[Haskell-cafe] Equality Contstraint?

[Alexey Shmalko]

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 at 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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