Re: compilation of pattern-matching?

Lennart Augustsson wrote:

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.

I completely agree. I think in an effort to be succinct I may have accidentally given the impression that I thought the ordering of case branches in Core mattered for some reason. I don't think that at all, I was just trying to explain why it would be difficult to make ordering meaningful. Cheers, Simon

-- Lennart

On Thu, Mar 26, 2009 at 9:09 AM, Simon Marlow wrote:

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Claus Reinke wrote:

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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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