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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