title: LET versus LET* in Common Lisp
body: <p>I understand the difference between LET and LET* (parallel versus sequential binding), and as a theoretical matter it makes perfect sense.  But is there any case where you've ever actually needed LET?  In all of my Lisp code that I've looked at recently, you could replace every LET with LET* with no change.</p>
  
  <p>Edit: OK, I understand <em>why</em> some guy invented LET*, presumably as a macro, way back when.  My question is, given that LET* exists, is there a reason for LET to stay around?  Have you written any actual Lisp code where a LET* would not work as well as a plain LET?</p>
  
  <p>I don't buy the efficiency argument.  First, recognizing cases where LET* can be compiled into something as efficient as LET just doesn't seem that hard.  Second, there are lots of things in the CL spec that simply don't seem like they were designed around efficiency at all.  (When's the last time you saw a LOOP with type declarations?  Those are so hard to figure out I've never seen them used.)  Before Dick Gabriel's benchmarks of the late 1980's, CL <em>was</em> downright slow.</p>
  
  <p>It looks like this is another case of backwards compatibility: wisely, nobody wanted to risk breaking something as fundamental as LET.  Which was my hunch, but it's comforting to hear that nobody has a stupidly-simple case I was missing where LET made a bunch of things ridiculously easier than LET*.</p>