Clojure: understanding dynamic vars and laziness

I spent a frustrating day figuring out why something wasn’t working as expected in a particular situation, even thought it was working just fine elsewhere. It turned out the problem was my flawed understanding of how Clojure’s dynamic vars behaved with respect to laziness. Thanks to Chouser and Chousuke on the #clojure channel in IRC, I think I have a slightly better grasp of the thing.

Update: Here’s a summary for those who don’t want to read the whole thing. What was happening is this – my assumption was that dynamic vars get captured in lazy objects – just like locals get captured in closures. They don’t – so if you later rebind the vars, when the lazy objects are realized, the vars will evaluate according to the latest binding. To baaad results 🙂

Here’s the chat transcript if you want to read the whole thing – it might help:

[10:24am] erohtar: chouser: u there?

[10:24am] Chouser: yep

[10:24am] erohtar: chouser: so i was thinking about our conversation yesterday about vars

[10:24am] erohtar: chouser: so vars in clojure arent the same as dynamically scoped variables in common lisp?

[10:25am] Chouser: I don’t know CL well enough to say for sure, but my understanding is that they’re pretty much equivalent.

[10:26am] Chouser: a possible difference being how threads interact with them — not sure what CL does there.

[10:26am] erohtar: chouser: the thing is… in common lisp, if i bing var-a to x, any thing down the call stack will see x…

[10:27am] erohtar: chouser: in clojure, it doesnt seem to behave like a closure… (laziness, during realization) is that correct? if var-a was bound to y elsewhere, realization of the old lazy stuff would now see y

[10:28am] erohtar: chouser: is that right?

[10:28am] Chouser: CL doesn’t do lazy seqs, right? so lets ignore that for now

[10:29am] erohtar: chouser: ok…

[10:29am] Chouser: let’s just look at how a closure works.

[10:30am] erohtar: chouser: i think i understand how a closure works, but when a closure is going to be evaluated lazily… what happens to a var that has since been rebound?

[10:31am] Chouser: (def var-a 2), (let [add (binding [var-a 5] (fn [n] (+ var-a n)))] (add 3))

[10:31am] Chouser: ok, here’s a little example that I would expect to work the same if translated to CL

[10:31am] Chouser: var-a has a root binding of 2.

[10:31am] erohtar: chouser: that should eval to 8, yes?

[10:31am] Chouser: no

[10:32am] erohtar: chouser: hmmm

[10:32am] Chouser: within the dynamic scope of the binding, a closure is created that refers to var-a

[10:32am] erohtar: chouser: ok… and that is 5?

[10:33am] Chousuke: nah, it’s var-a

[10:33am] erohtar: oh

[10:33am] Chouser: at the moment the closure is created, var-a has a thread-local binding of 5, but that doesn’t really matter because nothing is asking for var-a’s *value*, just grabbing var-a itself.

[10:33am] erohtar: i see… ok…

[10:34am] Chouser: so that closure is returned to outside the binding, and named ‘add’

[10:34am] Chousuke: I guess it’s a bit surprising though.

[10:34am] Chouser: and what does the closure have bundled inside? it knows it wants one arg n, and it has references to the vars var-a and +

[10:35am] erohtar: and it dynamically uses the value 2, for var-a when it needs it?

[10:35am] Chouser: right

[10:35am] erohtar: arent closures supposed to capture their environment?

[10:35am] Chouser: their lexical environment, yes.

[10:35am] erohtar: not the values?

[10:36am] Chouser: which is why this works: (def var-a 2), (let [m 7, add (binding [var-a 5] (fn [] (+ var-a m)))] (add))

[10:37am] Chouser: hm, not that’s not quite right, since m is still available when ‘add’ is called. But anyway, it wouldn’t have to be.

[10:38am] erohtar: thinking about this

[10:38am] Chouser: if you think about how dynamic vars are used in clojure.core, you’ll see it must be this way.

[10:38am] Chouser: let’s look at *in*

[10:38am] Chouser: er, *out*

[10:38am] Chousuke: ,(let [x 1 add (fn [] (+ 4 x))] [(binding [x 2] (add)) (binding [+ -] (add))])

[10:38am] clojurebot: java.lang.Exception: Unable to resolve var: x in this context

[10:38am] Chousuke: hmm

[10:39am] erohtar: ok – talking about *out* – is that the std-out ?

[10:39am] Chouser: somewhere there’s something like (def *out* System/out)

[10:39am] Chouser: right

[10:39am] Chousuke: ,(let [x 1] (let [add (fn [] (+ 4 x))] [(binding [x 2] (add)) (binding [+ -] (add))]))

[10:39am] erohtar: right

[10:39am] clojurebot: java.lang.Exception: Unable to resolve var: x in this context

[10:39am] Chouser: that’s the root binding of *out*

[10:39am] erohtar: ok, with u so far

[10:39am] Chousuke: hmm, I guess binding let-bound variables does not work

[10:40am] Chouser: then you’ve got functions like prn that use *out*

[10:40am] erohtar: yes… i think they need to be vars

[10:40am] erohtar: right

[10:40am] kotarak: let-bound locals are not Vars

[10:40am] Chousuke: hmm

[10:40am] Chousuke: ,(let [x 1] #’x)

[10:40am] clojurebot: java.lang.Exception: Unable to resolve var: x in this context

[10:41am] Chouser: we can pretend prn is defined something like (defn prn [& args] (.write *out* args))

[10:41am] erohtar: when u create a lazy something, that uses prn, what happens?

[10:41am] Chousuke: indeed.

[10:41am] Chouser: now, if prn took the value of the *out* when it was defined, there would be no way to get prn to write to any other stream — it would always use the root binding, since that’s what was in play when prn was defined.

[10:42am] Chouser: but that’s not how it works.

[10:42am] Chouser: prn has a reference to the Var *out*, not its value.

[10:42am] erohtar: so something like – (map #(prn %)
[1 2 3])

[10:42am] erohtar: chouser: i completely agree

[10:42am] Chouser: so instead, prn will use the dynamic value of *out* when you call it.

[10:42am] erohtar: chouser: hmmm

[10:42am] Chouser: ,(with-out-str (prn 5 10))

[10:42am] clojurebot: “5 10n”

[10:43am] Chouser: got it?

[10:43am] erohtar: i do

[10:43am] erohtar: im thinking about my confusion – and trying to figure out the question to ask next

[10:44am] erohtar: so lets take my example –

[10:44am] erohtar: (map #(prn %)
[1 2 3])

[10:45am] erohtar: now, somewhere else, i rebind *out* to str… and in there, if i use the above object, will it also use the new binding?

[10:45am] erohtar: it will…

[10:45am] erohtar: i think im getting it now

[10:46am] erohtar: ok – final thought –

[10:46am] Chouser: (def q (map #(prn %)
[1 2 3])), (with-out-str (doall q))

[10:47am] Chouser: so yes, even though you created the lazy seq and closure outside the with-out-str, since its not computed or realized until the doall which is within the with-out-str, all those prn’s will use the binding provided by with-out-str

[10:47am] erohtar: got it…

[10:48am] erohtar: so my final thought – what if instead of *out* we’re talking about a connection to a database… bad use of vars, right?

[10:48am] Chouser: now lazy seqs can be a bit tricky because they cache their results and can effectively contain multiple closures.

[10:49am] Chouser: hmm….

[10:49am] erohtar: if i read from a db a list of objects using a var that holds database config…. and then i rebind the config to write into another database

[10:49am] Chouser: if you’re going to open a connection to a database, do a batch of work in a single thread on it, and then close the connection — then that would be a fine use.

[10:50am] erohtar: since the initial read may be lazy… when it realizes… it will ‘read from the wrong place'”?

[10:50am] Chouser: right

[10:50am] Chouser: basically you want to be very careful about passing closures (and therefore lazy seqs) past ‘binding’ boundaries.

[10:50am] erohtar: chouser: on that thought – what i did was, i read from a db, and to write into the other, i created agents, binding the new db config in each

[10:51am] erohtar: chouser: it didnt work…

[10:51am] erohtar: chouser: ok… im beginning to understand …

[10:51am] erohtar: i need to rethink what im doing

[10:52am] erohtar: and avoid using vars for some of this stuff, since im doing a lot of agent related stuff

[10:52am] Chouser: yes, I’d recommend trying to have the behavior of as much of your code as possible depend only on the args passed directly in.

[10:53am] erohtar: chouser: this whole mess started when i wanted to avoid passing db-config around everywhere

[10:53am] Chouser: this is better for testing, reasoning about behavior, easier to work with threads, closures, laziness, etc.

[10:53am] Chouser: heh, yeah.

[10:53am] erohtar: yup – i hear you

[10:53am] Chouser: well, another thing that may be helpful — just guessing since I haven’t seen your code…

[10:53am] erohtar: thanks a ton for your time – ur incredibly helpful

[10:54am] erohtar: ok?

[10:54am] Chouser: would be to try to keep as much of your code purely functional as possible.

[10:54am] erohtar: yes – i understand

[10:54am] Chouser: so avoid writing a function that takes some args, does some computation, and then writes to a db. Instead, have a fn that takes args, does computation, returns result.

[10:55am] Chouser: then perhaps you don’t have to pass in db config at all — whoever calls this fn can to the db write itself.

[10:55am] erohtar: i think it mostly is… except for this stuff… where i ended up inadvertently depending on “global” db-config

[10:55am] erohtar: well, this is a persistence layer

[10:56am] erohtar: i take an object in, break it up etc., and put it into hbase

[10:56am] erohtar: the code is open-sourece… would u like to see?

[10:56am] Chouser: hm, sure. I may need to ramp up on hbase soon.

[10:56am] erohtar: http://github.com/amitrathore/capjure/blob/b939a7038ebb4aed0d068d6e86291cc881ecf72b/src/org/rathore/amit/capjure.clj

[10:57am] erohtar: its kind a messy – learning clojure and hbase while doing this…

[10:57am] Chouser:

[10:57am] Chousuke: erohtar: first thought: use some newlines.

[10:58am] danlarkin: Noooooooooooo -jure

[10:58am] erohtar: danlarkin: haha – u’ve told me that already

[10:58am] Chouser: danlarkin: hey, at least he’s got a name

[10:58am] Chousuke: erohtar: one newline after each def/defn would make it a lot nicer

[10:58am] danlarkin: erohtar, Chouser:

[10:58am] erohtar: ok – more newlines, check

[10:59am] Chousuke: well, the def group is fine I guess.

[10:59am] Chousuke: but (defn foo) (defn bar) without an empty line in the middle is a bit weird

[11:00am] erohtar: ok – those were sort of just quick utility functions… but i’ll put newlines

[11:00am] Chousuke: newlines never hurt anyone

[11:00am] Chouser: erohtar: this is what you get for posting code. unsolicited advice.

[11:01am] erohtar: haha

[11:01am] erohtar: its fine – maybe i’ll learn something too

[11:01am] Chouser: but while we’re at it — I much prefer to have the code arranged so that ‘declare’ is only needed in mutually-recusive situations.

[11:01am] Chouser: though others may disagree with me

[11:02am] erohtar: right – i struggled with that one – cause i dont like to have to order the functions in “reverse”

[11:02am] • Chouser nods

[11:02am] erohtar: see – capjure-insert is the only one that most people will call

[11:03am] Chousuke: erohtar: you could add more newlines and some kind of “headlines” for sections of code

[11:03am] erohtar: u bind the config stuff – *hbase-master* and *primary-keys-config* – and then u call capjure-insert with the params

[11:03am] Chousuke: so people can easily skip the helper function blocks

[11:03am] erohtar: most of the remaining functions are functional

[11:03am] erohtar: only the ones dealing with hbase have (obvious) side-effects

[11:04am] erohtar: and it works fine… and its in production too… ran into issues trying to use TWO hbase configs (to move data between them)

[11:04am] Chousuke: that’s a lot of functions for one namespace too, though

[11:05am] Chouser: I really don’t think more namespaces would be better.

[11:05am] Chousuke: if possible maybe it’d make sense to put the “side-effecting” functions (that deal with hbase) in a separate file or namespace?

[11:05am] erohtar: cause lazy lists of data from one hbase, seem to be getting messed up when i rebind the config to start writing into the other hbase

[11:06am] erohtar: do u know what i mean? (trying to get discussion back to lazy closures and bindings )

[11:06am] Chousuke: ah

[11:06am] Chousuke: right, I can see why that happens

[11:06am] Chousuke: the laziness gets evaluated in the context of the new hbase.

[11:06am] erohtar: well, im only starting to see it thanks to u guys…

[11:06am] erohtar: yea

[11:06am] erohtar: drove me crazy

[11:06am] erohtar: and i did all this only so i wouldnt need to pass the db config around all the functions

[11:06am] erohtar: like u said, there are a lot of them…

[11:07am] Chousuke: which means you will have to move away from using global vars for config and instead create something you explicitly pass around to the functions.

[11:07am] erohtar: so now what… ? pass the config around everywhere?

[11:08am] Chousuke: maybe only to a few core functions.

[11:08am] erohtar: yea

[11:08am] Chousuke: which then would bind the dynamic vars.

[11:09am] erohtar: well, if i pass it in, i dont need dynamic vars, rite

[11:09am] Chouser: or don’t use lazy seqs

[11:09am] Chouser: use doall or stuff them in a vector first

[11:09am] erohtar: hmmm

[11:09am] cp2 is now known as c|p.

[11:09am] erohtar: that might be one solution

[11:09am] Chousuke: right, if they are side-effecting that might be better.

[11:09am] Chousuke: though maybe not memory-efficient

[11:10am] erohtar: yea

[11:10am] erohtar: i think i will get rid of the dynamic vars

[11:10am] Chousuke: it’s a shame though

[11:10am] erohtar: what is the pattern for this kind of ‘static’ config being passed around?

[11:10am] Chousuke: they make the code much easier to write

[11:10am] Chousuke: I wonder if there’s a better solution

[11:11am] Chousuke: some way to “bind” the current config to when the lazy-seq is first created

[11:11am] erohtar: in typical OO, u can create an instance variable to hold the config – i wonder if in clojure u can create something like that… and since its immutable, it should be fine, rite?

[11:11am] Chouser: yes

[11:11am] Chousuke: maybe do (let [config *global*] (return-some-lazy-seq-qith config))?

[11:11am] erohtar: but there is no way to do that, rite?

[11:11am] Chousuke: would that fix the value?

[11:12am] erohtar: yea

[11:12am] Chouser: stuff all your config into a map, and pass that single thing into functions that need to use it

[11:12am] erohtar: i thought of doing something like that… but other functions couldnt access those values…

[11:12am] erohtar: i thought of simulating an ‘immutable object oriented system’ –

[11:12am] brianh joined the chat room.

[11:12am] erohtar: like how u do in javascript… but immutable

[11:13am] Chousuke: I did in clojurebot so that I pass a map of config stuff around with every function

[11:13am] erohtar: return a closure with the vars bound with a let… and then closures for all the other functions also

[11:13am] Chousuke: it’s a bit tedious though

[11:13am] erohtar: i dunno if im being clear

[11:13am] erohtar: very tedious

[11:14am] erohtar: macros can help

[11:14am] erohtar: essentially what id be doing is creating an immutable OO system

[11:14am] erohtar: something like that

[11:14am] erohtar: does that make sense?

[11:14am] Chousuke: erohtar: but your problem is only with lazy seqs right?

[11:14am] erohtar: lazy seqs and dynamic vars

[11:14am] Chousuke: erohtar: don’t you only need to fixate the config when returning a seq

[11:14am] erohtar: the interplay

[11:14am] erohtar: yes

[11:15am] Chouser: erohtar: clojure.zip does something like that

[11:15am] erohtar: chouser: like what?

[11:15am] Chouser: each node is a object with a several functions attached as metadata

[11:16am] erohtar: chouser: ah, i see

[11:16am] erohtar: got it

[11:16am] Chouser: most of the api fns are like (children my-node), where children calls fns from my-node’s metadata, passing in my-node

[11:16am] erohtar: i see –

[11:17am] erohtar: thats rather cool – so u can basically have my-node equivalent contain the config data

[11:17am] Chouser: sure

[11:18am] erohtar: alright folks, thanks a lot for this conversation – im going to experiment and see how to get this done with the least amount of work

[11:18am] Chouser: the only reason zip nodes have fns in metadata is so it can be polymorphic.

[11:18am] erohtar: yes- makes sense

[11:18am] erohtar: (then everyone can tell me how my code sucks )

[11:18am] Chouser: if you’re going to only have one kind of db config (seems likely) then you don’t need the metadata piece.

[11:19am] erohtar: yes – just need to take care of the fact that the config will be rebound… so just closures that capture the config, and the functions that use it

[11:19am] erohtar: something like that

P.S. erohtar is my handle on IRC… it’s the reverse of my last name. In case you were wondering.

More on Lisp syntax, and language extensions

Following my recent post on the topic, I thought of one more thing that the syntax of Lisp allows you to do. Being homoiconic, and the fact that code manipulation is so simple (it’s all lists), layering on “language extensions” becomes possible. For example, if Betty Programmer realizes that OO is a great way to design and write code but that Lisp by itself doesn’t provide an OO facility (there are no “class” constructs, no inheritance etc.) – she doesn’t need to despair.

She can write code to add an OOP system to the language. Yes, this means Lisp really blurs the distinction between the language designer and the programmer. In other words, while it’s fairly obvious that Lisp is very well suited to writing DSLs, it is also possible to fundamentally extend the language as well – like adding an OO system, or pattern-matching, or logic-programming (ala Prolog).

Now, obviously, I’m not proficient enough yet to do anything of this sort. But, as I said before, it is my intention to learn 🙂

Lisp. A language where being meta is something worth thinking about.

Lisp syntax, and when code is data

Like I said earlier, my friend Ravi introduced me to Lisp several years ago, but it has taken me many years to really want to learn it well enough. I’ll write about my reasons in another post. In any event, at the beginning of this year, I started to pick it up again, promising myself that I’d be serious. This time. So far so good.

I think I’ve started to grok one of the core ideas of Lisp. I had always read that the syntax of Lisp was one of its strengths. And I had always struggled with that idea, knowing it was important, yet was quite unable to really put my finger on it. I think I’m closer to it today.

If you had to create a programming language to write programs that wrote programs (as in, say DSLs) – what design choices would you make?

For one thing, you’d have to be able to generate and manipulate (walk parse-trees, compare and transform nodes etc.) code as though it were just another data-structure. Right? OK, so the code that was being generated would look like and behave like data.

You would then create an EVAL function that could run the generated code. Maybe your generated code would in turn produce generated code, so to keep things easy and simple, your language syntax would be the same as that of the generated language. In other words, you’d end up with a homoiconic programming language. Finally, you would bootstrap your language processor and arrive at your final metacircular evaluator.

To recap, this language would have syntax that looked and behaved like data and because of it could generate and manipulate that data, which itself could be code. What would this data structure look like? One obvious choice for this is a tree (because of parse-trees). If you think about it, XML is just like a tree. But it’s kludgey. What we want is something like XML but without all the cruft. For example –


<program>

<function name="add_to_stock">
<param name="counter" />
<call_function name="increment">
<argument value="counter"/>
</call_function>
</function>

<function name="remove_from_stock">
<param name="item"/>
<call_function  name="decrement_from_stock_file">
<argument value="item"/>
</call_function>
</funtion>
</program>

The syntax is truly disgusting, but useful – especially if you need to programmatically generate it. Let’s now try to make it easier for humans, too. I’m going to remove the ‘program’ tag, because all this stuff is code. I’m going to then change from XML tags to simple ‘(‘ and ‘)’ without the names – and make an assumption – the first word that appears is always a function call. Except for define – which I’ll use to denote a definition for a function. I’ll also lose the XML attribute names, assuming that words that follow the function name are always parameters (unless it’s a code block itself – which would get evaluated first). So, we’re left with –




(define (add_to_stock counter)
    (increment counter))

 (define (remove_from_stock item)
    (decrement_from_stock_file item))



Where does this leave us?

It’s the same exact XML syntax, but it’s just a bit modified and has a few rules thrown in. Importantly, it’s still as easy to generate as XML. It’s just a list of lists of words. As in, a unit of code in this format would always start and end with parenthesis, and they would enclose either a bunch of zero or more symbols, or other lists.

In fact, a language that was good at list processing and had an eval function would probably do a really good job with this stuff!

Beyond objects – I

… It’s in words that the magic is–Abracadabra, Open Sesame, and the rest–but the magic words in one story aren’t magical in the next. The real magic is to understand which words work, and when, and for what; the trick is to learn the trick.
… And those words are made from the letters of our alphabet: a couple-dozen squiggles we can draw with the pen. This is the key! And the treasure, too, if we can only get our hands on it! It’s as if–as if the key to the treasure is the treasure!

John Barth, Chimera

It’s a copy of a quotation that appears in The Structure and Interpretation of Programs. It speaks to the fact that if you know the name of something, as in a way to refer to it, then you have control over it. Like functions or closures for instance. If you can refer to it by name, you can tell it to someone else. Or pass it to another function. You can remember the name, so you can speak it when you’re ready. When the context is right. Like when all the variables, objects, and functions have aligned.

The idea of metalinguistic abstraction is a fundamental one. It’s a bottom-up way of thinking about your problem-space, and figuring out primitives and operations on those primitives. Rather than trying to build a system that satisfies a particular set of constraints in the given problem-space, the idea is to build a way to express more complex concepts using those very primitives and operations. That way, changes to the system can be made by changing higher level constructs – and going further down the layered stack of abstractions as needed, depending on how fundamental the changes are. This way, changes (or any new set of constraints) can be handled in a more clean and elegant fashion, and the entire system benefits automatically by a change made at a lower level.

If this sounds like building what these days is called a domain-specific language (DSL), then sure, but it is not a new way of doing things. It is however, possible to do a lot of this stuff easier in “enterprise-type” scenarios, because of a larger acceptance of more dynamic languages. Like Ruby, for example. It isn’t particularly easy to write software this way – it involves a change in the way most of us were taught programming. Everything is not imperative any more, code doesn’t have to be “written” before it runs, lines between data and code begin to blur. On that last point, what is a closure? Is it data or is it a procedure? It’s sometimes one, sometimes the other, and occasionally both, and indeed, sometimes it is neither. Here’s an example –


def create_coord(x, y)
Proc.new { |selector|
if selector == :x
x
elsif selector == :y
y
end
}
end

c = create_coord 10, 20
puts "X is " + c.call(:x).to_s
puts "Y is " + c.call(:y).to_s

So, the question here is, what is c? Is it data? Or is it code, that does something? Here, c is an object that represents a cartesian coordinate, but without any explicit data elements like variables or attributes… so, what is it?

Note – This is Ruby code, but a different language, (like Lua, for example, that has syntactic sugar for things like keys on a hash table) could make it unnecessary for the call(:symbol) syntax and instead, just calling x and y would translate it to a call on the object with the given method name.

The point, however, is that there are more ways to think about abstraction than just OO, and to paraphrase Paul Graham, some of them can transcend objects. The bottom-up approach itself can be implemented using plain objects and OO thinking, but there are other ways which can make for some powerful expressablilty. The code above approaches functional programming, and I’m interested in scaling that model to build large systems from pure or near-pure functional constructs. I’m nowhere close to being proficient in writing software this way, but it is my intention to learn.