What does the "@" symbol mean in reference to lists in Haskell?

Question

I've come across a piece of Haskell code that looks like this:

ps@(p:pt)

What does the @ symbol mean in this context? I can't seem to find any info on Google (it's unfortunately hard to search for symbols on Google), and I can't find the function in the Prelude documentation, so I imagine it must be some sort of syntactic sugar instead.

Answer 1

Yes, it's just syntactic sugar, with @ read aloud as "as". ps@(p:pt) gives you names for

1. the list: ps
2. the list's head : p
3. the list's tail: pt

Without the @, you'd have to choose between (1) or (2):(3).

This syntax actually works for any constructor; if you have data Tree a = Tree a [Tree a], then t@(Tree _ kids) gives you access to both the tree and its children.

Answer 2

The @ Symbol is used to both give a name to a parameter and match that parameter against a pattern that follows the @. It's not specific to lists and can also be used with other data structures.

This is useful if you want to "decompose" a parameter into it's parts while still needing the parameter as a whole somewhere in your function. One example where this is the case is the tails function from the standard library:

tails                   :: [a] -> [[a]]
tails []                =  [[]]
tails xxs@(_:xs)        =  xxs : tails xs

Answer 3

I want to add that @ works at all levels, meaning you can do this:

let a@(b@(Just c), Just d) = (Just 1, Just 2) in (a, b, c, d)

Which will then produce this: ((Just 1, Just 2), Just 1, 1, 2)

So basically it's a way for you to bind a pattern to a value. This also means that it works with any kind of pattern, not just lists, as demonstrated above. This is a very useful thing to know, as it means you can use it in many more cases.

In this case, a is the entire Maybe Tuple, b is just the first Just in the tuple, and c and d are the values contained in the first and second Just in the tuple respectively

Answer 4

To add to what the other people have said, they are called as-patterns (in ML the syntax uses the keyword "as"), and are described in the section of the Haskell Report on patterns.