And just to be clear - I get _why_ explicit parameter passing is a good
idea, or rather why dynamic binding is frowned upon, I am asking for
practical help with the consequences of that decision.
I also get the argument that a transaction changes at runtime state and so
shouldn't be part of the (Stuart Sierra's) component lifecycle, but I can't
be the only one to run into this; func-a calls func-b calls func-c which
needs some state - the answer can't be for func-a to pass it to func-b
particularly when neither of them know anything about the internals of
func-c.
On Wednesday, 4 March 2015 17:58:58 UTC, Colin Yates wrote:
>
> Hi,
>
> I am looking for the Clojure equivalent of:
>
> class Whatever {
> @Transactional
> void doSomething(IDoSomething one, IDoSomethingElse two) {
> one.doSomething()
> two.doSomething()
> }
> }
>
> where both one and two are dependency injected with a proxy which resolves
> to a thread local database connection. In addition, one might itself have a
> collaborator which itself has a collaborator which needs a datasource.
>
> So far I have two protocols:
>
> (defprotocol IDoSomething
> (do-something [this ...])
>
> (defprotocol IDoSomethingElse
> (do-something [this ...])
>
> Each protocol may have a number of implementations, one of which is a JDBC
> implementation:
>
> (defrecord JdbcIDoSomething [db]
> (do-something [this ...] ...))
>
> The problem is that the calling code only gets provided an IDoSomething
> and an IDoSomethingElse and it wants to do something like:
>
> (let [one (->JdbcDoSomething db) two (->JdbcDoSomethingElse db)]
> (with-transaction [tx db]
> (do-something one)
> (do-something-else two)))
>
> The problem here is that the implementations of do-something and
> do-something-else won't have access to the local bound 'tx', they will have
> their own 'db'.
>
> I realise the general argument is to be explicit and pass a db as the
> first argument to the protocol but this isn't appropriate in this case as
> there are validly multiple implementations. I could abstract a
> 'unit-of-work' and pass that as the first argument to the protocols but
> that seems a bit painful.
>
> Also, these protocols may be used quite far away from where the database
> code lives and passing a parameter all the way through the call stack is
> painful.
>
> I am using Stuart Sierra's components if that makes any difference.
>
> I can't be the first person to run into this but google is surprisingly
> unhelpful which makes me think I have missed something fundamental, and
> that I have something upside down.
>
> What do you all do?
>
>
>
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