[Haskell-cafe] Equality Contstraint?

[Ben]

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" 
To:"Michael Jones" , "" 
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
[1]   
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.

 Thanks
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Links:
------
[1]
http://en.wikibooks.org/wiki/Haskell/Laziness#Lazy_pattern_matching
[2] mailto:mike at proclivis.com
[3] mailto:Haskell-Cafe at haskell.org
[4] http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

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