Question

As probably all experienced elispers have found at some point, code like is broken:

(let ((a 3)
      (b 4)
      (c (+ a b)))
  c)

One should use the let* form instead when referring to a just-binded variable within the binding clauses.

I just wonder – why is a seemingly wrong behavior the default? Are there any risks on choosing always let* regardless of how is one gonna use it?

Answer 1

I was taught that the reason is mainly historical, but it might still hold: Since let does parallel assigment, having no data dependencies between the variables might give the compiler flexibility compile for speed or space and compile to something much faster than let*. I don’t know how much the elisp compiler uses this.

A bit of googling reveals a similar question for Common Lisp: LET versus LET* in Common Lisp