When messing about with another little game idea, I found myself retyping (for the umpteenth time) the same little bit of value bounding code I use a lot:

var x = Math.max(0, Math.min(y, 1))
Which basically translates to:
"Set x to be the value of y as long as it's between 0 and 1. 
If not, set it to 
  0 if it's smaller or 
  1 if it's larger."

Of course, 0 and 1 don't need to be the boundaries, I'm just using them for convenience.

Instead of continuing with the game, I decided to take a little tangent and see if there was any way I could rewrite this so that the syntax was a bit more obvious. I'd like to be able to use syntax like:

x = [0 < y < 1]

to mean the same. Written exactly as above, JS will try and evaluate left-to-right, changing the comparisons to booleans. The statement would become

y = 0.5

x = [0 < y < 1]
x = [0 < 0.5 < 1]
x = [true < 1]
x = [false]


y = -0.5

x = [0 < y < 1]
x = [0 < -0.5 < 1]
x = [false < 1]
x = [true]

My first thought was to be clever about it, I wanted to try and figure out how to partially evaluate the expression and take the different outcomes to figure out the logic required. If '0 < y' was false, then y is less than zero therefore outside our bounds, the return value should then be 0. If the first part is true and the second is false then we know the value is higher than our bounds....etc and so on.

This proved to be a logical dead-end as there was no good way to partially evaluate the statements. Not without preparsing the JS, anyway. Which leads me onto the second attempt...

Preparsing the JS

The next attack on the problem was the idea of reading the JS as a string, shuffling it around quickly and silently (not like a ninja, more like a speedy librarian in slippers) and put it back where it was.

So I began to look at ways to recreate that. I remembered from many, many years ago (two, actually) Alex Sexton creating the genius abomination that is goto.js and how that used some kind of preparsing. A quick skim through the code later and I ended up on James Padolsey's site looking at parseScripts.js.

In the end, all I needed to do was include parseScripts (which is a generalised form of the code I ended up using for the whitehat SEO question from last month) and provide a new parser function and script type.

parseScripts(/:bound/, function(unparsed) {
 return unparsed.replace(/\[(\d)\s*<\s*(\w)\s*<\s*(\d)\]/g, "Math.max($1, Math.min($2, $3))");

I'm not saying parseScript isn't clever because it most definitely is but I am saying it's simple. There's not always a need to branch off into deep technical investigations of partial evaluation when a simple search-and-replace does the job better and faster.

For someone always going on about bringing simplicity and pragmatism into development, you'd think I'd have gotten there faster...

Bounded Range

The final bounded range syntax code is available here (hint: view the console). It's silly but it was a fun half-hour.


Do you know of any better way to do this? Is there a clever parsing trick we can use instead? Is there, indeed, any language which has this syntax?