LET extends VAR with let expressions:

let a = 5 in -(a, 3)

A let expression evaluates the bound expression first, then binds its resulting value to a name while evaluating the body:

let a = 5 in -(a, 3)
-> VNumber 2

The binding's scope is the body. This means an inner let can shadow an outer binding:

let a = 5 in -(let a = 3 in a, a)
-> VNumber -2

The inner body sees a = 3, while the second operand still sees the outer a = 5.

For a closer look, read LET: Introducing Local Bindings and Scope.

You'll need Nix with flakes enabled.

nix develop
elm repl

Then:

import LET.Interpreter as I

I.run "let a = 5 in -(a, 3)"
-- Ok (VNumber 2)

I.run "let a = 5 in -(let a = 3 in a, a)"
-- Ok (VNumber -2)