[Haskell-cafe] Equality Contstraint?

[Michael Jones]

I am poking around in Arrow to understand it better. There is the following definition:

    second :: a b c -> a (d,b) (d,c)
    second f = arr swap >>> first f >>> arr swap
      where
        swap :: (x,y) -> (y,x)
        swap ~(x,y) = (y,x)

Can someone explain what the ~ is? Searching the net resulted in frustration.

I kind of think it is some kind of equality constraint but can’t find documentation. Perhaps if I knew what it was called I might succeed in finding something. If it is an equality constraint, please provide a reference if you have one. I did not find anything in a search including GHC, etc.

Thanks