Hi, Michael, It's irrefutable pattern - the pattern that matches lazily. You can get more info here [1]. Hope this helps, Alexey Shmalko [1]: http://en.wikibooks.org/wiki/Haskell/Laziness#Lazy_pattern_matching On Fri, May 15, 2015 at 4:17 AM Michael Jones wrote:

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.

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On Fri, 15 May 2015 03:17:07 +0200, Michael Jones wrote:

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

The Hoogle at haskell.org[0] can help you, it leads to: https://wiki.haskell.org/Keywords#.7E The Hoogle version at fpcomplete.com doesn't handle keywords or standard operators any more. Regards, Henk-Jan van Tuyl [0] https://www.haskell.org/hoogle/ -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --