Re: [Python-ideas] Tail Call Optimization -- natural? intuitive?

+1 for the example you used. On Mon, Jan 20, 2014 at 7:09 AM, Chris Angelico wrote:

On Mon, Jan 20, 2014 at 11:12 PM, spir wrote:

...

For instance, closed intervals are more intuitive or natural, obviously (but for some reason I don't know). If you ask someone to count from 1 to 9, you will probably be surprised to hear him/her start from 2 or stop after 8. If you are asked to choose a letter between c and g, you will probably be surprised to hear that 'c' or 'g' is no good choice.

I'm not so sure about that. The half-open interval makes as much sense as the fully closed - all you have to do is interpret the indices as being *between* elements. Take, for example, Scripture verses. (Quotes taken from THE HOLY BIBLE, NEW INTERNATIONAL VERSION®, NIV® Copyright © 1973, 1978, 1984, 2011 by Biblica, Inc.® Used by permission. All rights reserved worldwide. Copyright notice included for license compliance. Note that I'm using bracketed numbers to indicate the beginnings of verses - in a printed Bible, these would normally be in superscript.)

John 14: [31] To the Jews who had believed him, Jesus said, “If you hold to my teaching, you are really my disciples. [32] Then you will know the truth, and the truth will set you free.” [33]

This passage is normally referred to as "John 14:31-32", but as you see, the verse marker [32] is in the middle of the quote. Using a half-open interval, this would start at "John 14:31" and end at "John 14:33". Half-open means: "Begin at the beginning, go on till you come to the end, then stop", as the King of Hearts instructed the White Rabbit.

It's easy to indicate the beginning of a chapter: your start reference is verse 1. Here's the beginning of the account of the creation of the world:

[1] In the beginning God created the heavens and the earth. [2] Now the earth was formless and empty, darkness was over the surface of the deep, and the Spirit of God was hovering over the waters. [3] And God said, “Let there be light,” and there was light. [4] God saw that the light was good, and he separated the light from the darkness. [5] God called the light “day,” and the darkness he called “night.” And there was evening, and there was morning—the first day. [6]

Common parlance: Genesis 1:1-5. Half-open: Genesis 1:1-6. Conclusion: Tie. No argument to be made for either side. But what if you're looking at the *end* of a chapter? Here are a few verses from later on in Genesis 1:

[29] Then God said, “I give you every seed-bearing plant on the face of the whole earth and every tree that has fruit with seed in it. They will be yours for food. [30] And to all the beasts of the earth and all the birds of the air and all the creatures that move on the ground—everything that has the breath of life in it—I give every green plant for food.” And it was so. [31] God saw all that he had made, and it was very good. And there was evening, and there was morning—the sixth day.

Common parlance: Genesis 1:29-31. Half-open: Genesis 1:29-2:1. It's much more obvious by the latter that this passage extends exactly to the end of the chapter.

Obviously it's way WAY too late to change the way Bible references are written, any more than Melway could renumber their maps all of a sudden. Massive case of lock-in and backward-incompatibility with existing code. But I put it to you that the half-open would make at least as much sense as the closed, in any situation where there are boundaries with contents between them.

Note, by the way, that I'm not looking at anything involving backward scanning or wider strides, both of which Python's slice notation supports. Neither of those is inherently real-world intuitive, so the exact semantics can be defined as whatever makes sense in code. (And there was some discussion a little while ago about exactly that.) I'm just looking at the very simple and common case of referencing a subset of consecutive elements from a much larger whole.

The closed interval makes more sense when the indices somehow *are* the values being retrieved. When you count from 1 to 9, you expect nine numbers: 1, 2, ..., 8, 9. When you list odd numbers from 1 to 9, you expect 1, 3, 5, 7, 9. But what if you're looking at a container train and numbering the twenty-foot-equivalent-units (TEU) that it has? A 40-foot container requires 2 TEU, a 60-foot container requires 3 TEU. A "reefer" (refridgerated container) might require an extra slot, or at least it might be a 56-footer and consume 3 TEU. One wagon might, if you're lucky, carry 5 TEU; numbering them 1 through 5 would be obvious, but numbering the boundaries between them as 0 through 5 is better at handling the multiple TEU containers. (Even more so when you look at double-stacked containers. An over-height 40-foot container could consume 2 TEU horizontally and 2 TEU vertically, and be put in slots (0,0)-(2,2). This is, in fact, exactly how a GTK2 Table layout works.) Both types of intervals have their places.

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