Proposal: remove n+k patterns from the language

Pros

  • n+k patterns are a non-orthogonal feature:
    • no other datatype has this special notation
    • it requires excessive knowledge of library functions: the desugaring includes references to Integral, - and >=.
    • the symbol + is being abused in the syntax. It doesn't really mean +, and if + happens to be bound to something else that doesn't change the meaning of an n+k pattern.
  • the side-condition that n >= 0 is ugly, n is really a natural number
  • something like ViewPatterns might offer a more general solution
  • The report already admits "Many people feel that n+k patterns should not be used. These patterns may be removed or changed in future versions of Haskell." (Section 3.17.2)

Cons

  • it is a concise, natural, and familiar notation for recursion over naturals
  • it allows recursive functions over naturals to be defined using non-overlapping patterns
  • it would make a lot more sense if we had a natural number type, and restricted n+k patterns appropriately
  • some Haskell books use it (this was the main reason it was kept in Haskell 98)