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# Proposal: remove `n+k` patterns from the language
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## Pros
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- `n+k` patterns are a non-orthogonal feature:
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- no other datatype has this special notation
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- it requires excessive knowledge of library functions: the desugaring includes references to `Integral`, `-` and `>=`.
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- 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.
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- the side-condition that `n >= 0` is ugly, `n` is really a natural number
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- something like [ViewPatterns](view-patterns) might offer a more general solution
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- 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)
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## Cons
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- it is a concise, natural, and familiar notation for recursion over naturals
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- it allows recursive functions over naturals to be defined using non-overlapping patterns
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- it would make a lot more sense if we had a natural number type, and restricted `n+k` patterns appropriately
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- some Haskell books use it (this was the main reason it was kept in Haskell 98) |
