NPlusKPatterns

The NPlusKPatterns extension adds a new syntax to patterns. It allows patterns of the form (n+k) to be matched, where n is the identfier to bind to, k is any positive integer literal, and the plus sign represents mathematical addition (I know, shocking). The compiler just desugars the form into a subtraction.

NPlusKPatterns are largely of historical interest now. They were originally part of Haskell98, but subsequently removed from Haskell2010, and this extension turns them back on again. Anyone writing new code is directed to ViewPatterns, instead.