Thanks Stuart, glad to get a second opinion on it. It just felt like maybe I was missing something, but I guess not!

@meow: I did smth like this: - run transactor.cmd with dev-transactor-template.properties - run shell.cmd and import demo data in there - run console.cmd to see console in browser Then I was able to see smth in action But documentation is not clear here 😞 https://clojurians.slack.com/archives/datomic/p1453931405000091

@robert-stuttaford: greetings! I saw you asked @tonsky whether datomic<->datascript sync is a solved problem or not. What is the outcome of that? Are there any good approaches, or did it turn out to be way to specific to a particular project needs to have something useful to share? thanks

Has anyone ever had to implement something that’s just like :db.fn/cas, only retracts the fact if and only if the current value matches? Bascially, allowing “new value” to be effectively nil?

I basically want "compare-and-retract"

@jgdavey: You can simply issue a retraction.

@jgdavey: If the value in the [:db/retract E A V] doesnt match, you just get an empty transaction

That works when the fact exists.

It doesn’t “blow up” like cas if there is no fact to retract

i.e. you want something to throw or error if you try to retract a non-existant value?

The specific thing is: I have an expirable token. When that token is used in one transaction, and is retracted in the same transaction.

I want to ensure that token is valid (in an atomic sense) so that it can’t be used twice

I was thinking in terms of cas, but maybe there’s another way

Ah. I’d say probably a good candidate for a check and retract inside a custom tx function

And just throw if it’s gone?

if that’s your desired behavior

that will abort the whole transaction

Is that the correct way to abort a transaction?

http://docs.datomic.com/database-functions.html#uses-for-transaction-functions

specifies that "You can use them to ensure atomic read-modify-update processing, and integrity constraints. (To abort a transaction, simply throw an exception)."

Gotcha. Thanks!

sure

This page http://docs.datomic.com/exceptions.html also provides some additional info on how exceptions are propagated back to the peer from the transactor

FYI, here’s what I came up with:

https://gist.github.com/jgdavey/57208328312ebecbc6c6

seems reasonable to me