While hacking on some Ruby code today I started to miss
progn from my Common Lisp programming days. If you’re not familiar with
progn, it’s a special form which evaluates all of the contained expressions and returns the value of the last one.
progn construct is more important in the (more-or-less) functional1 programming world than in the imperative world because it allows us to insert multiple expressions where, syntactically, we could otherwise only insert one.
Let’s start with a trivial example:
(if (> x 0) x 0)
x is greater than zero, return
x, else return 0.
But what if we also wanted to output a message whenever
x is not greater than 0? We can’t just add our printing line in before (or after)
0 because the compiler knows what’s what based on the positions of the subexpressions in the
if expression: it has to be
(if test-form then-form [else-form]) 2.
(if (> x 0) x (progn (print "Returning zero!") 0))
Boom! It works. How very exciting. Ok, maybe not. In fact, I called
progn a “special form” earlier—which basically means that it’s something which defies the basic evaluation rule of the language—but the truth is it’s probably the least special of the special forms. In fact, implicit progns are all over the place in Lisp:
let forms, but most importantly, function bodies.