[Haskell-beginners] What is "~" ?

[TG]

Looking at the definition of "partition", it calls

    select :: (a -> Bool) -> a -> ([a], [a]) -> ([a], [a])
    select p x ~(ts,fs) | p x       = (x:ts,fs)
                        | otherwise = (ts, x:fs)

what does the "~" do? 
Can't seem to find it in the documentation, or see a difference in
output without it.
Thank you.
-- 
  TG
  cowscanfly at airpost.net

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