Parse more in tx-parser: id-literals and map shorthand notation

[ncalexan]

We need a way to declare temporary IDs. Datomic uses EDN read syntax like #db/id [:db.part/db -1]; DataScript uses negative integers; our CLJ implementation uses a function invocation like (id-literal :db.part/db -1). (Bare negative integers aren't appropriate for Mentat, since we're going to support parts.) I'm loathe to add read syntax just for id-literals and instants, but we need something.

While we're here, the initial tx-parser commit doesn't support the map shorthand; that is, the valid transaction

[{:db/id ENTID :temp/attr "value"}]

is not yet parsed.

[rnewman]

We need a way to declare temporary IDs.

The new Datomic way to do this, given client APIs that can't afford repeated network round-trips to allocate tempids, is to use placeholder strings and make tempids optional. The transactor knows each place that a string occurs where an entity ID or ident is expected, and transparently allocates one tempid per string. See <http://blog.datomic.com/2016/11/datomic-update-client-api-unlimited.html> for details. I suggest supporting tempids in the transact format itself (allowing their programmatic use), and allowing only string 'tempids' in the EDN input.

[ncalexan]

@rnewman correctly points out that using string identifiers for tempids is never ambiguous, because we can always differentiate a string from an entid based on the schema. During the discussion I pointed out that there is an ambiguity: two item lists are overloaded to be lookup-refs (so [[:db/attr "unique-value"] :db/other "some other value"]] is valid.

However, there's an ambiguity here. It's possible to use a lookup-ref as the value for a :db/valueType :db.type/ref which is itself :db/cardinality :db.cardinality/many! That conflicts with the shorthand for adding many datoms using a list. That is, if :db/attr has the above characteristics, what does

[ENTITY :db/attr [:user/unique-attr :user/some-enum]]

mean? Do you want to process the two-element list as a lookup-ref? Or do you want to insert two datoms?

[rnewman]

I'd be interested to know what Datomic and DataScript do. We can also detect this case and either define a precedence (pick which thing to do), log, or raise an error.

[ncalexan]

This is done: #180, #184.