syntax - 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.