let Prevents Confusion

The let special form prevents confusion. let creates a name for a local variable that overshadows any use of the same name outside the let expression (in computer science jargon, we call this binding the variable). This is like understanding that in your host’s home, whenever he refers to “the house”, he means his house, not yours. (The symbols used to name function arguments are bound as local variables in exactly the same way. See The defun Macro.)

Another way to think about let is that it defines a special region in your code: within the body of the let expression, the variables you’ve named have their own local meaning. Outside of the let body, they have other meanings (or they may not be defined at all). This means that inside the let body, calling setq for a variable named by the let expression will set the value of the local variable of that name. However, outside of the let body (such as when calling a function that was defined elsewhere), calling setq for a variable named by the let expression will not affect that local variable.9

let can create more than one variable at once. Also, let gives each variable it creates an initial value, either a value specified by you, or nil. (In the jargon, this is binding the variable to the value.) After let has created and bound the variables, it executes the code in the body of the let, and returns the value of the last expression in the body, as the value of the whole let expression. (“Execute” is a jargon term that means to evaluate a list; it comes from the use of the word meaning “to give practical effect to” (Oxford English Dictionary). Since you evaluate an expression to perform an action, “execute” has evolved as a synonym to “evaluate”.)



This describes the behavior of let when using a style called “lexical binding” (see How let Binds Variables).