What does the : infix operator do in Haskell?

Question

I'm reading A Gentle Introduction to Haskell (which is not so gentle) and it repeatedly uses the `:` operator without directly explaining what it does. So, what exactly does it do?

Accepted Answer

`:` is the “prepend” operator:

```
x : xs
```

Returns a list which has `x` as first element, followed by all elements in `xs`. In other functional languages, this is usually called `cons`, because it “cons”tructs a list recursively by repeated application from an empty list:

```
1 : 2 : 3 : 4 : []
```

is the list `[1, 2, 3, 4]`.

What does the : infix operator do in Haskell?

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