Why are constants declared with `let`?

Question

A few weeks ago, I went on Udemy course on Swift, the instructor mentioned that constants are called let in Swift because it's standard in OOP (JavaScript uses it for example).

I know this may seem like a really stupid question, but I think it is confusing that constants are declared by let. As, let doesn't stand for constant.

Where did the let keyword come from?

Answer 1

Using the keyword let to declare constants is not a general convention. The keyword let is used that way in Swift, Rust and Haskell, but in Javascript it denotes a regular variable (just one with a special scope).

As for where the use of let to declare variables come from, it's a very old tradition for mathematical proofs:

(Source, due to popular demand: https://www.maths.tcd.ie/~dwilkins/LaTeXPrimer/Theorems.html)

Answer 2

The use of let for a constant is simply a combination of two factors:

• The use of let for variables (as Michael Borgwardt explained)
• The belief that variables should be immutable (constants) unless specified otherwise.

In JavaScript, let declares a variable which is mutable (not a constant).
In Haskell, let declares a variable which is immutable. That's because all variables in Haskell are constant.
In Rust, let declares an immutable variable and let mut declares a mutable variable. This is because the designers of Rust wanted to make variables immutable by default - they wanted you to take extra time to think "do I really need mut here?" in the belief that this will reduce the number of bugs you create.

Answer 3

Well, if you want to go (koff, koff ...) much earlier than that, in the original Dartmouth BASIC language, "everything had to begin with a verb." So, originally, it had to be LET A=4. But it didn't take very long for people to realize that the grammar could just as easily be coded to make the keyword optional.

But – since that time – various subsequent language-designers have picked-up on that same word, and unfortunately they have used it in many and inconsistent ways. "So it goes," I guess.