Simon Peyton Jones

Well, then, that's just a formula that is evaluated, and you expect to compute the values of a2 and a3 automatically before computing a1.

It wouldn't make any sense to compute a1 from the old values of a2 and a3.

And there's no notion of a program counter.

There's no notion of do this and then do that.

It wouldn't be sensible to say equals print x plus 3.

because who knows when or even whether that cell, that formula, will ever be evaluated.

Now, the surprise is that it is possible to do useful things in such a limited language.

When I learned programming, I learned it by writing machine code, in which the fundamental mode of computation is registers and program counters, and mutation of registers as you go along, and memory.

Computation and mutation seem to be...

inextricably interwoven.

That's how computation gets done.

So it was a surprise to me when I learned that, well, it is possible to do useful things with functional programs.

I still remember that amazing revelation.

Now, in fact, one way to look at it is this.

Right back in the late 1920s, early 1930s,

Alan Turing was doing a PhD in Princeton under the supervision of Alonzo Church.

Now, Alan Turing, as we all know, invented the Turing machine, which is fundamentally about mutation.

It has a little tape, and you can read things from the tape, and then you write back new things onto the tape, and it has an immutable program.

Now, Alonzo Church, at the very same time, was working on something he called the lambda calculus, in which there's no motion of mutation.

The lambda calculus is like the essence of functional programming.