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[Chris Hathhorn <chathhorn AT gmail.com>]

On Sun, Aug 18, 2013 at 5:22 PM, Ömer Sinan Ağacan 
<omeragacan AT gmail.com>
 wrote:
> Thanks Chris, your solution worked(with a minor change, I had to add
> `...` after `rule incSomeMap(.Set, _) => .`).
>
> In your code, what's happening here:
>
>     incSomeMap((SetItem(K:K) => .Set) _:Set, I:Int) ...
>
> ? As far as I can understand `SetItem(K:K) => .Set)` matches an
> element of the set and replaces with unit(effectively removing the
> element), but what is that _ after this? eg. _:Set ? My guess is that
> _ unifies with rest of the set, is that correct? I think this is the
> first time I'm seeing this syntax. Is there also similar syntax to
> match match first element/rest of a map too?
>

Yeah that's right. The "_:Set" matches the rest of the set (actually
it should also work without the ":Set" and with "." for ".Set"). For
maps from K to Int, you could use "(K:K |-> I:Int => .) _" to peel off
the first key/value pair from a map.

Chris