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*View this file with results and syntax highlighting [here](https://mlochbaum.github.io/BQN/doc/pair.html).*
# Pair
The function `⋈` forms a list of all its arguments. When there's one argument, it's called "Enlist", and with two, it's called "Pair".
⋈ "enlist" # ⟨𝕩⟩
"pa" ⋈ "ir" # ⟨𝕨,𝕩⟩
It's usually preferable to use [list notation](arrayrepr.md#brackets) directly for such arrays, because it's easy to add or remove any number of elements. Pair is useful when a standalone function is needed, for example to be used as an operand.
2‿4‿1 ⋈⌜ "north"‿"south" # Cartesian product
⋈¨ "+-×÷" # Glyphs to strings
Another common pattern is to use Pair in a [train](train.md), giving the results from applying each of two functions.
'c' (+⋈-) 1‿2
For longer lists, this pattern can be extended with the function `<⊸∾`, which prepends a single element to a list.
"e0" <⊸∾ "e1" <⊸∾ "e2" ⋈ "e3"
However, before making a long list of this sort, consider that your goal might be more easily accomplished with a list of functions.
6 (+ <⊸∾ - <⊸∾ × ⋈ ÷) 3
{6𝕏3}¨ +‿-‿×‿÷
## Pair versus Couple
Enlist and Pair are closely related to [Solo and Couple](couple.md), in that `⋈` is equivalent to `≍○<` and `≍` is equivalent to `>∘⋈`. However, the result of `⋈` is always a list (rank 1) while Solo or Couple return an array of rank at least 1.
"abc" ≍ "def"
"abc" ⋈ "def"
And the arguments to Couple must have the same shape, while Enlist takes any two arguments.
"abc" ≍ "defg"
"abc" ⋈ "defg"
The difference is that Couple treats the arguments as [cells](array.md#cell), and adds a dimension, while Pair treats them as elements, adding a layer of depth. Couple is a "flat" version of Pair, much like [Cells](rank.md#cells) (`˘`) is a flat version of [Each](map.md#each) (`¨`). Pair is more versatile, but—precisely because of its restrictions—Couple may allow more powerful array operations on the result.
## Fill element
Enlist and Pair set the result's [fill](fill.md) element, while list notation doesn't have to. So the following result is guaranteed:
4 ↑ "a"‿5 ⋈ "b"‿7
This means that `⋈` may not always behave the same as the obvious implementation `{⟨𝕩⟩;⟨𝕨,𝕩⟩}`. However, `≍○<` and even `>∘{⟨𝕩⟩;⟨𝕨,𝕩⟩}○<` compute the result fill as `⋈` does and are identical implementations.
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