《Scheme程序设计语言》中let f绑定的f是什么？为何ls自赋值？

Great question—this is a super common point of confusion when you’re first getting comfortable with Scheme’s more expressive binding syntax. Let’s break this down clearly.

What exactly is f here? #

This isn’t a standard let binding—it’s a named let, a handy Scheme syntactic sugar for defining an inline recursive function. Here’s the breakdown:

• f is the name we’re giving to the recursive function we’re defining inside the let. This lets us call the function recursively without having to define it separately with define or letrec.
• The ([ls ls]) part sets up the function’s parameter and its initial value:
  • The first ls is the parameter name that f will accept every time it’s called (including recursively).
  • The second ls is the initial value we pass to that parameter when we first invoke f—it’s the original input list passed to the product function.

To make this more explicit, the named let is equivalent to writing this with letrec (which is used for recursive bindings):

(letrec ((f (lambda (ls)
              (cond [(null? ls) 1]
                    [(= (car ls) 0) (break 0)]
                    [else (* (car ls) (f (cdr ls)))]))))
  (f ls))

Why does it look like ls is assigned to itself? #

It’s not an assignment of a variable to itself—this is just how named let’s syntax works for specifying initial arguments. Let’s rewrite the line with different names to eliminate the confusion:
If we renamed the function’s parameter to current-list, the line would become:

(let f ([current-list ls])
  ...)

Now it’s obvious: current-list is the parameter for our recursive function f, and we’re initializing it with the original input list ls when we first run f. Each recursive call will pass (cdr current-list) to f, so the parameter takes on the remaining part of the list every iteration.

How this fits into your product function #

When you run (product '(1 2 3 4 5)):

1. call/cc captures the current continuation as the break function—this lets us exit early if we hit a 0.
2. The named let defines f as our recursive list iterator:
   • If the list is empty, return 1 (the multiplicative identity, which ends the recursion).
   • If we hit a 0, we use break 0 to immediately jump out of the entire product function and return 0 right away.
   • Otherwise, multiply the current element by the result of calling f on the rest of the list.

内容的提问来源于stack exchange，提问作者durandaltheta