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(defn make-adder [x]
(let [y x]
(fn [z] (+ y z))))
(def add2 (make-adder 2))
(add2 4)
-> 6
I am trying to figure out this let example in clojure. What is the y variable it never seems to be set to anything. I do not understand the let syntax.
877
Sign up. Clojure let is used to define new variables in a local scope. These local variables give names to values. In Clojure, they cannot be re-assigned, so we call them immutable.
def is a special form that associates a symbol (x) in the current namespace with a value (7). This linkage is called a var . In most actual Clojure code, vars should refer to either a constant value or a function, but it's common to define and re-define them for convenience when working at the REPL.
This is a perfect opportunity to enforce encapsulation to avoid drowning the client in board-implementation details. Clojure has closures, and closures are an excellent way to group functions (Crockford 2008) with their supporting data.
(let [y x]
<body>)
evaluates <body> in a lexical context where y is bound to the value of x.
See the Clojure documentation for the description of let syntax. The general form is:
(let [sym1 val1
sym2 val2
sym3 val3
... ]
<body>)
Each symN is bound to the corresponding valN.
143
This function:
(defn make-adder [x]
(let [y x]
(fn [z] (+ y z))))
Itself returns a first-class function (a function that can be returned, passed around and assigned to a name just like any other value). Inside the let, x is bound to y, and so the function above is equivalent to this function below:
(defn make-adder [x]
(fn [z] (+ x z)))
So that when make-adder is called with the value 2, it returns this first-class function:
(fn [z] (+ 2 z))
Remember, a first-class function can be assigned to a name, as in this line:
(def add2 (made-adder 2))
The line above is therefore equivalent to this code:
(def add2 (fn [z] (+ 2 z)))
And so is equivalent to this code also:
(defn add2 [z] (+ 2 z))
Which means that when add2 is called:
(add2 4)
The expression above evaluates to this:
(+ 2 4)
Which evaluates to 6.
11
Let expression is standard in many functional programming languages. What is the "let" keyword in functional languages like F# and OCaml for?
1
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