Does Clojure/Script offer a way to build a destructured map out of the arguments plus filled-in defaults in case the keys weren't supplied in the call?
Consider this example (that doesn't quite do what the code implies by a quick glance). Does clojure provide a way to build the map prompt with these four keys and values either from the caller or the defaults. I hate to think I have to repeat these key names two more times to get what I am after.
(re-frame/reg-event-db
:open-prompt
(fn [db [_ {title :title
text :text
label-yes :label-yes
label-no :label-no
:or {title "Confirm"
text "Are you sure?"
label-yes "Ok"
label-no "Cancel"}
:as prompt}]]
(-> db
(update :state conj :modal-prompt)
(assoc :prompt prompt))))
After reviewing the official documentation page about destructuring, I don't think that Clojure proposes a more convient way of doing that.
But just by curiosity, I was wondering what is the code generated by destructuring, because I'm expecting it relies on macro stuff. Let consider this toy example:
(def my-map {:text "Some text"})
(let
[{title :title
:or {title "Confirm"}
:as prompt} my-map]
(str "I got " title " from " prompt))
;; => "I got Confirm from {:text \"Some text\"}"
(macroexpand '(let
[{title :title
:or {title "Confirm"}
:as prompt} my-map]
(str "I got " title " from " prompt)))
;; => (let*
;; [map__12555
;; my-map
;; map__12555
;; (if
;; (clojure.core/seq? map__12555)
;; (clojure.lang.PersistentHashMap/create
;; (clojure.core/seq map__12555))
;; map__12555)
;; prompt
;; map__12555
;; title
;; (clojure.core/get map__12555 :title "Confirm")]
;; (str "I got " title " from " prompt))
So as you can see, after a macro expansion, the :or mechanism which allows to specifies default value relies on clojure.core/get.
In this particular example, title is affected by (clojure.core/get map__12555 :title "Confirm") form. It's a way to avoid repeating the title variable, but does it worth it?
You can also check the source code of the destructuring macro to get full details about it, but personally I found it pretty difficult to handle ^^'.
it is doable, maybe not very practical though, but nice for self education:
let's begin with making up the function what would be special binding case.
let's say, we want to pass vectors of length 2 or 3, where vector of 2 will represent the simple binding map key-value pair like [:as abc] or [a :a], and the vector of size 3 would be k-v-default triple: [a :a "my default"]. The example of it's usage:
(bindings-preproc [['a 1 "asd"]
['b 2 "ddd"]
[:or {'x 10}]
[:as 'whole]])
resulting to
{a 1, b 2, :or {x 10, a "asd", b "ddd"}, :as whole}
this function could look like this:
(defn bindings-preproc [decls]
(let [defaults (into {} (keep (fn [decl]
(when (and (not (keyword? (first decl)))
(= 3 (count decl)))
(let [[nm _ default] decl]
[nm default])))
decls))
all-kvs (apply assoc {} (mapcat (partial take 2) decls))]
(update all-kvs :or merge defaults)))
(this one doesn't include error checks for the sake of illustrative simplicity)
The next thing is to employ it inside the binding macros. The idea to make bindings-preproc a macro fails, because binding forms are checked for validity before the inner macros are evaluated.
But still we have a feature, that would help, namely reader tags. They are used for example when you use #inst syntax. Since these reader tags are processed at read-time, before any macros are getting expanded, we can plug our preprocessor in.
(here i will use actual reference update, to demonstrate it from repl, but in real projects you would declare these tags in a special file)
user> (alter-var-root
#'default-data-readers
assoc 'my/reader #'user/bindings-preproc)
;;=> {uuid #'clojure.uuid/default-uuid-reader,
;; inst #'clojure.instant/read-instant-date,
;; my/reader #'user/bindings-preproc}
so, now we can try to make it work:
(defn f [#my/reader [[a :a 10]
[b :b 20]
[z :z]
[:keys [k1 k2 k3]]
[[c1 c2 & cs] :c]
[:or {z 101
k3 :wooo}]
[:as whole]]]
{:a a :b b :c1 c1 :c2 c2 :cs cs :z z :k1 k1 :k2 k2 :k3 k3 :whole whole})
user> (f {:a 1000 :c [:one]})
;;=> {:cs nil,
;; :c2 nil,
;; :z 101,
;; :c1 :one,
;; :k3 :wooo,
;; :b 20,
;; :whole {:a 1000, :c [:one]},
;; :k1 nil,
;; :k2 nil,
;; :a 1000}
user> (let [a 10
b 20
#my/reader [[x :x 1]
[y :y 2]
[z :z 100]] {:z 432}]
[a b x y z])
;;=> [10 20 1 2 432]
I like to make a map of all default values, then use into or similar to fuse the user-supplied values into the map of default values. For example:
(ns tst.demo.core
(:use tupelo.core tupelo.test) )
(def stuff-default {:a 1 :b 2})
(defn apply-defaults
[arg]
(let [stuff (glue stuff-default arg)] ; or use `into`. Last one wins, so put defaults first
(with-map-vals stuff [a b]
(newline)
(spyx a)
(spyx b))
stuff))
(dotest
(is= (apply-defaults {}) ; no inputs => all default values
{:a 1, :b 2})
(is= (apply-defaults {:a 100}) ; some inputs => partial defaults
{:a 100, :b 2})
(is= (apply-defaults {:a 100, :b 200}) ; all inputs => no defaults used
{:a 100, :b 200}))
Here glue is like into but with more error checking. We also use tupelo.core/with-map-vals to destruct the map, with less repetition than native Clojure destructuring (vals->map does the reverse).
The output is:
-------------------------------
Clojure 1.10.1 Java 14
-------------------------------
a => 1
b => 2
a => 100
b => 2
a => 100
b => 200
Ran 2 tests containing 3 assertions.
0 failures, 0 errors.
Related
Is it possible to both destructure and rename keys in one go?
Consider this:
(let [{:keys [response]} {:response 1}]
(println response))
However, if I want to instead refer to 1 as my-response, I have to do something like:
(let [{:keys [my-response]} (clojure.set/rename-keys {:response 1} {:response :my-response})]
(println my-response))
Obviously this does not work with defn destructuring.
Is there any way in Clojure to both destructure and rename keys?
Use destructuring without :keys:
(let [{my-response :response} {:response 1}]
(println my-response))
{:keys [response]} is syntactic sugar for {response :response}.
Here you go:
(let [{:keys [response]} {:response 1}
my-response response]
(println my-response))
For a better answer refer to https://stackoverflow.com/a/57592661/2757027.
This answer is much closer to the question, but technically not single step. but this doesn't involve any complicated de-structuring.
If you don't mind using a library, a more powerful destructuring tool is available from tupelo.core/destruct. Here is an example:
(ns tst.demo.core
(:use demo.core tupelo.core tupelo.test))
(dotest
(let [info {:a 777
:b [2 3 4]}
mania [{:a 11} {:b 22} {:c [7 8 9]}]]
(let [z ::dummy]
(destruct [info {:a z
:b [d e f]}
mania [{:a ?} BBB {:c clutter}]]
(is= z 777)
(is= [d e f] [2 3 4])
(is= a 11)
(is= BBB {:b 22})
(is= clutter [7 8 9])))))
So you can see that inside the destruct expression, the symbols z, d, e, f, BBB, and clutter are given the corresponding values from the input variables info and mania. The special symbol ? is interpreted to mean that the keyword :a creates a symbol a to receive the value 11.
I was wondering if there was a way to access the arguments value of a thread-first macro in Clojure while it is being executed on.
for example:
(def x {:a 1 :b 2})
(-> x
(assoc :a 20) ;; I want the value of x after this step
(assoc :b (:a x))) ;; {:a 20, :b 1}
It has come to my attention that this works:
(-> x
(assoc :a 20)
((fn [x] (assoc x :b (:a x))))) ;; {:a 20, :b 20}
But are there any other ways to do that?
You can use as->:
(let [x {:a 1 :b 2}]
(as-> x it
(assoc it :a 20)
(assoc it :b (:a it))))
In addition to akond's comment, note that using as-> can get quite confusing quickly. I recommend either extracting a top level function for these cases, or trying to use as-> in -> only:
(-> something
(process-something)
(as-> $ (do-something $ (very-complicated $)))
(finish-processing))
I have a question regarding two functions, one taking a complete map and the other specific keywords like so:
(def mapVal
{:test "n"
:anotherKey "n"})
(defn functionTest
[& {:keys [test anotherKey] :or {:test "d" :anotherKey "d"}}]
(println :test :anotherKey))
(defn mapFunc
[map]
(functionTest (get-in map [:test :anotherKey])))
The goal would be that all the keys in the parameter map will be passed correctly to the functionTest. Is there anyway this could work? I tried a few things but i just cant get all the keywords and values passed to the functionTest. What i dont want is just the values of the map, it should be passed to the other function with keyword and value.
You're pretty close. A few things should clear it up.
First, when you declare parameters with [& varname] that means varname will be a list containing all the extra parameters. So you don't need to use that '&' here to destructure the input. Instead, you just name which keys you want to have become variables.
Try this:
(defn functionTest
[{:keys [test anotherKey]}]
(println test anotherKey))
And the other problem is using get-in. With get-in you're defining a "path" through nested data structures with that vector. For example, given:
{:first {:a 1 :b 2} :second {:c 3 :d 4}}
You could use get-in to get the value at :second :c with this:
(get-in {:first {:a 1 :b 2} :second {:c 3 :d 4}} [:second :c])
In your case, you don't need to use get-in at all. You just need to pass the entire map. The destructuring you defined in functionTest will handle the rest. Here is what I did that worked:
(defn mapFunc
[map]
(functionTest map))
I would also suggest you not name the that variable 'map' since it conflicts with the map function.
get-in is for accessing nested associative data structures.
(def m {:a {:x 1}})
(get-in m [:a :x]) ;;=> 1
After you destructure a map those values are in scope and are accessed via symbols. Your example should look like this:
(def mapVal
{:test "n"
:anotherKey "n"})
(defn functionTest
[& {:keys [test anotherKey]
:or {:test "d" :anotherKey "d"}}]
(println test anotherKey))
(defn mapFunc
[m]
(apply functionTest (apply concat (select-keys m [:test :anotherKey]))))
(mapFunc mapVal) ;;=> prints: n n
You have to go through this because functionTest is accepting bare key value pairs as optional parameters (the ones to the right of the &), as in:
(functionTest :test "n"
:anotherKey "n" )
;;=> Also prints: n n
select-keys returns a map with only the specified keys:
(select-keys mapVal [:test])
;; => {:test "n"}
applying concat to a map returns a flat seq of keys and values:
(apply concat (select-keys mapVal [:test :anotherKey]))
;; => (:test "n" :anotherKey "n")
apply applies a function to a seq, as though the seq were its argument list:
(+ [1 2 3]) ;;=> Error.
(apply + [1 2 3]) ;; => 6
As a side note, conventionally in Clojure code, snake-case is preferred to camelCase for most names.
I am struggling on how to construct a macro that lets me pass patterns and results to core.match/match in the form of a vector. I would like to be able to do this:
(let [x {:a 1}
patterns [[{:a 2}] :high
[{:a 1}] :low]]
(my-match x patterns))
> :low
I have tried the below and several other approaches which do not work, unless I pass patterns as a literal.
(defmacro my-match [e ems]
`(m/match [~e] ~#ems))
(let [x {:a 1}
patterns [[{:a 2}] :high
[{:a 1}] :low]]
(my-match x patterns))
=> CompilerException java.lang.IllegalArgumentException: Don't know how to create ISeq from: clojure.lang.Symbol, compiling:(*cider-repl kontrakt*:106:10)
(let [x {:a 1}]
(my-match x [[{:a 2}] :high
[{:a 1}] :low]))
=> :low
Macros are expanded at compile time, so you cannot rely on runtime information (the value of a parameter) during expansion. The root problem is that you can't apply a macro in the same way you can apply a function.
In clojure, how to apply a macro to a list?
So you have to either resort to using eval:
(defmacro functionize [macro]
`(fn [& args#] (eval (cons '~macro args#))))
(defmacro my-match [e ems]
`(apply (functionize m/match) [~e] ~ems))
Or approach the problem in a different way (do runtime pattern matching instead of compile time pattern matching).
The simplest way to solve your problem is with a plain old map:
(ns clj.core
(:use tupelo.core))
(def x {:a 1} )
(def patterns { {:a 2} :high
{:a 1} :low } )
(spyx (get patterns x))
;=> (get patterns x) => :low
Since you have no "wildcard values", you don't need core.match at all. If you would like to match on wild-card values, please see the function wild-match? in the Tupelo library. Samples:
(wild-match? {:a :* :b 2}
{:a 1 :b 2}) ;=> true
(wild-match? [1 :* 3]
[1 2 3]
[1 9 3] )) ;=> true
(wild-match? {:a :* :b 2}
{:a [1 2 3] :b 2}) ;=> true
Destructuring a map looks reversed to me. Can anybody explain what is happening?
I expect that this is the right form of destructuring a map
;=> (let [{:a a :b b} {:a 1 :b 2}] [a b])
which returns Exception Unsupported binding form: :a clojure.core/destructure/pb--4541 (core.clj:4029). Clojure documentations say that below is the right way. But it looks that keys and values are reversed.
This should be the right way:
;=> (let [{a :a b :b} {:a 1 :b 2}] [a b])
[1 2]
What is happening when destructuring a map?
It is not really reversed, actually it makes sense. It says: bind to symbol 'a' to value that is associated with the keyword :a
Are you aware of this when your map uses keywords as keys?
(let [{:keys [a b]} {:a 1 :b 2}] [a b])
Much neater and elegant!
Other variants exist if your keys are symbols or strings.
Also, it makes it possible to distinguish between these to cases:
cljs.user=> (let [{foo :foo :as bar} {:foo 3 :as 4}] [foo bar])
[3 {:foo 3, :as 4}]
cljs.user=> (let [{foo :foo bar :as} {:foo 3 :as 4}] [foo bar])
[3 4]