In clojure, I would like to apply a function to all the elements of a sequence and return a map with the results where the keys are the elements of the sequence and the values are the elements of the mapped sequence.
I have written the following function function. But I am wondering why such a function is not part of clojure. Maybe it's not idiomatic?
(defn map-to-object[f lst]
(zipmap lst (map f lst)))
(map-to-object #(+ 2 %) [1 2 3]) => {1 3, 2 4, 3 5}
Your function is perfectly idiomatic.
For a fn to be part of core, I think it has to be useful to most people. What is part of the core language and what is not is quite debatable. Just think about the amount of StringUtils classes that you can find in Java.
My comments were going to get too long winded, so...
Nothing wrong with your code whatsoever.
You might also see (into {} (map (juxt identity f) coll))
One common reason for doing this is to cache the results of a function over some inputs.
There are other use-cases for what you have done, e.g. when a hash-map is specifically needed.
If and only if #3 happens to be your use case, then memoize does this for you.
If the function is f, and the resultant map is m then (f x) and (m x) have the same value in the domain. However, the values of (m x) have been precalculated, in other words, memoized.
Indeed memoize does exactly the same thing behind the scene, it just doesn't give direct access to the map. Here's a tiny modification to the source of memoize to see this.
(defn my-memoize
"Exactly the same as memoize but the cache memory atom must
be supplied as an argument."
[f mem]
(fn [& args]
(if-let [e (find #mem args)]
(val e)
(let [ret (apply f args)]
(swap! mem assoc args ret)
ret))))
Now, to demonstrate
(defn my-map-to-coll [f coll]
(let [m (atom {})
g (my-memoize f m)]
(doseq [x coll] (g x))
#m))
And, as in your example
(my-map-to-coll #(+ 2 %) [1 2 3])
;=> {(3) 5, (2) 4, (1) 3}
But note that the argument(s) are enclosed in a sequence as memoize handles multiple arity functions as well.
Related
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.
I have functions that behave different depending on which keyword arguments have values supplied. For this question, I am wondering about functions that behave slightly differently depending on the type of argument supplied.
Example function, that increments each element of a list:
(defn inc-list [& {:keys [list-str list]}]
(let [prepared-list (if-not (nil? list) list (clojure.string/split list-str #","))]
(map inc prepared-list)))
Does it make sense to make a multimethod that instead tests for the type of argument? I have not used multimethods before, not sure about right time to use them. If it is a good idea, would the below example make sense?
Example:
(defn inc-coll [col] (map inc col))
(defmulti inc-list class)
(defmethod inc-list ::collection [col] (inc-col col))
(defmethod inc-list String [list-str]
(inc-col
(map #(Integer/parseInt %)
(clojure.string/split list-str #",")))
First things first: (map 'inc x) treats each item in x as an associative collection, and looks up the value indexed by the key 'inc.
user> (map 'inc '[{inc 0} {inc 1} {inc 2}])
(0 1 2)
you probably want inc instead
user> (map inc [0 1 2])
(1 2 3)
Next, we have an attempt to inc a string, the args to string/split out of order, and some spelling errors.
If you define your multi to dispatch on class, then the methods should be parameterized by the Class, not a keyword placeholder. I changed the multi so it would work on anything Clojure knows how to treat as a seq. Also, as a bit of bikeshedding, it is better to use type, which offers some distinctions for differentiating inputs in Clojure code that class does not offer:
user> (type (with-meta {:a 0 :b 1} {:type "foo"}))
"foo"
Putting it all together:
user> (defn inc-coll [col] (map inc col))
#'user/inc-coll
user> (defmulti inc-list type)
nil
user> (defmethod inc-list String [list-str]
(inc-coll (map #(Integer/parseInt %) (clojure.string/split list-str #","))))
#<MultiFn clojure.lang.MultiFn#6507d1de>
user> (inc-list "1,10,11")
(2 11 12)
user> (defmethod inc-list clojure.lang.Seqable [col] (inc-coll (seq col)))
#<MultiFn clojure.lang.MultiFn#6507d1de>
user> (inc-list [1 2 3])
(2 3 4)
Your first example is an obfuscated application of a technique called dispatching on type. It is obfuscated because in a message-passing style the caller must convey the type to your function.
Since in every case you only use one of the keyword args, you could as well define it as:
(defn inc-list
[m l]
(->> (case m ;; message dispatch
:list l
:list-str (map #(edn/read-string %) (str/split #",")) l)
(map inc)))
The caller could be relieved from having to pass m:
(defn inc-list
[l]
(->> (cond (string? l) (map ...)
:else l)
(map inc)))
This technique has the main disadvantage that the operation procedure code must be modified when a new type is introduced to the codebase.
In Clojure it is generally superseeded by the polymorphism construct protocols, e. g.:
(defprotocol IncableList
(inc-list [this]))
Can be implemented on any type, e. g.
(extend-type clojure.lang.Seqable
IncableList
(inc-list [this] (map inc this)))
(extend-type String
IncableList
(inc-list [this] (map #(inc ...) this)))
Multimethods allow the same and provide additional flexibility over message-passing and dispatching on type by decoupling the dispatch mechanism from the operation procedures and providing the additivity of data-directed programming. They perform slower than protocols, though.
In your example the intention is to dispatch based on type, so you don't need multimethods and protocols are the appropriate technique.
What is the "simplest"/shortest way to ensure a var is a vector? Self-written it could look like
(defn ensure-vector [x]
(if (vector? x)
x
(vector x))
(ensure-vector {:foo "bar"})
;=> [{:foo "bar"}]
But I wonder if there is already a core function that does this? Many of them (seq, vec, vector, list) either fail on maps or always apply.
I also wonder what would be the best name for this function. box, singleton, unit, v, cast-vector, to-vector, ->vector, !vector, vector!, vec!?
I further wonder if other languages, like Haskell, have this function built-in.
I think the function you want to use when the value is a collection is vec which turns any collection into a vector. The vector function receives the items of the resulting vector as its arguments, so you could use it when the value is neither a vector or a collection.
This is a possible approach:
(defn as-vector [x]
(cond
(vector? x) x
(sequential? x) (vec x)
:else (vector x)))
(map as-vector [[1] #{2 3} 1 {:a 1}])
I chose the name for the function based on the ones from the Coercions protocol in clojure.java.io (as-file and as-url).
Clojure is awesome, we all know this, but that's not the point. I'm wondering what the idiomatic way of creating and managing higher-order functions in a Haskell-like way is. In Clojure I can do the following:
(defn sum [a b] (+ a b))
But (sum 1) doesn't return a function: it causes an error. Of course, you can do something like this:
(defn sum
([a] (partial + a))
([a b] (+ a b)))
In this case:
user=> (sum 1)
#<core$partial$fn__3678 clojure.core$partial$fn__3678#1acaf0ed>
user=> ((sum 1) 2)
3
But it doesn't seem like the right way to proceed. Any ideas?
I'm not talking about implementing the sum function, I'm talking at a higher level of abstraction. Are there any idiomatic patterns to follow? Some macro? Is the best way defining a macro or are there alternative solutions?
Someone has already implememented this on the Clojure group. You can specify how many args a function has, and it will curry itself for you until it gets that many.
The reason this doesn't happen by default in Clojure is that we prefer variadic functions to auto-curried functions, I suppose.
I've played a bit with the functions suggested by amalloy. I don't like the explicit specification of the number of argument to curry on. So I've created my custom macro. This is the old way to specific an high order function:
(defn-decorated old-sum
[(curry* 3)]
[a b c]
(+ a b c))
This is my new macro:
(defmacro defn-ho
[fn-name & defn-stuff]
(let [number-of-args (count (first defn-stuff))]
`(defn-decorated ~fn-name [(curry* ~number-of-args)] ~#defn-stuff)))
And this is the new implicit way:
(defn-ho new-sum [a b c] (+ a b c))
As you can see there is no trace of (curry) and other stuff, just define your currified function as before.
Guys, what do you think? Ideas? Suggestions?
Bye!
Alfedo
Edit: I've modified the macro according the amalloy issue about docstring. This is the updated version:
(defmacro defhigh
"Like the original defn-decorated, but the number of argument to curry on
is implicit."
[fn-name & defn-stuff]
(let [[fst snd] (take 2 defn-stuff)
num-of-args (if (string? fst) (count snd) (count fst))]
`(defn-decorated ~fn-name [(curry* ~num-of-args)] ~#defn-stuff)))
I don't like the if statement inside the second binding. Any ideas about making it more succint?
This will allow you to do what you want:
(defn curry
([f len] (curry f len []))
([f len applied]
(fn [& more]
(let [args (concat applied (if (= 0 (count more)) [nil] more))]
(if (< (count args) len)
(curry f len args)
(apply f args))))))
Here's how to use it:
(def add (curry + 2)) ; read: curry plus to 2 positions
((add 10) 1) ; => 11
The conditional with the [nil] is meant to ensure that every application ensures some forward progress to the curried state. There's a long explanation behind it but I have found it useful. If you don't like this bit, you could set args as:
[args (concat applied more)]
Unlike JavaScript we have no way of knowing the arity of the passed function and so you must specify the length you expect. This makes a lot of sense in Clojure[Script] where a function may have multiple arities.
Suppose that I have a vector of key-value pairs that I want to put into a map.
(def v [k1 v1 k2 v2])
I do this sort of thing:
(apply assoc (cons my-map v))
And in fact, I've found myself doing this pattern,
(apply some-function (cons some-value some-seq))
several times in the past couple days. Is this idiomatic, or is there a nicer way to move arguments form sequences into functions?
apply takes extra arguments between the function name and the last seq argument.
user> (doc apply)
-------------------------
clojure.core/apply
([f args* argseq])
Applies fn f to the argument list formed by prepending args to argseq.
That's what args* means. So you can do this:
user> (apply assoc {} :foo :bar [:baz :quux])
{:baz :quux, :foo :bar}
user> (apply conj [] :foo :bar [:baz :quux])
[:foo :bar :baz :quux]