for setting up objects as you say, I'm fine using (doto Object ... ...
...). Records, as everything else in Clojure promote immutability and
so setting fields doesn't really make sense unless you're consuming
external java classes in which case 'doto' suffices...at least that is
the way I see it...I originally thought you were asking about how to
create a record at runtime via a macro or a function, which is actually
quite tricky (and I can certainly help), but what you're asking sort of
exists doesn't it?
Jim
On 04/08/12 17:44, Warren Lynn wrote:
My vague impression is this has been discussed somewhere, but I did
not find a ticket in Jira when searching on "record, constructor", so
I want to have some discussion here before considering entering a
ticket myself.
Often, when I create a record, I need to do some set-up more than just
pass the values to the fields (like to set the field values derived
from some arguments, or add extra map keys when needed. Regular common
OO constructor thing). Or course, I can do this:
(defn make-myrecord [....] ... )
But I don't like the ad-hoc nature of that. Besides personal taste,
whenever a common use case pattern is not captured and defined in the
language, it just makes further abstraction and writing generic code
more difficult.
So what I have now is this:
(defmulti make-record
"Create a new record. The first argument should be the record type (like
MyRecord). Kind of like a constructor for record.
Example method definition:
(defmethod make-record MyRecord
[rectype arg1 arg2 ...] ...)"
(fn [& args] (first args)))
This works but has one downside: it involves unnecessary run-time
dispatch on the type. Although there are cases we may need dynamic
creation of a record with run-time record type, most of time the
record type is just static that can be decided at compile time (again,
just like "new MyClass(a, b, c) as in common OO languages).
So, as you may have guessed out from the title and the above, I think
if we can have a mechanism to define constructors in "defrecord", that
seems a nice feature to me. What's on my mind is something like this:
(defrecord MyRecord
[x y z]
(make-record "My constructor 1" [arg1 arg2] ...)
(make-record "My constructor 2" [arg1 arg2 arg3 ...] ...))
(make-record MyRecord arg1 arg2 arg3 ...) ;; this creates a new
record. no need of run-time dispatching
Just like the constructors in common OO languages, constructors will:
1. Clearly communicates the designer's intention how the record should
be created and avoid insistent internal field values. By the way, the
doc strings in all constructors will also help, as everything in in
one place.
2. Provides more regularity in the language, which can smooth/help
further abstractions (for example, certain macros).
I know mentioning OO may be a big turn-off for certain people, but I
don't see a clash here. Immutable objects are still objects and
certain boring but widely accepted practice from OO may still apply here.
I had a brief look at "defrecord2"
(https://github.com/david-mcneil/defrecord2), which certainly enhanced
things. But because it is not part of the language itself, it feels
patchy (I think even the author is not happy about that). Also, in
general I would avoid creating functions behind the scene based on
naming manipulations, especially for low level constructs.
Any comments? Thank you.
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