Just in case it helps anyone's confusion, your equality constraint idea was on a reasonable track. When the ~ symbol appears *in a type signature* it is indeed an equality constraint. But in a pattern expression it's the marker for an irrefutable pattern.

Regads,
Ben

----- Original Message -----
From:
"Alexey Shmalko" <rasen.dubi@gmail.com>

To:
"Michael Jones" <mike@proclivis.com>, "<haskell-cafe@haskell.org>" <haskell-cafe@haskell.org>
Cc:

Sent:
Fri, 15 May 2015 01:26:23 +0000
Subject:
Re: [Haskell-cafe] Equality Contstraint?


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 <mike@proclivis.com> 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.

Thanks
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