Tuples

If you want a collection of values with differing types, you’ll need a tuple. Tuples look pretty much just like Lists, except they use parentheses instead of square brackets.

> (1, "one")
(1,"one") : ( number, String )
> ('a', 42, ["Life", "the Universe", "Everything"])
('a',42,["Life","the Universe","Everything"]) : ( Char, number, List String )

Just like with Lists, there isn't any generic tuple type in Elm. There is a potentially infinite number of tuple types, as a tuple's type depends on its member values. The type of two tuples will only match if all their contents also have matching types.

> (1, 2) == (3, 4)
False : Bool
> (1, 2) == ("three", "four")
-- TYPE MISMATCH --------------------------------------------- repl-temp-000.elm

The right argument of (==) is causing a type mismatch.

5│   (1, 2) == ("three", "four")
               ^^^^^^^^^^^^^^^^^
(==) is expecting the right argument to be a:

    ( number, number' )

But the right argument is:

    ( String, String )

But unlike Lists, the number of members in a tuple is a part of its type, too.

> [5, 6] == [7, 8, 9]
False : Bool
> (5, 6) == (7, 8, 9)
-- TYPE MISMATCH --------------------------------------------- repl-temp-000.elm

The right argument of (==) is causing a type mismatch.

5│   (7, 8, 9) == (10, 11, 12, 13, 14)
                  ^^^^^^^^^^^^^^^^^^^^
(==) is expecting the right argument to be a:

    ( number, number' )

But the right argument is:

    ( number, number', number'' )

Repeating type variables

Here we see some new type variables--number' and number''.

In short, those 's are a message that this number may or may not be of the same type as the other numbers. It prevents the problems we'd have with a type signature like (number, number, number).

A tuple with three numbers could only mean either of these:

• (Int, Int, Int)
• (Bool, Bool, Bool)

On the other hand, a tuple (number, number', number'') can act as either of those, as well as:

• (Int, Bool, Bool)
• (Int, Int, Bool)
• (Bool, Bool, Int)
• (Bool, Int, Int)

...and any other tuple with three number values!

Something similar happens with wildcard type variable a. You can see it if you make a tuple of empty Lists.

> ([], [], [])
([],[],[]) : ( List a, List b, List c )