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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?
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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. Save this answer.
Haskell guards are used to test the properties of an expression; it might look like an if-else statement from a beginner's view, but they function very differently. Haskell guards can be simpler and easier to read than pattern matching .
In Haskell, the colon operator is used to create lists (we'll talk more about this soon). This right-hand side says that the value of makeList is the element 1 stuck on to the beginning of the value of makeList .
The == operator means "is equal". The /= operator means "is not equal". It's supposed to be reminiscent of the mathematical "≠" symbol (i.e., an equals sign with a diagonal line through it).
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 ?
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