# Tuples (Pairs, Triples, ...)
# Extract tuple components
Use the fst and snd functions (from Prelude or Data.Tuple) to extract the first and second component of pairs.
fst (1, 2) -- evaluates to 1
snd (1, 2) -- evaluates to 2
Or use pattern matching.
case (1, 2) of (result, _) => result -- evaluates to 1
case (1, 2) of (_, result) => result -- evaluates to 2
Pattern matching also works for tuples with more than two components.
case (1, 2, 3) of (result, _, _) => result -- evaluates to 1
case (1, 2, 3) of (_, result, _) => result -- evaluates to 2
case (1, 2, 3) of (_, _, result) => result -- evaluates to 3
Haskell does not provide standard functions like fst or snd for tuples with more than two components. The tuple (opens new window) library on Hackage provides such functions in the Data.Tuple.Select (opens new window) module.
# Strictness of matching a tuple
The pattern (p1, p2) is strict in the outermost tuple constructor, which can lead to unexpected strictness behaviour (opens new window). For example, the following expression diverges (using Data.Function.fix):
fix $ \(x, y) -> (1, 2)
since the match on (x, y) is strict in the tuple constructor. However, the following expression, using an irrefutable pattern (opens new window), evaluates to (1, 2) as expected:
fix $ \ ~(x, y) -> (1, 2)
# Construct tuple values
Use parentheses and commas to create tuples. Use one comma to create a pair.
(1, 2)
Use more commas to create tuples with more components.
(1, 2, 3)
(1, 2, 3, 4)
Note that it is also possible to declare tuples using in their unsugared form.
(,) 1 2 -- equivalent to (1,2)
(,,) 1 2 3 -- equivalent to (1,2,3)
Tuples can contain values of different types.
("answer", 42, '?')
Tuples can contain complex values such as lists or more tuples.
([1, 2, 3], "hello", ('A', 65))
(1, (2, (3, 4), 5), 6)
# Write tuple types
Use parentheses and commas to write tuple types. Use one comma to write a pair type.
(Int, Int)
Use more commas to write tuple types with more components.
(Int, Int, Int)
(Int, Int, Int, Int)
Tuples can contain values of different types.
(String, Int, Char)
Tuples can contain complex values such as lists or more tuples.
([Int], String, (Char, Int))
(Int, (Int, (Int, Int), Int), Int)
# Pattern Match on Tuples
Pattern matching on tuples uses the tuple constructors. To match a pair for example, we'd use the (,) constructor:
myFunction1 (a, b) = ...
We use more commas to match tuples with more components:
myFunction2 (a, b, c) = ...
myFunction3 (a, b, c, d) = ...
Tuple patterns can contain complex patterns such as list patterns or more tuple patterns.
myFunction4 ([a, b, c], d, e) = ...
myFunction5 (a, (b, (c, d), e), f) = ...
# Apply a binary function to a tuple (uncurrying)
Use the uncurry function (from Prelude or Data.Tuple) to convert a binary function to a function on tuples.
uncurry (+) (1, 2) -- computes 3
uncurry map (negate, [1, 2, 3]) -- computes [-1, -2, -3]
uncurry uncurry ((+), (1, 2)) -- computes 3
map (uncurry (+)) [(1, 2), (3, 4), (5, 6)] -- computes [3, 7, 11]
uncurry (curry f) -- computes the same as f
# Apply a tuple function to two arguments (currying)
Use the curry function (from Prelude or Data.Tuple) to convert a function that takes tuples to a function that takes two arguments.
curry fst 1 2 -- computes 1
curry snd 1 2 -- computes 2
curry (uncurry f) -- computes the same as f
import Data.Tuple (swap)
curry swap 1 2 -- computes (2, 1)
# Swap pair components
Use swap (from Data.Tuple) to swap the components of a pair.
import Data.Tuple (swap)
swap (1, 2) -- evaluates to (2, 1)
Or use pattern matching.
case (1, 2) of (x, y) => (y, x) -- evaluates to (2, 1)