Tuples

A pair is a type

A tuple groups values whose types may differ. The arity is part of the type, so a pair and a triple are different things and never mix:

person :: (String, Int) person = ("Ada", 36) point3 :: (Double, Double, Double) point3 = (1.0, 2.0, 3.0)

Destructuring happens in a pattern, so a function can take the parts directly instead of receiving the pair:

birthday :: (String, Int) -> (String, Int) birthday (name, age) = (name, age + 1)

fst and snd

Two standard functions read the halves of a pair. There is no general accessor for larger tuples, because a tuple with many fields is a record waiting to be written:

fst ("Ada", 36) -- "Ada" snd ("Ada", 36) -- 36 -- swap is a one-liner with a pattern swap (a, b) = (b, a)

fst and snd are partial in spirit: they only exist for pairs. For triples, write the pattern.

zip and unzip

zip walks two lists in step and stops at the shorter one. unzip takes a list of pairs back apart:

zip [1,2,3] ["a","b"] -- [(1,"a"),(2,"b")] zipWith (+) [1,2,3] [10,20,30] -- [11,22,33] unzip [(1,"a"),(2,"b")] -- ([1,2],["a","b"])

The silent truncation of zip is convenient and occasionally a bug. When the lengths must agree, check them or use zipWith with a combining function that reports the mismatch.

Tuples or records

A tuple is right for a value that travels a short distance: a function returning two results, a dictionary entry, an accumulator. It becomes wrong when the meaning of each slot must be remembered:

-- hard to read at the call site register ("Ada", 36, True) -- self-documenting, lesson 26 register (Person { name = "Ada", age = 36, active = True })

The rule of thumb: two or three slots used locally are fine, and anything that crosses a module boundary deserves names.

Next: Lists

Article author: Arthur Isaev