ViewPatterns

The ViewPatterns extension adds a new syntax to existing patterns. Instead of writing a pattern that the argument will be matched against, you write two things in the form (e -> p). Here e is an arbitrary expression and p is a pattern. Where without a view pattern the corresponding argument would be matched against a pattern directly, instead the expression will be evaluated with the pattern’s argument as its arguemnt, and its result will then be matched against the given pattern.

One interesting use is to replace NPlusKPatterns by using the subtract function as a view.