Give relation and value constraints explicit keywords

[flyingsilverfin]

Problem to Solve

All constraints in TypeQL are explicitly declared with a keyword a few exceptions.

One is the use of a relation variable and a relation tuple:

$r (friend: $x) isa friendship, has start-date...;

and the other is implicit attribute/value assignment:

$x 10;
?x 10;

These points make TypeQL harder to interpret to the user. Instead, we should consider making all constraints have an explicit keyword.

Proposed Solution

For attributes, we will simply require that we use predicates:

$x == 10;
?x == 10;

For relations, we will introduce of to denote the constraint between a variable and a role players tuple, and ban the usage of implicit value predicates.

So relations will look like the following:

$r of (friend: $x, friend: $y) isa friendship, has name ...;

Which will continue to be syntactic sugar over:

$r of (friend $x, $friend: $y);
$r isa friendship;
$r has name...

Open question: would we allow splitting of and isa into a comma separated list in the same multiple has constraints can be chained? We will need to come up with a consistent pattern for describing contractions constraints into a single comma-separated line.

$r isa friendship, of (friend: $x, friend: $y);

Additional information

Dropping a relation variable would generate an implicit variable with the of:

(friend: $x, friend: $y) isa friendship;

Should represent:

$_ of (friend: $x, friend: $y) isa friendship;