ViewPatterns and multiple calls in Haskell

Question

I read this:

http://hackage.haskell.org/trac/ghc/wiki/ViewPatterns

I like the idea, want to use the extension. I however would like to make sure as to one thing: whether the view function is evaluated once for a single matching.

So let's say we have:

{-# LANGUAGE ViewPatterns #-}
...

f (view -> Nothing) = ...
f (view -> Just x) = ...

view :: a -> Maybe b

Now let's say I invoke f a. Is view invoked twice or just once for the given argument a?

EDIT:

I tried to find out whether this is the case and wrote the following:

{-# LANGUAGE ViewPatterns #-}

import System.IO.Unsafe

blah (ble -> Nothing) = 123
blah (ble -> Just x) = x

ble x = unsafePerformIO $ do
    putStrLn $ "Inside ble: " ++ show x
    return x

main :: IO ()
main = do
    putStrLn $ "Main: " ++ show (blah $ Just 234)

Output using GHC:

Inside ble: Just 234
Inside ble: Just 234
Main: 234

Output using GHC (with optimization)

Inside ble: Just 234
Main: 234

Output using GHCi:

Main: Inside ble: Just 234
Inside ble: Just 234
234