CODEWITHSUNDEEP
dictionaryclojuredestructor
Morteza R
Morteza R

Reputation: 2319

Destructing a map (using the let keyword)

As I understand it, the let keyword, locally binds variables to values (supporting some sort of pattern matching). It receives two arguments. First is a vector with the symbol we want to bind and the value we want bound. Then comes an expression that uses that value.

In this example, first the variable person is defined:

user=> (def person {:name "Jabba" :profession "Gangster"})
#'user/person

now suppose we want to destruct the map using the let function:

user=> (let [{name :name} person] (str "The person's name is " name))
"The person's name is Jabba"

Why is it that in [{name :name} person], :name should necessarily appear after the variable name? This actually wouldn't work:

user=> (let [{:name name} person] (str "The person's name is " name))
"The person's name is "

Why is the order like this? I thought that maps could be defined in either order:

user=> (def map1 {:a 1})
#'user/map1
user=> (def map2 {1 :a})
#'user/map2

Upvotes: 0

Views: 237

Answers (2)

Alfred Xiao
Alfred Xiao

Reputation: 1788

  1. in a map, ordering of entry values is VERY IMPORTANT; first guy is key, second is value.

  2. if you don't like to repeat yourself, you can use below syntax to destructure one or more entries in a map:

    (let [{:keys [name profession]} person] (str "The person's name is " name ", and job is " profession))

Upvotes: 1

sloth
sloth

Reputation: 101162

I thought that maps could be defined in either order:

user=> (def map1 {:a 1})
#'user/map1
user=> (def map2 {1 :a})
#'user/map2

No.

map1 has one element; with the key :a and the value 1.
map2 has one element; with the key 1 and the value :a.

It's not the same.

Upvotes: 1

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