Version 2 (modified by 8 years ago) (diff)  ,

Proposal: NoNPlusKPatterns
Ticket  #130 
Dependencies  none 
Related  ViewPatterns 
Compiler support
GHC  full (XNoNPlusKPatterns from 6.12)

nhc98  full (nonplusk )

Hugs  none? 
UHC  full (doesn't support n+k patterns at all) 
JHC  none? 
LHC  none? 
Summary
Remove n+k
patterns.
Description
n+k
patterns provide a concise, natural, and familiar notation for recursion over naturals. When used with nonnegative types, such as Word
, they allow functions to be cleanly defined using nonoverlapping patterns.
However, n+k
patterns are a nonorthogonal feature. No other pattern has special notation like this, although there is a ViewPatterns extension which does something not entirely dissimilar in a general way.
The +
symbol is being abused here; it doesn't really mean +
, and if +
happens to be bound to something other than Prelude.+
the notation's behaviour doesn't change. Counterintuitively, the desugaring does include references to a number of other Prelude
definitions: Integral
, 
and >=
.
Working on all Integral
types is inconsistent with the n >= 0
condition, which means that n
is really a natural number.
There has long been a general consensus in the community that n+k
patterns should be removed. Indeed, 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." (Haskell 98 Revised Report, Section 3.17.2).
References
Haskell 98 Revised Report, Section 3.17.2
Report Delta
In Section 3.17.1 replace:
pat > var + integer (successor pattern)  pat0
with:
pat > pat0
In Section 3.17.2 remove:
 Matching an
n+k
pattern (wheren
is a variable andk
is a positive integer literal) against a valuev
succeeds ifx >= k
, resulting in the binding ofn
tox  k
, and fails otherwise. Again, the functions>=
and
are overloaded, depending on the type of the pattern. The match diverges if the comparison diverges.
The interpretation of the literal k
is the same as in numeric literal patterns, except that only integer literals are allowed.
and renumber.
In Section 3.17.2 remove:
 An
n+k
pattern can only be matched against a value in the classIntegral
.
Many people feel that n+k
patterns should not be used. These patterns may be removed or changed in future versions of Haskell.
In Section 3.17.3 remove:
(s) case v of { x+k > e; _ > e' } = if v >= k then (\x > e) (vk) else e' where k is a numeric literal
In Section 4.4.3.2 remove:
A note about syntax.
It is usually straightforward to tell whether a binding is a pattern binding or a function binding, but the existence of n+k
patterns sometimes confuses the issue. Here are four examples:
x + 1 = ...  Function binding, defines (+)  Equivalent to (+) x 1 = ... (x + 1) = ...  Pattern binding, defines x (x + 1) * y = ...  Function binding, defines (*)  Equivalent to (*) (x+1) y = ... (x + 1) y = ...  Function binding, defines (+)  Equivalent to (+) x 1 y = ...
The first two can be distinguished because a pattern binding has a pat0 on the left hand side, not a pat — the former cannot be an unparenthesised n+k
pattern.