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.
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?
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 #-} This could indeed be useful, and since it subsumes your other proposal without complicating the phase declaration experience this looks like a good extension to have.
As to the numeric phase ids, I'm all in favour of dropping them altogether. For legacy code, you could provide wired-in names 0, 1 and 2 for now but I would deprecate them. Also, I'm one of the probably very few people who have ever used phase 3 and beyond and dropping support for that is perfectly fine with me.
Agreed: we should deprecate numeric ids if we have this alternative. Thanks for your interesting comments! Max