Max Bolingbroke wrote:
Hi Roman,
Three things. Firstly, what would lenient ordering be useful for? You
probably had a specific use case in mind?
I suspect that when you have multiple plugins all specifying
constraints on the phase ordering independently it is possible to end
up in a situation where using each plugin individually results in a
consistent phase ordering but a combination of multiple plugins causes
a cycle to appear in the order. Hence it is useful to let plugin
authors flag up dependencies they don't >need< to have so we can
intellegently break the cycle using those constraints rather than just
giving up. Admittedly I only have a superstition that this will be a
practical problem.
If you don't need a dependency and it can be ignored anyway, why would
you want to specify it in the first place? I just can't quite imagine a
situation in which I would use this.
As to cycles, I think just aborting if they occur would be ok.
Secondly, I think it is quite
important to be able to specify dependencies for already declared phases.
That is, I (probably) want to be able to say in module C that phase X from
module A should come before phase Y from module B.
This in interesting. You're right that it's not possible with this
system: to support this you would need a more class + instance rather
than single declaration flavour for phases. Do you have a practical
example in mind of how you would use it?
Yeah, I'd prefer the class/instance model. An example: suppose you have
two independent libraries which implement two optimisations and you want
one of them to run before another. I think we really need this if we
want to compose optimisations.
I suspect that's not
quite possible with your current design. Lastly, in addition to < and > I'd
also like to have = (i.e., phase X = phase Y which would make X an alias for
Y).
Ah! This would also let you do what you wanted with your second point:
module C where
{-# PHASE XAlias = A.X, < B.Y #-}
True. It's a bit of a hack, though :-)
Roman
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