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Bit more draft RFC about type normalization

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ajeffrey@roblox.com 2022-03-10 16:07:14 -06:00
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rfcs/type-normalization.md
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@ -2,7 +2,7 @@
## Summary ## Summary
Normalize types, for example removing redundant unions and We should normalize types, for example removing redundant unions and
intersections, to minimize memory usage and make user-visible inferred intersections, to minimize memory usage and make user-visible inferred
types easier to read. types easier to read.
@ -59,7 +59,9 @@ or `Animal&Cat`.
## Design ## Design
In this section, we outline a number of possible designs. These fall into two broad camps: syntactic subtyping uses rewrite rules on the syntax of types, whereas semantic subtyping uses semantic models of types. In this section, we outline some possible designs. These fall into two
broad camps: syntactic subtyping uses rewrite rules on the syntax of
types, whereas semantic subtyping uses semantic models of types.
### Syntactic subtyping ### Syntactic subtyping
@ -74,6 +76,8 @@ Advantages: easy to implement; fast.
Disadvantage: pointer equality is a very brittle test; it is easy to still get Disadvantage: pointer equality is a very brittle test; it is easy to still get
types `T|T`, caused by having two clones of `T`. types `T|T`, caused by having two clones of `T`.
This is currently what we do.
#### Alternative: check for suptyping before adding to a union/intersection #### Alternative: check for suptyping before adding to a union/intersection
Ditto, but don't add a type if there's already a supertype in the Ditto, but don't add a type if there's already a supertype in the
@ -161,6 +165,10 @@ Tree automata have:
(Luau has namned properties rather than positional arguments, but otherwise this is standard). (Luau has namned properties rather than positional arguments, but otherwise this is standard).
An automaton accepts a tree if its initial state does. A state `q` accepts a tree `t`
if there is a transition `q0 -> f(p1 = q1, ..., pN = qN)` where `t` has node label `f`,
and children `t -p1-> t1`,..., `t -pN-> tN` where each `qI` accepts `tI`.
Draw automata where the transition `q0 -> f(p1 = q1, ..., pN = qN)` as Draw automata where the transition `q0 -> f(p1 = q1, ..., pN = qN)` as
``` ```
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