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Stephan Renatus
87e6b1e38e
Before, we had been planing `every x in xs { BODY }` as, roughly,
NOT
SCAN xs
NOT
BODY
This poses a problem: scans never fail -- they iterate _their_
bodies, and break out of their block when they are done iterating.
As far as the NOT is concerned, it always looks like the SCAN was
successful.
We had worked around that for the "outer" SCAN by breaking out of
the NOT's block at the end. However, the same trick can't be used
when BODY contains another SCAN, since we can't tell beforehand how
deeply we're nested, and doesn't know how far to break out again.
So in this commit, we replace the NOTs by some custom "condition
var" construct that works similar to how NOT gets compiled, but
sets the "inner" condition variable when the BODY's plan succeeds.
Likewise, the "outer" condition variable is tied to that result.
We're practically using the condition variables to encode
every x, y in xs { p(x,y) }
~> p(x1, y1) AND p(x2, y2) AND ... AND p(xn, yn)
~> NOT (NOT p(x1, y1) OR NOT p(x2, y2) OR ... OR NOT p(xn, yn))
The conditon variables are now implemented in the IR. (When using
ir.NotStmt, they are an artifact of compiling the IR to wasm.) This
is achieved by resetting a new local, and assigning it a dummy value
(true) to signal the condition was met.
----
This path was taken because I could not come up with a way to deal
with the "inner SCAN" situation just by using NOT blocks. The "outer"
NOT/SCAN could perhaps be salvaged, but I found it easier to reason
about one mechanism applied twice than to mix and match.
Another abandoned path was using ir.ReturnLocalStmt in the query's
plan iterator: while it worked well for simple cases, the problems
considered were that we might do the wrong thing in nested sitations,
like comprehensions. Also, it's not something we'd done in any other
place.
Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>