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Comment by thsksbd | Hacker News Reader

Very cool, but I wonder why did he define foot as:

(setq foot (* 0.3048 m))

And not

(setq foot (* 12 inch))

It comes to the same thing, but inch is defined in metric as 2.54 cm and the foot is a derived unit of the inch. But this way it clearly spells out the dependency.

Im not criticizing, it was his library for his use, I'm just wondering if there is there a deeper meaning beyond "God enough"?

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He wanted to work in meters? Closer to his final result?

  (setq km (* m 1000.0))
  (setq cm (* m 0.01))
  (setq foot (* 0.3048 m))
  (setq ft (* 0.3048 m))
  (setq mile (* 5280 foot))

The very snippet you gave there is a counterexample to your argument. He defines miles in terms of feet (which is in turn defined by meters) allowing him to use commonly known conversion factors as a sanity check, while still keeping all values in meters. If he had used his already present definition of an inch as 0.0254 meters to define feet, he could have compounded this even further. The true answer is almost certainly that he simply did whatever came to mind first, and didn't think of defining feet in terms of inches because he hadn't defined inches yet.

I mean the legal definition of the inch is 25.4 mm. All other imperial length units are derived from the inch.

Furthermore, there are more than one definition of the inch. If he worked on astronomical data from the 19th century UK, all these units would have to be changed. By tying the foot to the inch he'd only have to redefine the inch to the old British inch.

Heh, the creator of Lisp probably was "God enough" to redef constants as he saw fit.

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