Generally speaking, why does it exist? It exists for autoboxing as Google suggests. If you have code like this: Integer foo = 0; foo++; Java internally makes this into this helper expression: Integer foo = 0; let int foo_helper = foo.intValue() in foo_helper++; Source: http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6614974 That expression obviously has no syntax representation, it’s just an … Read more
let creates a lexically scoped immutable alias for some value. binding creates a dynamically scoped binding for some Var. Dynamic binding means that the code inside your binding form and any code which that code calls (even if not in the local lexical scope) will see the new binding. Given: user> (def ^:dynamic x 0) … Read more
This is a good example of where a simple ternary assignment could suffice: const classes = withBorder ? `${styles.circularBorder} ${styles.dimensions} ${styles.circularPadding} row flex-items-xs-middle flex-items-xs-center` : `${styles.dimensions} ${styles.circularPadding} row flex-items-xs-middle flex-items-xs-center` As specified in other comments/answers let and const are block scoped, so that’s why they don’t work in your example. For DRYer code, you can … Read more
With var you have a function scope, and only one shared binding for all of your loop iterations – i.e. the i in every setTimeout callback means the same variable that finally is equal to 6 after the loop iteration ends. With let you have a block scope and when used in the for loop … Read more
Short answer: Use let without in in the body of a do-block, and in the part after the | in a list comprehension. Anywhere else, use let … in …. The keyword let is used in three ways in Haskell. The first form is a let-expression. let variable = expression in expression This can be … Read more
TL;DR In theory, an unoptimized version of this loop: for (let i = 0; i < 500; ++i) { doSomethingWith(i); } might be slower than an unoptimized version of the same loop with var: for (var i = 0; i < 500; ++i) { doSomethingWith(i); } because a different i variable is created for each … Read more
1: The problem in the example f :: State s a f = State $ \x -> y where y = … x … is the parameter x. Things in the where clause can refer only to the parameters of the function f (there are none) and things in outer scopes. 2: To use a … Read more
let and const have two broad differences from var: They are block scoped. Accessing a var before it is declared has the result undefined; accessing a let or const before it is declared throws ReferenceError: console.log(aVar); // undefined console.log(aLet); // Causes ReferenceError: Cannot access ‘aLet’ before initialization var aVar = 1; let aLet = 2; … Read more
@thefourtheye is correct in saying that these variables cannot be accessed before they are declared. However, it’s a bit more complicated than that. Are variables declared with let or const not hoisted? What is really going on here? All declarations (var, let, const, function, function*, class) are “hoisted” in JavaScript. This means that if a … Read more
Let is a mathematical statement that was adopted by early programming languages like Scheme and Basic. Variables are considered low level entities not suitable for higher levels of abstraction, thus the desire of many language designers to introduce similar but more powerful concepts like in Clojure, F#, Scala, where let might mean a value, or … Read more