Given I have this action to perform
(def structure (atom [{:id "an-id"} {:id "another-id"}]))
(def job {:type "some-job"})
(reset! structure (map #(if (= "an-id" (:id %)) (update-in % [:performed-jobs] (fnil conj []) job) %) #structure))
next structure:
[{:id "an-id" :performed-jobs [{:type "some-job"}]} {:id "another-id"}]
How can I use swap! to change a single occurrence in my structure instead of resetting it all?
Replace reset! by swap! by giving it a function that takes the old value of the atom and returns a new value to store in the atom.
Replace dereferencing of the atom with the function's argument, the old value.
(swap! structure
(fn [old]
(map #(if (= "an-id" (:id %))
(update-in % [:performed-jobs]
(fnil conj []) job) %)
old)))
Related
Let's say I have several vectors
(def coll-a [{:name "foo"} ...])
(def coll-b [{:name "foo"} ...])
(def coll-c [{:name "foo"} ...])
and that I would like to see if the names of the first elements are equal.
I could
(= (:name (first coll-a)) (:name (first coll-b)) (:name (first coll-c)))
but this quickly gets tiring and overly verbose as more functions are composed. (Maybe I want to compare the last letter of the first element's name?)
To directly express the essence of the computation it seems intuitive to
(apply = (map (comp :name first) [coll-a coll-b coll-c]))
but it leaves me wondering if there's a higher level abstraction for this sort of thing.
I often find myself comparing / otherwise operating on things which are to be computed via a single composition applied to multiple elements, but the map syntax looks a little off to me.
If I were to home brew some sort of operator, I would want syntax like
(-op- (= :name first) coll-a coll-b coll-c)
because the majority of the computation is expressed in (= :name first).
I'd like an abstraction to apply to both the operator & the functions applied to each argument. That is, it should be just as easy to sum as compare.
(def coll-a [{:name "foo" :age 43}])
(def coll-b [{:name "foo" :age 35}])
(def coll-c [{:name "foo" :age 28}])
(-op- (+ :age first) coll-a coll-b coll-c)
; => 106
(-op- (= :name first) coll-a coll-b coll-c)
; => true
Something like
(defmacro -op-
[[op & to-comp] & args]
(let [args' (map (fn [a] `((comp ~#to-comp) ~a)) args)]
`(~op ~#args')))
Is there an idiomatic way to do this in clojure, some standard library function I could be using?
Is there a name for this type of expression?
For your addition example, I often use transduce:
(transduce
(map (comp :age first))
+
[coll-a coll-b coll-c])
Your equality use case is trickier, but you could create a custom reducing function to maintain a similar pattern. Here's one such function:
(defn all? [f]
(let [prev (volatile! ::no-value)]
(fn
([] true)
([result] result)
([result item]
(if (or (= ::no-value #prev)
(f #prev item))
(do
(vreset! prev item)
true)
(reduced false))))))
Then use it as
(transduce
(map (comp :name first))
(all? =)
[coll-a coll-b coll-c])
The semantics are fairly similar to your -op- macro, while being both more idiomatic Clojure and more extensible. Other Clojure developers will immediately understand your usage of transduce. They may have to investigate the custom reducing function, but such functions are common enough in Clojure that readers can see how it fits an existing pattern. Also, it should be fairly transparent how to create new reducing functions for use cases where a simple map-and-apply wouldn't work. The transducing function can also be composed with other transformations such as filter and mapcat, for cases when you have a more complex initial data structure.
You may be looking for the every? function, but I would enhance clarity by breaking it down and naming the sub-elements:
(let [colls [coll-a coll-b coll-c]
first-name (fn [coll] (:name (first coll)))
names (map first-name colls)
tgt-name (first-name coll-a)
all-names-equal (every? #(= tgt-name %) names)]
all-names-equal => true
I would avoid the DSL, as there is no need and it makes it much harder for others to read (since they don't know the DSL). Keep it simple:
(let [colls [coll-a coll-b coll-c]
vals (map #(:age (first %)) colls)
result (apply + vals)]
result => 106
I don't think you need a macro, you just need to parameterize your op function and compare functions. To me, you are pretty close with your (apply = (map (comp :name first) [coll-a coll-b coll-c])) version.
Here is one way you could make it more generic:
(defn compare-in [op to-compare & args]
(apply op (map #(get-in % to-compare) args)))
(compare-in + [0 :age] coll-a coll-b coll-c)
(compare-in = [0 :name] coll-a coll-b coll-c)
;; compares last element of "foo"
(compare-in = [0 :name 2] coll-a coll-b coll-c)
I actually did not know you can use get on strings, but in the third case you can see we compare the last element of each foo.
This approach doesn't allow the to-compare arguments to be arbitrary functions, but it seems like your use case mainly deals with digging out what elements you want to compare, and then applying an arbitrary function to those values.
I'm not sure this approach is better than the transducer version supplied above (certainly not as efficient), but I think it provides a simpler alternative when that efficiency is not needed.
I would split this process into three stages:
transform items in collections into the data in collections you want to operate
on - (map :name coll);
Operate on transformed items in collections, returning collection of results - (map = transf-coll-a transf-coll-b transf-coll-c)
Finally, selecting which result in resulting collection to return - (first calculated-coll)
When playing with collections, I try to put more than one item into collection:
(def coll-a [{:name "foo" :age 43} {:name "bar" :age 45}])
(def coll-b [{:name "foo" :age 35} {:name "bar" :age 37}])
(def coll-c [{:name "foo" :age 28} {:name "bra" :age 30}])
For example, matching items by second char in :name and returning result for items in second place:
(let
[colls [coll-a coll-b coll-c]
transf-fn (comp #(nth % 1) :name)
op =
fetch second]
(fetch (apply map op (map #(map transf-fn %) colls))))
;; => false
In transducers world you can use sequence function which also works on multiple collections:
(let
[colls [coll-a coll-b coll-c]
transf-fn (comp (map :name) (map #(nth % 1)))
op =
fetch second]
(fetch (apply sequence (map op) (map #(sequence transf-fn %) colls))))
Calculate sum of ages (for all items at the same level):
(let
[colls [coll-a coll-b coll-c]
transf-fn (comp (map :age))
op +
fetch identity]
(fetch (apply sequence (map op) (map #(sequence transf-fn %) colls))))
;; => (106 112)
I am new to Clojure.
I have used to change value of identifier using swap! and reset!.
reset!
(def item (atom "Apple"))
user=> #item
Out Put ;;=> "Apple"
(reset! item "Grapes")
user=> #item
Out Put ;;=> "Grapes"
swap!
(def item (atom "Apple"))
user=> #item
Out Put ;;=> "Apple"
(swap! item (#(str %) "PineApple"))
Out Put ;;=> ClassCastException java.lang.String cannot be cast to clojure.lang.IFn
How can I change value of item by using swap!?
(swap! item (fn [old] "PineApple"))
or:
(swap! item (fn [_] "PineApple"))
But as you are discarding the input, reset! is better here:
(reset! item "PineApple")
As per swap! syntax (swap! atom f). swap should require function.
So I just tried to solve the issue in this way.
(swap! item (fn[s] "Banana"))
Output ;;=> Banana
I have an atom called app-state that holds a map. It looks like this:
{:skills [{:id 1 :text "hi"} {:id 2 :text "yeah"}]}
What is the idiomatic way to remove the element inside the vector with :id = 2 ? The result would look like:
{:skills [{:id 1 :text "hi"}]}
...
So far, I have this:
(defn new-list [id]
(remove #(= id (:id %)) (get #app-state :skills)))
swap! app-state assoc :skills (new-list 2)
It works, but I feel like this isn't quite right. I think it could be something like:
swap! app-state update-in [:skills] remove #(= id (:id %))
But this doesn't seem to work.
Any help is much appreciated!
Try this:
(defn new-list [app-state-map id]
(assoc app-state-map :skills (into [] (remove #(= id (:id %)) (:skills app-state-map)))))
(swap! app-state new-list 2)
swap! will pass the current value of the atom to the function you supply it. There's no need to dereference it yourself in the function.
See the docs on swap! for more details.
(swap! state update :skills (partial remove (comp #{2} :id)))
(def skills {:skills [{:id 1 :text "hi"} {:id 2 :text "yeah"}]})
(defn remove-skill [id]
(update skills :skills (fn [sks] (vec (remove #(= id (:id %)) sks)))))
You would then be able to call say (remove-skill 1) and see that only the other one (skill with :id of 2) is left.
I like your way better. And this would need to be adapted for use against an atom.
You can use filter to do this. Here is a function that takes an id and the map and let's you filter out anything that doesn't match your criteria. Of course, you can make the #() reader macro check for equality rather than inequality depending on your needs.
user=> (def foo {:skills [{:id 1 :text "hi"} {:id 2 :text "yeah"}]})
#'user/foo
user=> (defn bar [id sklz] (filter #(not= (:id %) id) (:skills sklz)))
#'user/bar
user=> (bar 1 foo)
({:id 2, :text "yeah"})
user=> (bar 2 foo)
({:id 1, :text "hi"})
Here is a code example from Clojure Programming
(defn character
[name & {:as opts}]
(ref (merge {:name name :itmes #{} :health 500}
opts)))
(def smaug (character "Smaug" :health 500 :strength 400 :items (set (range 50))))
(def bilbo (character "Bilbo" :health 100 :strength 100))
(def gandalf (character "Gandalf" :health 75 :mana 750))
(defn loot
[from to]
(dosync
(when-let [item (first (:items #from))]
(alter to update-in [:items] conj item)
(alter from update-in [:items] disj item))))
(wait-futures 1
(while (loot smaug bilbo))
(while (loot smaug gandalf)))
(map (comp count :items deref) [bilbo gandalf])
(filter (:items #bilbo) (:items #gandalf))
Everything works fine until the last line, which brings up an error:
ClassCastException clojure.lang.PersistentList cannot be cast to clojure.lang.IFn clojure.core/filter/fn--4264 (core.clj:2605)
The constructor function "character" typos the key for :items as :itmes, making the conj on alter get nil as the initial value. Conjing nil and a value gives you a list -- thus the ClassCastException.
Looks like you've got a typo in your definition of character - :itmes instead of :items.
This means that when you loot, you're calling
(conj nil 0), which turns the entry under :items into a list of (0). A list doesn't implement IFn, hence the error.
I am working on updating counters in a map ref in Clojure.
(defn increment-key [this key]
(dosync
(let [value (get #this key)]
(if (= value nil)
(alter this assoc key (ref 1))
(alter this assoc key (alter value inc))))))
However, it looks like the alter value inc statement is losing the reference:
(defn -main [& args]
(def my-map (ref {}))
(increment-key my-map "yellow")
(println my-map)
(increment-key my-map "yellow")
(println my-map))
Which prints:
$ lein run
#<Ref#65dcc2a3: {yellow #<Ref#3e0d1329: 1>}>
#<Ref#65dcc2a3: {yellow 2}>
How can I keep the same reference while updating it in this scenario?
You were almost there. Below is the solution, check the last line of increment-key, you just need to alter the value (not alter the key in the map as you were doing, coz that was causing the key to be updated with the alter return value which in you example was 2, remember alter returns the new value of the ref, not the ref itself). Also don't use def inside a def, you should use let (in your -main function)
(defn increment-key [this key]
(dosync
(let [value (get #this key)]
(if (= value nil)
(alter this assoc key (ref 1))
(alter value inc)))))
(defn -main [& args]
(let [my-map (ref {})]
(increment-key my-map "yellow")
(println my-map)
(increment-key my-map "yellow")
(println my-map)))