There is macros. This macro is convenient because it allows you
to see the result of executing the Clojure script.
But often it gives out a result that does not suit me.
I use Babashka for running scripts.
(defn symbol-several
"returns a symbol with the concatenation of the str values of the args"
[& x]
(symbol (apply str x)))
(defmacro ! [& forms]
(cons
`symbol-several
(for [form forms]
`(str '~form " ;-> " ~form "\n"))))
,that outputs:
c:\clj>bb "~#.clj"
(do (def v [3 4]) (def l (quote (1 2))) (clojure.core/sequence (clojure.core/seq (clojure.core/concat (clojure.core/list 0) l v)))) ;-> (0 1 2 3 4)
The ~#.clj file contains the macro "!" and the code:
"(! (do (def v [3 4]) (def l '(1 2)) `(0 ~#l ~#v)))"
How to rewrite a macro so that it outputs the original code, i.e. something like this:
(do (def v [3 4]) (def l '(1 2)) `(0 ~#l ~#v))) ;-> (0 1 2 3 4)
Also, instead of the result (list), the macro outputs LazySeq:
c:\clj>bb map.clj
(apply map vector [[1 2] [3 4]]) ;-> clojure.lang.LazySeq#8041
I use Babashka (bb.exe) for running script.
The map.clj file contains the macro "!" and the code:
(apply map vector [[1 2] [3 4]])
Using pr-str helps printing Clojure data structures properly.
(defn symbol-several
"returns a symbol with the concatenation of the str values of the args"
[& x]
(symbol (apply str x)))
(defmacro ! [& forms]
`(symbol-several
~#(for [form forms]
`(str '~form " ;-> " (pr-str ~form) "\n"))))
(! (apply map vector [[1 2] [3 4]]))
;; => (apply map vector [[1 2] [3 4]]) ;-> ([1 3] [2 4])
Also see https://clojuredocs.org/clojure.core/pr-str
Related
I need to quote without namespace and combine it with unquoting. Something like:
'[a b ~c]
Unfortunately, unquoting works only with syntactic quoting:
`[a b ~c]
But then it expands to
[user/a user/b 7]
I would like to expand without namespaces.
What was suggested on clojurians slack channel is as follows:
Use a combination of "quote unquote" for symbols to get rid of namespaces:
`[~'a ~'b ~c]
and this works perfectly.
As a reference, I have been working on a similar capability that doesn't require defensive treatment like ~'a for each symbol you wish to remain unchanged. It isn't published yet, but here is the technique:
;-----------------------------------------------------------------------------
(defn unquote-form?
[arg]
(and (list? arg)
(= (quote unquote) (first arg))))
(defn unquote-splicing-form?
[arg]
(and (list? arg)
(= (quote unquote-splicing) (first arg))))
(defn quote-template-impl
[form]
(walk/prewalk
(fn [item]
(cond
(unquote-form? item) (eval (xsecond item))
(sequential? item) (let [unquoted-vec (apply glue
(forv [it item]
(if (unquote-splicing-form? it)
(eval (xsecond it))
[it])))
final-result (if (list? item)
(t/->list unquoted-vec)
unquoted-vec)]
final-result)
:else item))
form))
(defmacro quote-template
[form]
(quote-template-impl form))
and unit tests to show it in action:
;-----------------------------------------------------------------------------
(def vec234 [2 3 4])
(dotest
(is (td/unquote-form? (quote (unquote (+ 2 3)))))
(is (td/unquote-splicing-form? (quote (unquote-splicing (+ 2 3)))))
(is= (td/quote-template {:a 1 :b (unquote (+ 2 3))})
{:a 1, :b 5})
(is= (td/quote-template {:a 1 :b (unquote (vec (range 3)))})
{:a 1, :b [0 1 2]})
(is= (td/quote-template {:a 1 :b (unquote vec234)})
{:a 1, :b [2 3 4]})
(let [result (td/quote-template (list 1 2 (unquote (inc 2)) 4 5))]
(is (list? result))
(is= result (quote (1 2 3 4 5))))
(is= (td/quote-template [1 (unquote-splicing vec234) 5]) ; unqualified name OK here
[1 2 3 4 5])
(is= (td/quote-template [1 (unquote-splicing (t/thru 2 4)) 5])
[1 2 3 4 5])
(is= (td/quote-template [1 (unquote (t/thru 2 4)) 5])
[1 [2 3 4] 5])
)
So, instead of Clojure's syntax-quote (i.e. backquote), you can use quote-template. Then, use either unquote or unquote-splicing to insert values into the quoted template without having the namespace prepended to other symbols.
I know it is probably not the answer you are looking for by why not use code to construct that thing you want? Why force it to be all done in one expression? You could for example use
(conj '[a b] c)
I want to write a function that concatenates vectors or matrices, which can take arbitrary inputs. To combine two vectors I've written the follow code. It also also matrices to be combined such that columns are lengthened.
(defn concats
([x y] (vec (concat x y))))
Where I am stuck is extending the input to n vectors or matrices, and combining matrices to make longer rows.
Ex) (somefunction [[:a :b] [:c :d]] [[1 2] [3 4]] 2]
[[:a :b 1 2] [:c :d 3 4]]
The 2 in the input designates level to concatenate.
If you're not interested in "how it works", here's the solution right up front (note that level is zero-indexed, so what you've called the 1st level I'm calling the 0th level):
(defn into* [to & froms]
(reduce into to froms))
(defn deep-into*
[level & matrices]
(-> (partial partial mapv)
(iterate into*)
(nth level)
(apply matrices)))
The short answer for how it works is this: it iteratively builds up a function that will nest the call to into* at the correct level, and then applies it to the supplied matrices.
Regular old into, given a vector first argument, will concatenate the elements of the second argument onto the end of the vector. The into* function here is just the way I'm doing vector concatting on a variable number of vectors. It uses reduce to iteratively call into on some accumulated vector (which starts as to) and the successive vectors in the list froms. For example:
user> (into* [1 2] [3 4] [5 6])
> [1 2 3 4 5 6]
Now for deep-into*, I had to recognize a pattern. I started by hand-writing different expressions that would satisfy different "levels" of concatenation. For level 0, it's easy (I've extrapolated your example somewhat so that I can make it to level 2):
user> (into* [[[:a :b] [:c :d]]] [[[1 2] [3 4]]])
> [[[:a :b] [:c :d]] [[1 2] [3 4]]]
As for level 1, it's still pretty straightforward. I use mapv, which works just like map except that it returns a vector instead of a lazy sequence:
user> (mapv into* [[[:a :b] [:c :d]]] [[[1 2] [3 4]]])
> [[[:a :b] [:c :d] [1 2] [3 4]]]
Level 2 is a little more involved. This is where I start using partial. The partial function takes a function and a variable number of argument arguments (not a typo), and returns a new function that "assumes" the given arguments. If it helps, (partial f x) is the same as (fn [& args] (apply f x args)). It should be clearer from this example:
user> ((partial + 2) 5)
> 7
user> (map (partial + 2) [5 6 7]) ;why was six afraid of seven?
> (7 8 9)
So knowing that, and also knowing that I'll want to go one level deeper, it makes some sense that level 2 looks like this:
user> (mapv (partial mapv into*) [[[:a :b][:c :d]]] [[[1 2][3 4]]])
> [[[:a :b 1 2] [:c :d 3 4]]]
Here, it's mapping a function that's mapping into* down some collection. Which is kind of like saying: map the level 1 idea of (mapv into* ...) down the matrices. In order to generalize this to a function, you'd have to recognize the pattern here. I'm going to put them all next to each other:
(into* ...) ;level 0
(mapv into* ...) ;level 1
(mapv (partial mapv into*) ...) ;level 2
From here, I remembered that (partial f) is the same as f (think about it: you have a function and you're giving it no additional "assumed" arguments). And by extending that a little, (map f ...) is the same as ((partial map f) ...) So I'll re-write the above, slightly:
(into* ...) ;level 0
((partial mapv into*) ...) ;level 1
((partial mapv (partial mapv into*)) ...) ;level 2
Now an iterative pattern is becoming clearer. We're calling some function on ... (which is just our given matrices), and that function is an iterative build-up of calling (partial mapv ...) on into*, iterating for the number of levels. The (partial mapv ...) part can be functionalized as (partial partial mapv). This is a partial function that returns a partial function of mapving some supplied arguments. This outer partial isn't quite necessary because we know that the ... here will always be one thing. So we could just as easily write it as #(partial mapv %), but I so rarely get a chance to use (partial partial ...) and I think it looks pretty. As for the iteration, I use the pattern (nth (iterate f initial) n). Perhaps another example would make this pattern clear:
user> (nth (iterate inc 6) 5)
> 11
Without the (nth ...) part, it would loop forever, creating an infinite list of incrementing integers greater than or equal to 5. So now, the whole thing abstracted and calculated for level 2:
user> ((nth (iterate (partial partial mapv) into*) 2)
[[[:a :b][:c :d]]] [[[1 2][3 4]]])
> [[[:a :b 1 2] [:c :d 3 4]]]
Then, using the -> macro I can factor out some of these nested parantheses. This macro takes a list of expressions and recursively nests each into the second position of the successive one. It doesn't add any functionality, but can certainly make things more readable:
user> ((-> (partial partial mapv)
(iterate into*)
(nth 2))
[[[:a :b][:c :d]]] [[[1 2][3 4]]])
> [[[:a :b 1 2] [:c :d 3 4]]]
From here, generalizing to a function is pretty trivial--replace the 2 and the matrices with arguments. But because this takes a variable number of matrices, we will have to apply the iteratively-built function. The apply macro takes a function or macro, a variable number of arguments, and finally a collection. Essentially, it prepends the function or macro and the supplied arguments onto the final list, then evaluates the whole thing. For example:
user> (apply + [1 5 10]) ;same as (+ 1 5 10)
> 16
Happily, we can stick the needed apply at the end of the (-> ...). Here's my solution again, for the sake of symmetry:
(defn deep-into*
[level & matrices]
(-> (partial partial mapv)
(iterate into*)
(nth level)
(apply matrices)))
Using the concats function you listed in the question:
user=> (map concats [[:a :b] [:c :d]] [[1 2] [3 4]])
([:a :b 1 2] [:c :d 3 4])
this doesn't take into account the level as you listed, but it handles the input given
Taking arbitrary number of arguments needs a replacement concats function
(defn conc [a b & args]
(if (nil? (first args))
(concat a b)
(recur (concat a b) (first args) (rest args))))
Here are two examples
user=> (map conc [[:a :b] [:c :d]] [[1 2] [3 4]] [["w" "x"] ["y" "z"]])
((:a :b 1 2 "w" "x") (:c :d 3 4 "y" "z"))
user=> (map conc [[:a :b] [:c :d] [:e :f]] [[1 2] [3 4] [5 6]] [["u" "v"] ["w" "x"] ["y" "z"]])
((:a :b 1 2 "u" "v") (:c :d 3 4 "w" "x") (:e :f 5 6 "y" "z"))
Here are two different solutions for a function which will return a vector that's the concatenation of an arbitrary number of input collections:
(defn concats [& colls]
(reduce (fn [result coll]
(into result coll))
[]
colls))
(defn concats [& colls]
(vec (apply concat colls)))
The [& arg-name] notation in the argument lists is how you specify that the function is "variadic" - meaning it can accept a variable number of arguments. The result is that colls (or whatever name you pick) will be a sequence of all the arguments in excess of the positional arguments.
Functions can have multiple arities in Clojure, so you can also do things like this:
(defn concats
([x]
(vec x))
([x y]
(vec (concat x y)))
([x y & colls]
(vec (apply concat (list* x y colls)))))
However, only one of the overloads can be variadic, and its variadic part must come last (i.e. you can't do [& more n], only [n & more].
The Clojure.org page on special forms has more useful information on argument lists in Clojure (in the section on fn).
The function below correctly handles the example input/output you provided. Unfortunately I don't think I understand how you want the levels (and associated numeric input) to work well enough to generalize it as far as you're looking for.
(defn concats [x y]
;; only works with two inputs
(vec (map-indexed (fn [i v] (into v (nth y i)))
x)))
(concats [[:a :b] [:c :d]] [[1 2] [3 4]]) ;=> [[:a :b 1 2] [:c :d 3 4]]
But maybe it will give you some ideas anyway, or if you can add more information (especially examples of how different levels should work) I'll see if I can be more help.
So I have a list of functions and a list of data:
[fn1 fn2 fn3] [item1 item2 item3]
What can I do to apply each function to its corresponding data item:
[(fn1 item1) (fn2 item2) (fn3 item3)]
Example:
[str #(* 2 %) (partial inc)] [3 5 8]
=> ["3" 10 9]
You can use map
(map #(%1 %2) [str #(* 2 %) (partial inc)] [3 5 8])
("3" 10 9)
If you need a vector back, you can (apply vector ...)
(apply vector (map #(%1 %2) [str #(* 2 %) (partial inc)] [3 5 8]))
["3" 10 9]
Disclaimer: I don't know much Clojure, so there would probably be better ways to do this.
An alternative, not necessarily better:
user=> (for [[f x] (map vector [neg? pos? number?] [1 2 "foo"])]
#_=> (f x))
(false true false)
To make the map version suitable for varargs:
user=> (map (fn [f & args] (apply f args)) [+ - *] [1 2 3] [4 5 6] [7 8 9])
(12 -11 162)
I'm trying to write a function adjacents that returns a vector of a sequence's adjacent pairs. So (adjacents [1 2 3]) would return [[1 2] [2 3]].
(defn adjacents [s]
(loop [[a b :as remaining] s
acc []]
(if (empty? b)
acc
(recur (rest remaining) (conj acc (vector a b))))))
My current implementation works for sequences of strings but with integers or characters the REPL outputs this error:
IllegalArgumentException Don't know how to create ISeq from: java.lang.Long clojure.lang.RT.seqFrom (RT.java:494)
The problem here is in the first evaluation loop of (adjacents [1 2 3]), a is bound to 1 and b to 2. Then you ask if b is empty?. But empty? works on sequences and b is not a sequence, it is a Long, namely 2. The predicate you could use for this case here is nil?:
user=> (defn adjacents [s]
#_=> (loop [[a b :as remaining] s acc []]
#_=> (if (nil? b)
#_=> acc
#_=> (recur (rest remaining) (conj acc (vector a b))))))
#'user/adjacents
user=> (adjacents [1 2 3 4 5])
[[1 2] [2 3] [3 4] [4 5]]
But, as #amalloy points out, this may fail to give the desired result if you have legitimate nils in your data:
user=> (adjacents [1 2 nil 4 5])
[[1 2]]
See his comment for suggested implementation using lists.
Note that Clojure's partition can be used to do this work without the perils of defining your own:
user=> (partition 2 1 [1 2 3 4 5])
((1 2) (2 3) (3 4) (4 5))
user=> (partition 2 1 [1 2 nil 4 5])
((1 2) (2 nil) (nil 4) (4 5))
Here is my short answer. Everything becomes a vector, but it works for all sequences.
(defn adjacent-pairs [s]
{:pre [(sequential? s)]}
(map vector (butlast s) (rest s)))
Testing:
user=> (defn adjacent-pairs [s] (map vector (butlast s) (rest s)))
#'user/adjacent-pairs
user=> (adjacent-pairs '(1 2 3 4 5 6))
([1 2] [2 3] [3 4] [4 5] [5 6])
user=> (adjacent-pairs [1 2 3 4 5 6])
([1 2] [2 3] [3 4] [4 5] [5 6])
user=>
This answer is probably less efficient than the one using partition above, however.
The following code executes as expected but gives a NullPointerException at the end. What am I doing wrong here?
(ns my-first-macro)
(defmacro exec-all [& commands]
(map (fn [c] `(println "Code: " '~c "\t=>\tResult: " ~c)) commands))
(exec-all
(cons 2 [4 5 6])
({:k 3 :m 8} :k)
(conj [4 5 \d] \e \f))
; Output:
; Clojure 1.2.0-master-SNAPSHOT
; Code: (cons 2 [4 5 6]) => Result: (2 4 5 6)
; Code: ({:k 3, :m 8} :k) => Result: 3
; Code: (conj [4 5 d] e f) => Result: [4 5 d e f]
; java.lang.NullPointerException (MyFirstMacro.clj:0)
; 1:1 user=> #<Namespace my-first-macro>
; 1:2 my-first-macro=>
(For properly syntax highlighted code, go here.)
Take a look at the expansion that is happening:
(macroexpand '(exec-all (cons 2 [4 5 6])))
=>
((clojure.core/println "Code: " (quote (cons 2 [4 5 6])) "\t=>\tResult: " (cons 2 [4 5 6])))
As you can see, there is an extra pair of parentheses around your expansion, which means that Clojure tries to execute the result of the println function, which is nil.
To fix this I'd suggest modifying the macro to include a "do" at the front, e.g.
(defmacro exec-all [& commands]
(cons 'do (map (fn [c] `(println "Code: " '~c "\t=>\tResult: " ~c)) commands)))
Since the OP asked for other possible ways of writing this macro (see comments on the accepted answer), here goes:
(defmacro exec-all [& commands]
`(doseq [c# ~(vec (map (fn [c]
`(fn [] (println "Code: " '~c "=> Result: " ~c)))
commands))]
(c#)))
This expands to something like
(doseq [c [(fn []
(println "Code: " '(conj [2 3 4] 5)
"=> Result: " (conj [2 3 4] 5)))
(fn []
(println "Code: " '(+ 1 2)
"=> Result: " (+ 1 2)))]]
(c))
Note that the fn forms whose values will be bound to c are collected in a vector at macro-expansion time.
Needless to say, the original version is simpler, thus I think (do ...) is the perfect fix. :-)
Example interaction:
user=> (exec-all (conj [2 3 4] 5) (+ 1 2))
Code: (conj [2 3 4] 5) => Result: [2 3 4 5]
Code: (+ 1 2) => Result: 3
nil