Scopes

[vpetrovykh]

The current state of understanding of scope:

DETACHED completely ignores all current scope. Only schema-level symbols are recognized and it is equivalent to declaring a schema-level view.

Explicit SELECT makes whatever it wraps into a SET OF. Other than that normal nested scope rules apply.

Aliases created by := always imply an explicit SELECT for their expression part, but they do define a new symbolic name for the resulting set behaves as any other named set from this point in the scope and in any nested scopes. The same reasoning that forbids FILTER to alter the meaning of a path used in SELECT, also applies to aliases. Once defined they become opaque and cannot be filtered or otherwise affected by symbols referring to their components. The difference is that it's not illegal, but the aliased sets are no longer related to their components anywhere where the aliases are visible.

Problems we're avoiding:

Lexical substitution is great when it's pure and without special cases. Unfortunately even simple aliasing can wreak havok on "just substitution: approach:

WITH A := (SELECT Issue FILTER Issue.owner.age > 3)
SELECT A
FILTER A.number > 3;

# if we perform _only_ lexical substitution
SELECT (SELECT Issue FILTER Issue.owner.age > 3)
FILTER (SELECT Issue FILTER Issue.owner.age > 3).number > 3;
# the outer filter is largely irrelevant

Also aliasing literals is all but useless under "pure substitution":

WITH A := {1,2,3}
SELECT A
FILTER A > 2;

So we know we cannot "just substitute".

Non obvious interference from other parts of an expression:

WITH Stuff := Issue
SELECT (Issue, Stuff);

# is it same or different from
WITH 
   Stuff1 := Issue,
   Stuff2 := Issue
SELECT (Stuff1, Stuff2);

Depending on the "substitution" and "implicit SELECT" doctrine there are really subtle differences in how the above expressions are treated.