On Mar 8, 12:53 pm, Rich Hickey <richhic...@gmail.com> wrote:
> Recently it was said in a thread:
>
> "You can do a lot of that in Clojure, too, but, unless I'm mistaken,
> there are some arbitrary limits as things stand right now. ... you can
> have any kind of structure you want, as long as it's a map, set, or
> sequence."
I said that.
>From my point of view there are two different topics here that might
tend to get confused. One is a sort of academic musing about protocol
versus structure, how languages like Smalltalk and C++ conflate the
two, and how it is possible, in langauges like Haskell and, to a
lesser extent, Clojure, to completely separate them. On that spectrum,
I prefer the Haskell/Clojure end over the Smalltalk/C++ end, have have
preferred it for at least 15 years--since before there were clear
examples of language support for that approach.
The other topic is about the role of user-defined datatypes in Clojure
code. Maps and sequences are great and I use them all over the place.
Sometimes I want a convenient way to say that a datum will have these,
and *only* these, fields in it. I can do that in Clojure; the code to
write to do it for a particular case isn't very hard to write. On the
other hand, if two or three such cases arise in a project, then that
code becomes boilerplate. The boilerplate ends up getting factored out
into common functions or macros, and those functions or macros then
amount to an ad hoc typesystem. So, one way or another, Clojure
projects end up with user-defined types, whether Clojure provides the
mechanisms for defining them or the users do.
Clojure doesn't have an analog of defclass or of keyword arguments in
functions. I read and understood your reasons for not including those
features. I have no quibble with your decisions in those areas, as
long as Clojure doesn't actively prevent me from implementing things
like them.
I end up reinventing things with the flavor of defclass because I want
to explicitly spell out the layout of a set of structured values. I
want to be able to say, "exactly these fields, and no others", because
I'm describing values that some other code is going to be using, and I
want myself and other programmers to be able to look at the code later
and see those structures explicitly spelled out. When we have to
change some of them, I want code that depends on them to be able to
say, "hey! fix me!"
Similarly, I end up reinventing keyword arguments because I want to be
able to spell out explicitly in the source that a function will accept
any of a specified set of arguments and no others. I want to be able
to straightforwardly and definitely answer the question, "hey, what
were the allowed set of inputs to this function again?"
Clojure doesn't have to provide these facilities (though I wouldn't
mind if it did); it just needs to stay out of my way when I decide I
need to add them.
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