Is it possible to convert the below code so it uses defprotocol and defrecord instead of defmulti and defmethod?
(defmulti test-multimethod (fn [keyword] keyword))
(defmethod test-multimethod :foo [a-map]
"foo-method was called")
(defmethod test-multimethod :bar [a-map]
"bar-method was called")
(defmulti perimeter (fn [shape] (:shape-name shape)))
(defmethod perimeter :circle [circle]
(* 2 Math/PI (:radius circle)))
(defmethod perimeter :rectangle [rectangle]
(+ (* 2 (:width rectangle)) (* 2 (:height rectangle))))
(def some-shapes [{:shape-name :circle :radius 4}
{:shape-name :rectangle :width 2 :height 2}])
(defmulti area (fn [shape] (:shape-name shape)))
(defmethod area :circle [circle]
(* Math/PI (:radius circle) (:radius circle)))
(defmethod area :rectangle [rectangle]
(* (:width rectangle) (:height rectangle)))
(defmethod perimeter :square [square]
(* 4 (:side square)))
(defmethod area :square [square]
(* (:side square) (:side square)))
(def more-shapes (conj some-shapes
{:shape-name :square :side 4}))
(for [shape more-shapes] (perimeter shape))
(for [shape more-shapes] (area shape))
Yes, you declare your functions in the protocol definition Shape, and then you define your implementations in your various record implementations Square, Circle, etc.
(defprotocol Shape
(area [this])
(perimeter [this]))
(defrecord Square [side] Shape
(area [this] (* (:side this) (:side this)))
(perimeter [this] (* 4 (:side this))))
(defrecord Rect [w l] Shape
(area [this] (* (:l this) (:w this)))
(perimeter [this] (+ (:l this) (:l this) (:w this) (:w this))))
(def s (->Square 4))
(def r (->Rect 2 5))
(map area [s r]) ; '(16 10)
(map :side [s r]) ; '(4 nil)
(map :l [s r]) ; '(nil 5)
Essentially this is just like OOP (but immutable) if you're familiar with that.
One nice thing about the defmulti implementation for things like this though, are you can often just serialize and deserialize your maps and use them as-is, without having to reify them into the specific record class.
Related
I saw a talk about railroad oriented programming (https://www.youtube.com/watch?v=fYo3LN9Vf_M), but i somehow do not get how to work this out, if i use reduce, because reduce has two or even three arguments.
How am i able to to put the following code like a railroad? I seems to me hard, because of reduce taking a function as an argument in addition to the game object.
(defn play-game-reduce []
(let [game-init
(->>
(io/initialize-cards-and-players)
(shuffle-and-share-cards myio/myshuffle)
(announce))
play-round
(reduce play-card (assoc-in game-init [:current-trick] '()) [:p1 :p2 :p3 :p4])]
(reduce play-round game-init (range (get game-init :round-count)))))
The whole code is here:
https://github.com/davidh38/doppelkopf/blob/master/src/mymain.clj
The code should more look like this:
(->> (io/initialize-cards-and-players)
(shuffle-and-share-cards myio/myshuffle)
(announce)
reduce (play-round .. )
reduce (play-card ...))
That would look to me much more explicit.
That video was made for a different language and you can't directly transfer these ideas to Clojure.
I looked at your source code and there are some things to improve:
(defn play-card-inp []
(eval (read-string (read-line))))
You shouldn't use eval in production code.
Read-string is unsafe and you should use clojure.edn/read-string instead. I'm not sure what is expected input here and what is the result of the evaluation, maybe you should use just clojure.edn/read here.
(defn myshuffle [cards]
(shuffle cards)
)
(defn initialize-cards-and-players []
; init cards
(def cards '([0 :c], [1 :c],[2 :c], [3 :c], [0 :s], [1 :s], [2 :s], [3 :s]))
(def players '(:p1 :p2 :p3 :p4))
;(def round-players (take 4 (drop (who-won_trick tricks) (cycle (keys players)))))
; mix and share cards
{:players (zipmap players (repeat {:cards () :tricks ()}))
:current-trick ()
:round-start-player :p1
:cards cards
:round-count (/ (count cards) (count players))
:mode ""
})
You should delete myshuffle and use directly shuffle where needed. Ending parenthesis shouldn't be on a separate line.
Don't use def (creates global variable) inside defn, use let (creates local variables). I would rewrite this as:
(defn new-deck []
(for [letter [:c :s]
number (range 4)]
[number letter]))
(defn new-game []
(let [cards (new-deck)
players [:p1 :p2 :p3 :p4]]
{:players (zipmap players (repeat {:cards () :tricks ()}))
:current-trick ()
:round-start-player :p1
:cards cards
:round-count (/ (count cards) (count players))
:mode ""}))
Notes for mymain.clj:
(defn who-won-trick [trick]
(eval (read-string (read-line))))
Some unused function, same problems as above.
(defn share-card-to-player [game players-cards]
(assoc game
:players
(assoc
(get game :players)
(first players-cards)
(assoc (get (game :players) (first players-cards))
:cards
(second players-cards)))))
Use assoc-in and some destructuring, something like this:
(defn share-card-to-player [game [player cards]]
(assoc-in game [:players player :cards] cards))
Your next function:
(defn shuffle-and-share-cards [myshuffle game]
(reduce share-card-to-player game
(map vector
(keys (get game :players))
(->> (get game :cards)
(myshuffle)
(partition (/ (count (get game :cards))
(count (get game :players))))))))
You can also destructure hash-maps, so I would rewrite this as:
(defn shuffle-and-share-cards [{:keys [players cards] :as game}]
(let [card-piles (->> cards
shuffle
(partition (/ (count cards)
(count players))))]
(reduce share-card-to-player game
(map vector
(keys players)
card-piles))))
Next functions:
(defn announce [game]
game)
(defn play-card [game curr-player]
(println curr-player)
(println game)
(let [played-card (io/play-card-inp)]
(->
(assoc-in game [:players curr-player :cards]
(remove #(= played-card %) (get-in game [:players curr-player :cards])))
(assoc-in [:current-trick]
(conj (game [:current-trick]) played-card)))))
announce is useless and update and update-in are better here:
(defn play-card [game curr-player]
(println curr-player)
(println game)
(let [played-card (io/play-card-inp)]
(-> game
(update-in [:players curr-player :cards] #(remove #{played-card} %))
(update :current-trick conj played-card))))
And finally, the last two functions:
(defn play-game-reduce []
(let [game-init
(->>
(io/initialize-cards-and-players)
(shuffle-and-share-cards myio/myshuffle)
(announce))
play-round
(reduce play-card (assoc-in game-init [:current-trick] '()) [:p1 :p2 :p3 :p4])]
(reduce play-round game-init (range (get game-init :round-count)))))
(defn play-game []
(let [game-init
(->>
(io/initialize-cards-and-players)
(shuffle-and-share-cards io/myshuffle)
(announce))]
(loop [round 1 game game-init]
(let [game-next (loop [curr-player 1 game-next game]
(if (> curr-player 4)
game-next
(recur (inc curr-player)
(play-card game-next (keyword (str "p" curr-player))))))]
(if (> round 2)
game-next
(recur (inc round) game-next))))))
loop/recur will be probably more readable, but two reduce should also work:
(defn play-game-reduce []
(let [game-init (-> (io/new-game)
shuffle-and-share-cards)]
(reduce (fn [game round]
(reduce play-card (assoc-in game [:current-trick] '()) [:p1 :p2 :p3 :p4]))
game-init
(range (get game-init :round-count)))))
(play-game-reduce)
Version with one reduce:
(defn play-game-reduce []
(let [game-init (-> (io/new-game)
shuffle-and-share-cards)
turns (for [round (range (:round-count game-init))
player [:p1 :p2 :p3 :p4]]
[round player])]
(reduce (fn [game [round player]]
(let [state (cond-> game
(= player (:round-start-player game)) (assoc-in [:current-trick] '()))]
(play-card state player)))
game-init
turns)))
And I also noticed that there's no validation of whether the current player can really play inserted card.
OK, I watched the talk (for the record, it gives a 5 minute overview of FP, then discusses error handling in pipelines in F#.
I didn't really care for the content of the video.
Clojure uses Exceptions for error handling, so a Clojure function always has only one output. Therefore the whole bind and map thing in the video doesn't apply.
I haven't looked at F# much before, but after watching that video I think it over-complicates things without much benefit.
I'm writing an agar.io clone. I've lately seen a lot of suggestions to limit use of records (like here), so I'm trying to do the whole project only using basic maps.*
I ended up creating constructors for different "types" of bacteria like
(defn new-bacterium [starting-position]
{:mass 0,
:position starting-position})
(defn new-directed-bacterium [starting-position starting-directions]
(-> (new-bacterium starting-position)
(assoc :direction starting-directions)))
The "directed bacterium" has a new entry added to it. The :direction entry will be used to remember what direction it was heading in.
Here's the problem: I want to have one function take-turn that accepts the bacterium and the current state of the world, and returns a vector of [x, y] indicating the offset from the current position to move the bacterium to. I want to have a single function that's called because I can think right now of at least three kinds of bacteria that I'll want to have, and would like to have the ability to add new types later that each define their own take-turn.
A Can-Take-Turn protocol is out the window since I'm just using plain maps.
A take-turn multimethod seemed like it would work at first, but then I realized that I'd have no dispatch values to use in my current setup that would be extensible. I could have :direction be the dispatch function, and then dispatch on nil to use the "directed bacterium"'s take-turn, or default to get the base aimless behavior, but that doesn't give me a way of even having a third "player bacterium" type.
The only solution I can think of it to require that all bacterium have a :type field, and to dispatch on it, like:
(defn new-bacterium [starting-position]
{:type :aimless
:mass 0,
:position starting-position})
(defn new-directed-bacterium [starting-position starting-directions]
(-> (new-bacterium starting-position)
(assoc :type :directed,
:direction starting-directions)))
(defmulti take-turn (fn [b _] (:type b)))
(defmethod take-turn :aimless [this world]
(println "Aimless turn!"))
(defmethod take-turn :directed [this world]
(println "Directed turn!"))
(take-turn (new-bacterium [0 0]) nil)
Aimless turn!
=> nil
(take-turn (new-directed-bacterium [0 0] nil) nil)
Directed turn!
=> nil
But now I'm back to basically dispatching on type, using a slower method than protocols. Is this a legitimate case to use records and protocols, or is there something about mutlimethods that I'm missing? I don't have a lot of practice with them.
* I also decided to try this because I was in the situation where I had a Bacterium record and wanted to create a new "directed" version of the record that had a single field direction added to it (inheritance basically). The original record implemented protocols though, and I didn't want to have to do something like nesting the original record in the new one, and routing all behavior to the nested instance. Every time I created a new type or changed a protocol, I would have to change all the routing, which was a lot of work.
You can use example-based multiple dispatch for this, as explained in this blog post. It is certainly not the most performant way to solve this problem, but arguably more flexible than multi-methods as it does not require you to declare a dispatch-method upfront. So it is open for extension to any data representation, even other things than maps. If you need performance, then multi-methods or protocols as you suggest, is probably the way to go.
First, you need to add a dependency on [bluebell/utils "1.5.0"] and require [bluebell.utils.ebmd :as ebmd]. Then you declare constructors for your data structures (copied from your question) and functions to test those data strucutres:
(defn new-bacterium [starting-position]
{:mass 0
:position starting-position})
(defn new-directed-bacterium [starting-position starting-directions]
(-> (new-bacterium starting-position)
(assoc :direction starting-directions)))
(defn bacterium? [x]
(and (map? x)
(contains? x :position)))
(defn directed-bacterium? [x]
(and (bacterium? x)
(contains? x :direction)))
Now we are going to register those datastructures as so called arg-specs so that we can use them for dispatch:
(ebmd/def-arg-spec ::bacterium {:pred bacterium?
:pos [(new-bacterium [9 8])]
:neg [3 4]})
(ebmd/def-arg-spec ::directed-bacterium {:pred directed-bacterium?
:pos [(new-directed-bacterium [9 8] [3 4])]
:neg [(new-bacterium [3 4])]})
For each arg-spec, we need to declare a few example values under the :pos key, and a few non-examples under the :neg key. Those values are used to resolve the fact that a directed-bacterium is more specific than just a bacterium in order for the dispatch to work properly.
Finally, we are going to define a polymorphic take-turn function. We first declare it, using declare-poly:
(ebmd/declare-poly take-turn)
And then, we can provide different implementations for specific arguments:
(ebmd/def-poly take-turn [::bacterium x
::ebmd/any-arg world]
:aimless)
(ebmd/def-poly take-turn [::directed-bacterium x
::ebmd/any-arg world]
:directed)
Here, the ::ebmd/any-arg is an arg-spec that matches any argument. The above approach is open to extension just like multi-methods, but does not require you to declare a :type field upfront and is thus more flexible. But, as I said, it is also going to be slower than both multimethods and protocols, so ultimately this is a trade-off.
Here is the full solution: https://github.com/jonasseglare/bluebell-utils/blob/archive/2018-11-16-002/test/bluebell/utils/ebmd/bacteria_test.clj
Dispatching a multimethod by a :type field is indeed polymorphic dispatch that could be done with a protocol, but using multimethods allows you to dispatch on different fields. You can add a second multimethod that dispatches on something other than :type, which might be tricky to accomplish with a protocol (or even multiple protocols).
Since a multimethod can dispatch on anything, you could use a set as the dispatch value. Here's an alternative approach. It's not fully extensible, since the keys to select are determined within the dispatch function, but it might give you an idea for a better solution:
(defmulti take-turn (fn [b _] (clojure.set/intersection #{:direction} (set (keys b)))))
(defmethod take-turn #{} [this world]
(println "Aimless turn!"))
(defmethod take-turn #{:direction} [this world]
(println "Directed turn!"))
Fast paths exist for a reason, but Clojure doesn't stop you from doing anything you want to do, per say, including ad hoc predicate dispatch. The world is definitely your oyster. Observe this super quick and dirty example below.
First, we'll start off with an atom to store all of our polymorphic functions:
(def polies (atom {}))
In usage, the internal structure of the polies would look something like this:
{foo ; <- function name
{:dispatch [[pred0 fn0 1 ()] ; <- if (pred0 args) do (fn0 args)
[pred1 fn1 1 ()]
[pred2 fn2 2 '&]]
:prefer {:this-pred #{:that-pred :other-pred}}}
bar
{:dispatch [[pred0 fn0 1 ()]
[pred1 fn1 3 ()]]
:prefer {:some-pred #{:any-pred}}}}
Now, let's make it so that we can prefer predicates (like prefer-method):
(defn- get-parent [pfn x] (->> (parents x) (filter pfn) first))
(defn- in-this-or-parent-prefs? [poly v1 v2 f1 f2]
(if-let [p (-> #polies (get-in [poly :prefer v1]))]
(or (contains? p v2) (get-parent f1 v2) (get-parent f2 v1))))
(defn- default-sort [v1 v2]
(if (= v1 :poly/default)
1
(if (= v2 :poly/default)
-1
0)))
(defn- pref [poly v1 v2]
(if (-> poly (in-this-or-parent-prefs? v1 v2 #(pref poly v1 %) #(pref poly % v2)))
-1
(default-sort v1 v2)))
(defn- sort-disp [poly]
(swap! polies update-in [poly :dispatch] #(->> % (sort-by first (partial pref poly)) vec)))
(defn prefer [poly v1 v2]
(swap! polies update-in [poly :prefer v1] #(-> % (or #{}) (conj v2)))
(sort-disp poly)
nil)
Now, let's create our dispatch lookup system:
(defn- get-disp [poly filter-fn]
(-> #polies (get-in [poly :dispatch]) (->> (filter filter-fn)) first))
(defn- pred->disp [poly pred]
(get-disp poly #(-> % first (= pred))))
(defn- pred->poly-fn [poly pred]
(-> poly (pred->disp pred) second))
(defn- check-args-length [disp args]
((if (= '& (-> disp (nth 3) first)) >= =) (count args) (nth disp 2)))
(defn- args-are? [disp args]
(or (isa? (vec args) (first disp)) (isa? (mapv class args) (first disp))))
(defn- check-dispatch-on-args [disp args]
(if (-> disp first vector?)
(-> disp (args-are? args))
(-> disp first (apply args))))
(defn- disp*args? [disp args]
(and (check-args-length disp args)
(check-dispatch-on-args disp args)))
(defn- args->poly-fn [poly args]
(-> poly (get-disp #(disp*args? % args)) second))
Next, let's prepare our define macro with some initialization and setup functions:
(defn- poly-impl [poly args]
(if-let [poly-fn (-> poly (args->poly-fn args))]
(-> poly-fn (apply args))
(if-let [default-poly-fn (-> poly (pred->poly-fn :poly/default))]
(-> default-poly-fn (apply args))
(throw (ex-info (str "No poly for " poly " with " args) {})))))
(defn- remove-disp [poly pred]
(when-let [disp (pred->disp poly pred)]
(swap! polies update-in [poly :dispatch] #(->> % (remove #{disp}) vec))))
(defn- til& [args]
(count (take-while (partial not= '&) args)))
(defn- add-disp [poly poly-fn pred params]
(swap! polies update-in [poly :dispatch]
#(-> % (or []) (conj [pred poly-fn (til& params) (filter #{'&} params)]))))
(defn- setup-poly [poly poly-fn pred params]
(remove-disp poly pred)
(add-disp poly poly-fn pred params)
(sort-disp poly))
With that, we can finally build our polies by rubbing some macro juice on there:
(defmacro defpoly [poly-name pred params body]
`(do (when-not (-> ~poly-name quote resolve bound?)
(defn ~poly-name [& args#] (poly-impl ~poly-name args#)))
(let [poly-fn# (fn ~(symbol (str poly-name "-poly")) ~params ~body)]
(setup-poly ~poly-name poly-fn# ~pred (quote ~params)))
~poly-name))
Now you can build arbitrary predicate dispatch:
;; use defpoly like defmethod, but without a defmulti declaration
;; unlike defmethods, all params are passed to defpoly's predicate function
(defpoly myinc number? [x] (inc x))
(myinc 1)
;#_=> 2
(myinc "1")
;#_=> Execution error (ExceptionInfo) at user$poly_impl/invokeStatic (REPL:6).
;No poly for user$eval187$myinc__188#5c8eee0f with ("1")
(defpoly myinc :poly/default [x] (inc x))
(myinc "1")
;#_=> Execution error (ClassCastException) at user$eval245$fn__246/invoke (REPL:1).
;java.lang.String cannot be cast to java.lang.Number
(defpoly myinc string? [x] (inc (read-string x)))
(myinc "1")
;#_=> 2
(defpoly myinc
#(and (number? %1) (number? %2) (->> %& (filter (complement number?)) empty?))
[x y & z]
(inc (apply + x y z)))
(myinc 1 2 3)
;#_=> 7
(myinc 1 2 3 "4")
;#_=> Execution error (ArityException) at user$poly_impl/invokeStatic (REPL:5).
;Wrong number of args (4) passed to: user/eval523/fn--524
; ^ took the :poly/default path
And when using your example, we can see:
(defn new-bacterium [starting-position]
{:mass 0,
:position starting-position})
(defn new-directed-bacterium [starting-position starting-directions]
(-> (new-bacterium starting-position)
(assoc :direction starting-directions)))
(defpoly take-turn (fn [b _] (-> b keys set (contains? :direction)))
[this world]
(println "Directed turn!"))
;; or, if you'd rather use spec
(defpoly take-turn (fn [b _] (->> b (s/valid? (s/keys :req-un [::direction])))
[this world]
(println "Directed turn!"))
(take-turn (new-directed-bacterium [0 0] nil) nil)
;#_=> Directed turn!
;nil
(defpoly take-turn :poly/default [this world]
(println "Aimless turn!"))
(take-turn (new-bacterium [0 0]) nil)
;#_=> Aimless turn!
;nil
(defpoly take-turn #(-> %& first :show) [this world]
(println :this this :world world))
(take-turn (assoc (new-bacterium [0 0]) :show true) nil)
;#_=> :this {:mass 0, :position [0 0], :show true} :world nil
;nil
Now, let's try using isa? relationships, a la defmulti:
(derive java.util.Map ::collection)
(derive java.util.Collection ::collection)
;; always wrap classes in a vector to dispatch off of isa? relationships
(defpoly foo [::collection] [c] :a-collection)
(defpoly foo [String] [s] :a-string)
(foo [])
;#_=> :a-collection
(foo "bob")
;#_=> :a-string
And of course we can use prefer to disambiguate relationships:
(derive ::rect ::shape)
(defpoly bar [::rect ::shape] [x y] :rect-shape)
(defpoly bar [::shape ::rect] [x y] :shape-rect)
(bar ::rect ::rect)
;#_=> :rect-shape
(prefer bar [::shape ::rect] [::rect ::shape])
(bar ::rect ::rect)
;#_=> :shape-rect
Again, the world's your oyster! There's nothing stopping you from extending the language in any direction you want.
I am trying to use 'extend' to define record methods with a map in Clojure. The following works in Clojure 1.4.0:
(defprotocol PointMethods
(add [self other])
(distance [self]))
(defrecord Point [x y]
PointMethods
(add [self other]
(Point. (+ (:x self) (:x other)) (+ (:y self) (:y other))))
(distance [self]
(Math/sqrt (+ (* (:x self) (:x self)) (* (:y self) (:y self))))))
(def p1 (Point. 2 3))
(def p2 (Point. 1 1))
(def p3 (add p1 p2))
(println p3)
(println (distance p3))
But this version fails:
(defprotocol PointMethods
(add [self other])
(distance [self]))
(defrecord Point [x y])
(extend Point
PointMethods
{:add
(fn [self other] (Point. (+ (:x self) (:x other)) (+ (:y self) (:y other))))
:distance
(fn [self] (Math/sqrt (+ (* (:x self) (:x self)) (* (:y self) (:y self)))))})
(def p1 (Point. 2 3))
(def p2 (Point. 1 1))
(def p3 (add p1 p2))
(println p3)
(println (distance p3))
Clojure Compiler: java.lang.IllegalArgumentException: No implementation of method: :add
of protocol: #'user/PointMethods found for class: user.Point, compiling:(records.clj:16)]
What is wrong with the second version?
As others have noted in their comments to your code, your code as written works fine in Clojure 1.4.0 (I just verified it myself). You are likely running into some sort of IDE misconfiguration, or perhaps there was some stale cached bytecode.
I have the following code, defining a type that has an atom in there.
(defprotocol IDeck
(vec-* [dk] "Output to a persistent vector")
(count-* [dk] "Number of elements in the deck")
(conj1-* [dk & es] "Adding multiple elements to the deck"))
(deftype ADeck [#^clojure.lang.Atom val]
IDeck
(vec-* [dk] (->> (.val dk) deref (map deref) vec))
(count-* [dk] (-> (.val dk) deref count))
(conj1-* [dk & es]
(try
(loop [esi es]
(let [e (first esi)]
(cond
(nil? e) dk
:else
(do
(swap! (.val dk) #(conj % (atom e)))
(recur (rest esi))))))
(catch Throwable t (println t)))))
(defn new-*adeck
([] (ADeck. (atom [])))
([v] (ADeck. (atom (vec (map atom v))))))
(defn conj2-* [dk & es]
(try
(loop [esi es]
(let [e (first esi)]
(cond
(nil? e) dk
:else
(do
(swap! (.val dk) #(conj % (atom e)))
(recur (rest esi))))))
(catch Throwable t (println t))))
;; Usage
(def a (new-*adeck [1 2 3 4]))
(count-* a)
;=> 4
(vec-* a)
;=> [1 2 3 4]
(conj1-* a 1 2) ;; The deftype case
;=> IllegalArgumentException java.lang.IllegalArgumentException: Don't know how to create ISeq from: java.lang.Long
(vec-* a)
;=> [1 2 3 4]
(conj2-* a 1 2) ;; The defn case
(vec-* a)
;=> [1 2 3 4 1 2]
Even though the two conj-* methods are exactly the same, except that one is in a deftype and the other is a normal defn, the first gives an error while the second succeeds. Why is this?
This is because protocols doesn't support variable number of arguments.
What you can do is make:
(conj1-* [dk & es] "Adding multiple elements to the deck"))
into
(conj1-* [dk es] "Adding multiple elements to the deck"))
such that the es param will be vector and called like:
(conj1-* a [1 2])
What's the problem with following code:
the func expression get expression that contains term that can contain expression...
(defn term[]
(def mytmp (zip/xml-zip {:tag :term}))
(cond
(= (first(:content(first vecTok))) "(")
(do
(def mytmp (popVecTo mytmp))
(def mytmp (zip/append-child mytmp (expression)))
(def mytmp (popVecTo mytmp)))
:else
(def mytmp (popVecTo mytmp)))
(zip/node mytmp))
(defn expression[]
(def mytmp (zip/xml-zip {:tag :expression}))
(def mytmp (zip/append-child mytmp (term)))
(while (contains? #{"+", "-", "*","/", "&", "|", "<", ">", "="} (first(:content(first vecTok))) )
(do
(def mytmp (popVecTo mytmp))
(def mytmp (zip/append-child mytmp (term)))))
(zip/node mytmp))
(def vecTok (vec (:content(first(xml-seq (parse "C:/Users/User/Desktop/forHekronot/BallT.xml"))))))
In the file :
<a><symbol>(</symbol><identifier>dy</identifier><symbol>-</symbol><identifier>dx</identifier><symbol>)</symbol></a>
Notwithstanding #jszakmeister's comment on how to better solve the problem, let me try to give and answer to the question:
you can first (def expression) and then (defn term [] ...) and finally (defn expression [] ...).
The classic example for indirect recursion is of course the poor man's odd/even function for positive numbers:
clojurec.core=> (def even)
#'clojurec.core/even
clojurec.core=> (defn odd [x] (and (not (= x 0)) (even (dec x))))
#'clojurec.core/odd
clojurec.core=> (defn even [x] (or (= x 0) (odd (dec x))))
#'clojurec.core/even
clojurec.core=> (even 10)
true
clojurec.core=> (odd 10)
false
clojurec.core=> (odd 10000)
StackOverflowError clojure.lang.Numbers.equal (Numbers.java:214)
Ooops, depending on the size (or rather depth) of your file this could be a problem. But not all is lost, we can redefine even to use trampoline internally:
(defn even [n]
(letfn [(evenrec [x] (or (= x 0) #(oddrec (dec x))))
(oddrec [x] (and (not (= x 0)) #(evenrec (dec x))))]
(trampoline evenrec n)))