Transacting API for Datomic

Question

Certainly a dumb question, but hopefully should clear things up.

I am following along https://docs.datomic.com/on-prem/getting-started/transact-schema.html, and the step to transact the schema, in this tutorial, is

user=> (d/transact conn {:tx-data movie-schema})

However, when I try this, I get

ClassCastException clojure.lang.PersistentArrayMap cannot be cast to java.util.List  datomic.api/transact 

Instead, when I just do

(d/transact conn schema)

it works for me. Is the tutorial missing some subtlety here? Am I missing something? (the only difference is that I'm using the Free version instead of the Starter Pro version).

Edit: the initial version of this post mentioned the schema, but this is generally true for just adding new facts too (i.e. using {:tx-data foo} doesn't work, but just using foo does).

Answer 1

I think the cause of your problem here is an inconsistency between the Peer API (in which transact accepts a list) and the Client API (in which transact accepts a map containing a :tx-data key). I suspect you tried to run your REPL commands in the REPL of a Peer process, whereas the tutorial you linked to expects you to run commands in the REPL of a Client process.

Why the inconsistency between Peers and Clients? Not being part of the Datomic team, I can only speculate:

1. The Peer API was designed before the Client API historically, at which time the 'transaction requests as lists' format was sufficient
2. When the Client API was designed, because of the more expensive nature of calling d/transact (I believe it incurs an additional I/O roundtrip from client to server), the authors left room for additional data in transaction requests (e.g for templating), hence the more extensible map-based format.