I've tried googling but come up short. I am furthering my Haskell knowledge by reading some articles and I came across one that uses a syntax I've never seen before. An example would be:
reconstruct node@(Node a b c l r) parent@(Node b d le ri)
I've never seen these @'s before. I tried searching online for an answer but came up short. Is this simply a way to embed tags to help make things clearer, or do they have an actual impact on the code?
It is used in pattern matching. Now node variable will refer to the entire Node data type for the argument Node a b c l r. So instead of passing to the function as Node a b c l r, you can use node instead to pass it up.
A much simpler example to demonstrate it:
data SomeType = Leaf Int Int Int | Nil deriving Show
someFunction :: SomeType -> SomeType
someFunction leaf@(Leaf _ _ _) = leaf
someFunction Nil = Leaf 0 0 0
The someFunction can also be written as:
someFunction :: SomeType -> SomeType
someFunction (Leaf x y z) = Leaf x y z
someFunction Nil = Leaf 0 0 0
See how simpler was the first version ?