What does the as operator mean in hasekll?

Question

Haskell

Just started learning this AP. I'm reading a tutorial: anton-k.github.io/ru-haskell-book/book/2.html
There is such a code. As I understand it, "as" is not just a variable name, but some kind of keyword, because in the textbook, it is not highlighted like a regular variable.

instance Show Time where
    show (Time h m s) = show h ++ ":" ++ show m ++ ":" ++ show s

instance Show Hour where
    show (Hour h) = addZero (show h)

instance Show Minute where
    show (Minute m) = addZero (show m)

instance Show Second where
    show (Second s) = addZero (show s)

addZero :: String -> String
addZero (a:[]) = '0' : a : []
addZero as     = as

Actually, what kind of operator is this at the end of the code, and why is it needed?

Answer 1

It's not an operator, but a variable name. If the first match did not work (where there is 0 at the beginning of the string), then the function will return the string unchanged.
You can replace 'as' with any other identifier.