite minimalistic. fst and snd are library functions for the common case of pairs; you can define all the others yourself, or better, define record types for your data so that your data members have appropriate names.

(It may be that GHC has an extension to do this, but I haven't encountered one; check the docs or ask on the mailing list to be sure.)