Okay, I think I understand that.
Does that mean this code could never work as intended in a Clojure program,
only at the repl (which seems a bit of a limitation) or is there a way to
make it work as intended, generating a different id each time? Or is the
whole approach taken in this code flawed?
On Sunday, 3 March 2013 21:59:45 UTC, Michał Marczyk wrote:
>
> #db/id ... is a "tagged literal". It gets read in as an id object with
> some integer inside. The way this happens is that the reader looks up
> a "data reader" function associated with the symbol db/id in the map
> held by the var *data-readers* and passes to it the result of parsing
> the rest of the literal (a vector holding the keyword :db.part/db in
> this case). Importantly, this function is not pure and the integer it
> uses will be different on each invocation. (Given this form of the
> #db/id literal; you can also specify a particular number yourself --
> #db/id [:db.part/db some-number-here].)
>
> In any case, once the reader reads in your code including some
> particular id, the compiler will compile the result preserving the
> particular values it sees, so it will embed code to construct *exactly
> this* id in the compiled output. Thus you end up with a particular id
> hardwired into the first version of your function.
>
> With the second version, if you invoke it multiple times at the REPL,
> you ask the reader for a new id at each invocation, so it works as you
> expect. If instead you were to use it inside a function like so:
>
> (defn foo []
> (make-column-schema #db/id [:db.part/db] :results/subject
> :db.type/string)),
>
> then again the same id would be used for every (foo) call.
>
> Cheers,
> Michał
>
>
> On 3 March 2013 21:58, <edw...@kenworthy.info <javascript:>> wrote:
> > So, I am studying a piece of code from the web. I've dissected most of
> it
> > and am in the process of re-factoring it.
> >
> > What I don't understand is why one version works and the other doesn't.
> >
> > So for both:
> >
> > (ns gcse-results.core (:use [datomic.api :only [q db] :as d]) (:use
> > quil.core))
> >
> > This doesn't work:
> >
> > (defn make-column-schema [db-ident db-type]
> > {:db/id #db/id[:db.part/db]
> > :db/ident db-ident
> > :db/valueType db-type
> > :db/cardinality :db.cardinality/one
> > :db.install/_attribute :db.part/db})
> >
> > Each call to:
> >
> > (make-column-schema :results/subject :db.type/string)
> >
> > The value of #db/id[:db.part/db] is the same.
> >
> > And this works:
> >
> > (defn make-column-schema [db-id db-ident db-type]
> > {:db/id db-id
> > :db/ident db-ident
> > :db/valueType db-type
> > :db/cardinality :db.cardinality/one
> > :db.install/_attribute :db.part/db})
> >
> > Each call to: (make-column-schema #db/id[:db.part/db] :results/subject
> > :db.type/string)
> >
> > The value of #db/id[:db.part/db] is the different, as expected.
> >
> > Now I thought that I understood #db/id[:db.part/db] (call to a Java
> > constructor) but obviously my understanding is flawed as I would expect
> both
> > of these functions to produce the same thing, but they don't so there's
> > obviously some gap in my understanding.
> >
> > Help?
> >
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