compilation of pattern-matching?

Lennart Augustsson lennart at
Thu Mar 26 09:23:40 EDT 2009

I find this "reordering" discussion somewhat nonsensical.
Haskell specifies top-to-botton, left-to-right matching.
This specifies exactly which tests that have to be made and in what order,
and ghc does exactly those and in the correct order.

One can have a perception that when there are multiple arms in a case
decided by a single test,
then the first arm should somehow be reached quicker than the second one etc
But that is something that the Haskell standard has never promised,
nor has any compiler ever promised this.
And to me such a perception is counter-intuitive; Haskell is about
specifying functions abstractly so order should only matter when it's
a matter of semantics.

On the other hand, adding some kind of pragma that indicates the
likelyhood of a branch seems quite sensible to me.

  -- Lennart

On Thu, Mar 26, 2009 at 9:09 AM, Simon Marlow <marlowsd at> wrote:
> Claus Reinke wrote:
>> Strange. I don't think it is my idea (older implementations
>> used to work that way, and iirc, it also matches what Prolog
>> systems used to do), and I didn't think it was anything but
>> straightforward to avoid case transformations unless there
>> is a clear benefit, so I doubt there is a useful paper in there
>> (also, I can't afford to plan that far ahead atm).
>> What is the benefit of changing the ordering (not just joining paths to
>> avoid redundant tests, but actually modifying the order of tests, to sort by
>> their order in the data type declaration)? Is there any documentation of
>> these case transformations that I could look up?
> It's not that GHC deliberately re-orders case alternatives, it's that it
> doesn't deliberately not do it.
> That's quite an important difference.  To check whether case alternatives
> ever get reordered, we'd have to look at the whole compiler.  It's a new
> constraint on which transformations are valid, and global constraints should
> not be added lightly.  I some kind of annotation is a much more promising
> avenue to explore.
> Cheers,
>        Simon
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