hi,
I'm new to lisp/clojure
in general. I was reading the free whitepaper from Amit
Rathore's 'Clojure in Action' where he gives a simple
example on macros.
In the example on page 9 he shows how you might do an
audited connection in java:
public void addExpense(long userId, Date date, BigDecimal amount) {
AuditLogger.logLookup(userId, AuditCode.ADD_EXPENSES);
ExpensesConnection connection = new ExpensesConnection(userId);
connection.saveNewExpense(date, amount);
connection.flush();
connection.close();
}
In clojure he shows a macro (without showing the code[1])
that boils it down to:
(defn add-expense [user-id date amount]
(with-audited-connection [user-id connection]
(save-new-expense connection date amount)))
So in ruby you can be pretty concise here - in fact
I differ with the author who says it isn't as good:
def add_expense user_id , date , amount
with_audited_connection(user_id,'audit code') do |conn|
conn.save_new_expense date , amount
end
end
where with_audited_connection might be something like:
def with_audited_connection user_id , audit_code
puts "logging conn for '#{user_id}' (#{audit_code})"
conn = ... # Get a connection.
yield conn # Expose conn in block - see above.
puts "closing conn or whatever"
end
('puts' just means print to stdout)
The second function above organises the connection 'conn'
and then yields it for use elsewhere in a block (see the
do-block in the first function). After that block finishes,
the yield returns and the the 2nd function tidies up and
does whatever else it needs to.
Bear in mind, I'm really new to lisp in general.
So is 'connection' in
(with-audited-connection [user-id connection] ... )
being filled in by the macro or is the example assuming that
'connection' has already been defined elsewhere?
Can clojure do something like the 'yield' in the ruby
example above (which I think is one of ruby's strengths
coming from smalltalk)?
I know you might say something like: "go learn some lisp
and macros and you'll know what the author is trying to
say", but I was hoping for just a quick indication on what is
going on here - I'll continue learning clojure in the
background anyway so I guess it'll eventually twig.
Cheers,
Daniel Bush
[1] I know the example is just there to whet the appetite
and is not part of a particular lesson or chapter.
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