Hello,

actually the definition with data keyword is right there:

infixr 5 :-:  
data List a = Empty | a :-: (List a) deriving (Show, Read, Eq, Ord)  

it could be written in prefix form as

data List a = Empty | Cons a (List a) deriving (....)


Petr




On Mon, Dec 21, 2015 at 7:56 PM, Olumide <50295@web.de> wrote:
Hello,

On chapter 7 of LYH there's an example of a user-defined operator .++

infixr 5  .++
(.++) :: List a -> List a -> List a
Empty .++ ys = ys
(x :-: xs) .++ ys = x :-: (xs .++ ys)

which is used as follows

let a = 3 :-: 4 :-: 5 :-: Empty
let b = 6 :-: 7 :-: Empty
a .++ b
(:-:) 3 ((:-:) 4 ((:-:) 5 ((:-:) 6 ((:-:) 7 Empty))))

Following this the text reads:

"Notice how we pattern matched on (x :-: xs). That works because pattern matching is actually about matching constructors. We can match on :-: because it is a constructor for our own list type ..."
Source: http://learnyouahaskell.com/making-our-own-types-and-typeclasses#recursive-data-structures

Is the operator :-: a constructor? I'm confused because the definition of :-: is not prefixed by the data keyword?

- Olumide

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