[Stretch] Support Float type in classical expressions.
#13832
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This reverts commit e2b3221.
Types that have some natural order no longer have an ordering when one of them is strictly greater but has an incompatible const-ness (i.e. when the greater type is const but the other type is not).
We need to reject types with const=True in QPY until it supports them. For now, I've also made the Index and shift operator constructors lift their RHS to the same const-ness as the target to make it less likely that existing users of expr run into issues when serializing to older QPY versions.
This is probably a better default in general, since we don't really have much use for const except for timing stuff.
Since we're going for using a Cast node when const-ness differs, this will be fine.
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I wasn't going to have this, but since we have DANGEROUS Float => Int, and we have Int => Bool, I think this makes the most sense.
kevinhartman
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Also removes the assumption that a const-type can never be an l-value in favor of just restricting l-values with const types from being added to circuits for now. We will (in a separate PR) add support for adding stretch variables to circuits, which are const. However, we may track those differently, or at least not report them as variable when users query the circuit for variables.
This one I'd added thinking I ought to block store from using a const var target. But since I figured it's better to just restrict adding vars to the circuit that are const (and leave the decision of whether or not a const var can be an l-value till later), this test no longer makes sense.
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Summary
Adds a new
Floattype to the classical expression system. This is necessary to express timing relationships between durations and stretches in expressions.In a follow-up(s), I'll add
DurationandStretchtypes as well as binary operators for+,-,*and/, and then plumb all of this into the existing circuit variable tracking infra, integrate with theDelayinstruction, support serialization, etc.Details and comments
Currently based on #13811. Here's a readable diff in the meantime.
Floattype is considered of an "unspecified width", in accordance with the machine-precisionfloatin OpenQASM3. If we add a fixed-width float (similar to what we've got forUint), then it shouldn't be difficult to add an optional constructor argument toFloatforwidth, defaulted toNone. I didn't opt to do this since I imagine fixed-width floats aren't desperately needed for stretches, and they would've taken too long to stamp out.expr.liftnow supports Python floats.UintorBooltoFloatto be unsafe.Floatcannot be mixed withUintin binary operations. You must cast one of them, first.Floatfor one reason or another (e.g. not losslessly coercable toBool). The arithmetic operations to come (+,-,*and/) will support them.To-do