I want to create a function that takes in a required argument x, and either a optional argument opt1 OR a keyword argument opt2.
Right now I have
(defn foo x & [opt1 {:keys [opt2]}]
...
But the above signature only lets me pass in keyword argument opt2 when both x and opt1 is present like
(foo 'x 'opt1 {:opt2 'opt2})
not like this
(foo 'x {:opt2 'opt2})
Please help me create a function that takes a required argument X and either opt1 or opt2, where opt2 is a keyword argument.
Thank you.
EDIT: I want to do the same for other macros as well. So I still need to use the defmacro.
The problem is ambiguity. Consider a function (fn foo [x y & args]) that takes two optional arguments and then any number of keyword arguments. If you then call it like (foo :bar :baz), how does your program handle it? x => :bar, y => :baz? Or x and y not provided, with a single :bar => :baz keyword argument?
Even in Common Lisp, which has arguably even more flexibility than Clojure in parsing function parameters, mixing optional and keyword arguments is not recommended, according to at least one popular book.
Your best bet is to change all of your arguments to positional arguments, or all of your parameters to keyword arguments. If you use keyword arguments, you can use hash-map destructuring to provide defaults for "optional" keyword parameters.
user> (defn foo [& {:keys [x y bar]
:or {x 1 y 2 bar 3}}]
(prn [x y bar]))
#'user/foo
user> (foo)
[1 2 3]
nil
user> (foo :bar :baz)
[1 2 :baz]
nil
you have to check if the aditional arguments are keyword arguments or not anyway (I assume your or is an exclusive or) so you can do it like this:
(defn foo [& args]
(if (= (count args) 1)
(let [[opt1] args] (println opt1))
(let [{:keys [opt2]} args] (println opt2))))
check the arguments if they are keyword arguments or not. As you only have one optional parameter it's easy: check if there's only one as keyword arguments require two.
Related
In JavaScript, one can do the following:
function foo(arg1, arg2, arg3) {
...
}
var others = [ 'two', 'three' ];
foo('one', ...others); // same as foo('one', 'two', 'three')
In Clojure, "variable args" can be accepted like so:
(defn foo [arg1 & others]
...)
But to pass them in combination with other args, you have to do this:
(apply foo (concat '("one") others))
Which is frankly really ugly. It's also impossible when what you need to do is recur:
(apply recur (concat '("one") others)) ;; doesn't work
Is there a better way to do this? And if not, is there any way at all to accomplish it in the recur case?
But to pass them in combination with other args, you have to do this:
(apply foo (concat '("one") others))
You don't have to do that: apply is also a variadic function that can take arguments before the final sequence argument e.g.
(apply foo "one" others)
You can pass any number of individual arguments before the final sequence argument to apply.
user=> (defn foo [arg1 & args] (apply println arg1 args))
#'user/foo
user=> (apply foo "one" 2 "three" [4 "five" 6.0])
one 2 three 4 five 6.0
To further demonstrate, these calls to + are functionally equivalent:
(apply + 1 2 [3])
(apply + 1 [2 3])
(apply + [1 2 3])
It's also impossible when what you need to do is recur
recur is a special form, and apply doesn't work with it like it would a typical Clojure function.
is there any way at all to accomplish it in the recur case?
Not with apply. You can recur with variadic args, but you can't (apply recur ...).
I was writing an answer for this challenge, when I needed to give a recursive function an optional parameter. I ended up with something kind of equivalent to:
(defn func [a & [b?]]
(if b?
b?
(recur a a)))
My intent was for b? to act as an optional parameter. If it wasn't supplied, it would be defaulted to nil via destructuring.
Instead of running though, it gave me an error:
(func 1)
UnsupportedOperationException nth not supported on this type: Long clojure.lang.RT.nthFrom (RT.java:947)
After some debugging, I realized that for some reason the rest parameter wasn't a list as I'd expect, but just the passed number! The error was coming about because it tried to destructure the number.
I can fix it by getting rid of the wrapper list in the parameter list:
(defn func [a & b]
...
But this just looks wrong. I know the rest parameter should be a list, but b is actually just a number. If I use "unoptimized" recursion, it works as I'd expect:
(defn func2 [a & [b?]]
(if b?
b?
(func2 a a)))
(func2 1)
=> 1
Can anyone explain what's going on here?
This appears to be a known difference
; Note that recur can be surprising when using variadic functions.
(defn foo [& args]
(let [[x & more] args]
(prn x)
(if more (recur more) nil)))
(defn bar [& args]
(let [[x & more] args]
(prn x)
(if more (bar more) nil)))
; The key thing to note here is that foo and bar are identical, except
; that foo uses recur and bar uses "normal" recursion. And yet...
user=> (foo :a :b :c)
:a
:b
:c
nil
user=> (bar :a :b :c)
:a
(:b :c)
nil
; The difference arises because recur does not gather variadic/rest args
; into a seq.
It's the last comment that describes the difference.
In a function definition:
(defn ^boolean =
;;other arities omitted...
([x y]
(if (nil? x)
(nil? y)
(or (identical? x y)
^boolean (-equiv x y))))
what does the ^boolean part in function definition mean? Does it only extend the metadata and signify the type of return, or does it have any deeper meaning? In other words, does it add any more value than simply making the code more self-described?
It is a type hint. See
https://www.safaribooksonline.com/library/view/clojure-programming/9781449310387/ch09s05.html
http://clojure-doc.org/articles/language/functions.html
or your favorite book. PLEASE NOTE: the compiler does not enforce that the actual type matches the type hint! Example w/o type hint:
(defn go []
"banana" )
(println (go))
;=> banana
(defn ^long go []
"banana" )
(println (go))
;=> Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Number,
I am doing the closure tutorial at http://clojurescriptkoans.com and I am stuck here: http://clojurescriptkoans.com/#functions/9
It looks like this
Higher-order functions take function arguments
(= 25 ( _ (fn [n] (* n n))))
I am supposed to fill in something at the underscore to make the expression true. I have no clue what to do.
The syntax simply consists of binding the function, and then calling it.
Since this is an exercise, I will show a similar situation rather than showing the exercise's solution:
user> ((fn [f] (f "abc")) (fn [s] (str s s s)))
"abcabcabc"
here I bind the argument of the first function to f, and call f with the argument "abc".
or you can use the short-hand notation:
#(%1 5)
Higher order functions takes functions as arguments.
Defining two functions
user=> (defn multiply [n] (* n n))
#'user/multiply
user=> (defn add [n] (+ n n))
#'user/add
Defining higher order function
user=> (defn highorderfn [fn number] (fn number))
#'user/highorderfn
Calling the higher order function
user=> (highorderfn multiply 5)
25
user=> (highorderfn add 5)
10
Say I have a function that needs two arguments and where the order of arguments affects the results.
Is it possible to pass the first argument into a partial or comp function and the other aside of it, like this:
(defn bar [arg1 arg2] (- arg1 arg2))
(def baz (partial (bar arg1)))
(def qux (comp str (bar arg1)))
(baz arg2)
(qux arg2)
If I want to pass arg2 into the function, could I do something like this?
(def baz2 (partial (bar _ arg2)))
(def qux2 (comp str (bar _ arg2)))
(baz arg1)
(qux arg1)
partial only "fills in" arguments on the left side, so if you need to skip arguments you must use fn:
(def baz2 #(bar %1 arg2))
Note also that comp requires that all its arguments be functions, so your qux and qux2 are actually nonsense. They should be:
(def qux (comp str baz))
(def qux2 (comp str baz2))
In general, Clojure core functions will place the variable most likely to change last to make composition with comp and partial more natural. (E.g., the collection argument is almost always last in Clojure, except for things like into where it makes sense to put it first.) When you design your own functions you should stick to this convention so you can compose your functions more easily.
Scheme SRFI 26 has a useful macro cut for slotting parameters like this.
Usage would be like so for your subtracting bar:
((cut bar <> 1) 2)
;=> 1
((comp str (cut - <> 1)) 2)
;=> "1"
Where the <> symbol represents the slot to be filled.
It is a fun exercise to implement a cut in Clojure yourself, but here is one port by #Baishampayan Ghose.
Here's what I got in a repl:
user=> (defn bar [arg1 arg2] (- arg1 arg2))
#'user/bar
user=> (def baz (partial bar 5))
#'user/baz
user=> (def qux (comp str baz))
#'user/qux
user=> (baz 2)
3
user=> (qux 2)
"3"
It's the closest I could get from your first example.
For the second example, maybe a simple defn is better than a partial def:
user=> (defn baz2 [x] (bar x 5))
#'user/baz2
user=> (def qux2 (comp str baz2))
#'user/qux2
user=> (baz2 2)
-3
user=> (qux2 2)
"-3"
I'd suggest you start a repl and try it yourself, it's the second best way to discover a language that I know of (TDD being the first).
Cheers!