How to change a value of sub maps of a map? - clojure

Question

I am new to Clojure and functional programming and now I am stuck with a problem. I get such a data structure:
{
:service1 \a
:service2 \b
:service3 \c
:default \d
:alert-a {
:duration "00:00-23:59"
:if-alert true
:continuous-times 2
:time-interval [2 6 9 15 30 60]
:times -1
}
:alert-b {
:duration "09:00-23:00"
:if-alert true
:continuous-times 2
:time-interval [2 6 9 15 30 60]
:times -1
}
:alert-c {
:duration "00:00-23:59"
:if-alert true
:continuous-times 5
:time-interval [5]
:times 1
}
:alert-d {
:duration "00:00-23:59"
:if-alert true
:continuous-times 5
:time-interval [5 15 30 60]
:times -1
}
}
This is something read from a config file. I want to change all the :duration value to a DateTime object using clj-time. So I can get something like:
{
:service1 \a
:service2 \b
:service3 \c
:default \d
:alert-a {
:duration DateTime Object
:if-alert true
:continuous-times 2
:time-interval [2 6 9 15 30 60]
:times -1
}
:alert-b {
:duration DateTime Object
:if-alert true
:continuous-times 2
:time-interval [2 6 9 15 30 60]
:times -1
}
:alert-c {
:duration DateTime Object
:if-alert true
:continuous-times 5
:time-interval [5]
:times 1
}
:alert-d {
:duration DateTime Object
:if-alert true
:continuous-times 5
:time-interval [5 15 30 60]
:times -1
}
}
But the data structure is immutable. This is an easy problem in other languages but now I don't know how to do it after a whole afternoon.
So can anyone give me some suggestions? Am I using a bad data structure? Or this problem can be somehow solved in a functional way.

Answer 1

Although you are working with immutable datastructures, you can easily and efficiently return new datastructures that are based on the originals.
In this case, the simplest (if repetitive) solution would be:
(-> m
(update-in [:alert-a :duration] parse-duration)
(update-in [:alert-b :duration] parse-duration)
(update-in [:alert-c :duration] parse-duration)
(update-in [:alert-d :duration] parse-duration))
The important thing to realize here is that update-in does not mutate the datastructure it's working on. Instead it returns a new datastructure with the modifications applied.
The threading macro -> allows the new datastructure to be threaded through the update-in operations, so that the final returned value is the original datastructure with all of the updates applied.
The parse-duration function would probably look a bit like this:
(defn parse-duration
"Convert duration in HH:MM-HH:MM format"
[s]
(let [[t1 t2] (clojure.string/split s #"-"))
(Period. (clj-time.coerce/to-date-time t1)
(clj-time.coerce/to-date-time t2)))

Answer 2

In functional programming you don't modify collection, but instead create new collection with needed values substituted by new ones. Fortunately, Clojure comes with a bunch of useful functions for this. For your case update-in should work well. It takes a collection (e.g. map), sequence of nested keys and a function to apply to the most nested value defined by key sequence. For example:
> (def m {:a 1 :b 2 :c {:c1 1 :c2 2}})
#'sandbox5448/m
> m
{:a 1, :c {:c1 1, :c2 2}, :b 2}
> (update-in m [:c :c1] str)
{:a 1, :c {:c1 "1", :c2 2}, :b 2}
Note how value 1 from key sequence [:c :c1] was converted to "1".
So, converting :duration field of :alert-a to DateTime is as easy as writing:
> (update-in your-map [:alert-a :duration] string-to-date)
where string-to-date is you converter function.