问题
I'm reading through Real World Haskell, and am trying to understand the as-pattern.
From the book (Chapter 4):
suffixes :: [a] -> [[a]]
suffixes xs@(_:xs') = xs : suffixes xs'
suffixes _ = []
The book explains the @ symbol thus,
"...bind the variable xs to the value that matches the right side of the @ symbol."
I'm having trouble understanding this explanation. Supposing I call
suffixes "hello"
Explicitly, what would the above line with the @ do to this (on the first iteration)? I know what the result of the function is, but cannot see how we get there from the above code.
回答1:
xs' would be bound to the string "ello".
xs would be bound to the string "hello".
The @ pattern lets you give a name to a variable while also matching its structure and possibly giving name to the components.
回答2:
Perhaps an actual "de-sugaring" will make it easier to understand:
suffixes xs@(_:xs') = xs : suffixes xs'
is equivalent to
suffixes xs
| (_:xs') <- xs = xs : suffixes xs'
i.e. you're firstly binding the entire argument to the variable xs, but you also do pattern matching on the same argument (or, equivalently, on xs) to (_:xs').
来源：https://stackoverflow.com/questions/27467650/understanding-the-haskell-as-pattern