# Re: n+k patterns

```Original-Via: uk.ac.ox.prg; Wed, 6 Nov 91 11:44:16 GMT

> | | Which semantics did you use?
> |
> | The following seemed sensible to me (Your first choice in each case):
>
> [ ... my attempt at a semantics for c*p and p+k patterns ... ]
>
>    I had hoped not, since then a match can fail and fail the entier program,
> even if a "later" patterns really matches, since in the above the "next"
> pattern is not tested upon such a fail.

I have been persuaded and would go along with Kent's proposed semantics
for c*p and p+k patterns if these were introduced into the language.
Specifically:

|          case e0 of {p+k -> e; _ -> e'}
| either   = if e0 >= k then case e0-k of {p -> e; _ -> e'} else e'
| or       =                 case e0-k of {p -> e; _ -> e'}

But let's not use the second option here ... I'm a firm believer that (p+k)
patterns should match only positive values.

Note also that this includes the current semantics for Haskell n+k patterns
as a special case (writing p as a variable which is guaranteed to match and
simplifying the case expr on the rhs).

|          case e0 of {c*p -> e; _ -> e'}
| either   = if e0 >= 0 then case e0 `divRem` c of (p, 0) -> e; _ -> e'} else e'| or
|    =                 case e0 `divRem` c of (p, 0) -> e; _ -> e'}

The choice here isn't so clear.  Should c*p patterns match only positive
values (for uniformity with with p+k patterns, although there is no real
need for the restriction in this case), or should we allow them to match
arbitrary multiples of c?  Perhaps someone with lots of examples of the
use of c*p+k patterns could comment on which would be best?  Tony?

Thanks to Kent for straightening this out.

Mark

```