hash-map initialization issue

[Ryan]

Hello!

I am having a small issue with a hash-map initialization and I am failing to understand why. I have the following situation:

(def a-list '({:BAR_KEY bar-value}, {:BAR_KEY another-value}))

(defn my-function [foo-id a-keyword a-list] 

  (map #({:foo_id foo-id (keyword a-keyword) (:BAR_KEY %)}) a-list))

So, by running the above function like this:

(my-function 5 "my_keyword" a-list) 

I get the following error:

clojure.lang.ArityException: Wrong number of args (0) passed to: PersistentArrayMap

 

I am trying to get the following result:

({:foo_id 5 :my_keyword bar-value}, {:foo_id 5 :my_keyword another-value})

Any ideas? I have played around in repl for the last 2 hrs but I haven't found the proper way to do this.

Thank you for your time :)

[Jonathan Fischer Friberg]

It's because the #() syntax always calls the content as a function.

So #(...) is the same as (fn [] (...)). In your case,

#({:foo_id foo-id (keyword a-keyword) (:BAR_KEY %)})

is the same as:
(fn [%] ({:foo_id foo-id (keyword a-keyword) (:BAR_KEY %)}))
Note the extra () around {}. In other words, your map is called
as a function.

Maps can normally be called as functions, like this:
({:hello :world} :hello)
=> :world
That's why you get the "Wrong number of args" error
(and not a "a map is not a function" error).
Hope that makes sense.

Btw, hyphen is normally used instead of underscore
in both variables and keywords. Just a slight style
"issue", but maybe you had your reasons. :)

Jonathan

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[Ryan]

Thanks for your explanation Jonathan. I am still a bit confused however what is the proper solution here. Should i use an anonymous function instead to do what I want or can it be done with the #() syntax?

Hyphens is my preferred way as well, but, those keys represent sql columns which they use underscore so I gotta go with underscores in order code to match them :)

Ryan

[Jonathan Fischer Friberg]

It can still be done with the #(), with for example the hash-map function.
It's basically the same as the {} but as a function, like this:
(hash-map :a 3 :b 4)
=> {:a 3, :b 4}

So you should be able to write the function as:
#(hash-map :foo_id foo-id (keyword a-keyword) (:BAR_KEY %))

I think you should use the standard (fn []) syntax though, since it's shorter.

> Hyphens is my preferred way as well, but, those keys represent sql columns which they use underscore so I gotta go with underscores in order code to match them :)

I see. :)

Jonathan

[Ryan]

Thanks for all your help Jonathan :) I went with the standard fn syntax, its a two-liner anyway so not a big of deal :)

The important part here was that I learned that #() executes the content as a function, very helpful!

Ryan

[Jonathan Fischer Friberg]

No problem, glad to be of help. :)

Jonathan

[JvJ]

Here's a cheezy hack, use identity.

#(identity {:foo %})

[JvJ]

...even cheezier, use do
#(do {:foo %})

[Alex Baranosky]

There's also 'for'

(defn my-function [foo-id a-keyword a-list]  

  (for [m a-list]

    {:foo_id foo-id (keyword a-keyword) (:BAR_KEY m)})) 

[Ryan]

Thanks for your input guys

Ryan

[Gary Verhaegen]

And there's the threading macro : #(-> {:foo %})

[Ryan T.]

Thanks Gary, it didn't cross my mind that i can use the # reader with the threading macro :)

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[Ryan]

Thanks Gary, it didn't cross my mind that i can use the # reader with the threading macro :)

[Michał Marczyk]

On 28 March 2013 22:51, Ryan <areka...@gmail.com> wrote:
> Thanks for your explanation Jonathan. I am still a bit confused however what
> is the proper solution here. Should i use an anonymous function instead to
> do what I want or can it be done with the #() syntax?

Just wanted to add that the #() syntax is just shorthand notation
expanded by the reader into fn* forms, so what it produces is an
anonymous function, with no special magic. To see the exact expansion,
you can quote a #() form:

user=> '#(foo)
(fn* [] (foo))

fn* is the special form behind #() and fn (which is a macro);
basically an implementation detail.

Cheers,
Michał