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2 points by rocketnia 2227 days ago | link | parent

"pos seems a reasonable case for keeping 0 truth-y"

While I personally like 0 being truthy, I don't see this as a convincing reason.

I'd treat 'pos exactly the same way as 'find. They're even conceptually similar, one finding the key and the other finding the value. For 'find, the value we find might be falsy, so truthiness isn't enough to distinguish success from failure. The same might as well be true for 'pos.

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"But you're right, I might yet make empty strings and tables false-y."

What if the table is mutable? That's an interesting can of worms. :)

JavaScript has 7 falsy values, all of which are immutable. If we know something's always falsy, we also know it encodes a maximum of ~2.8 bits of information--and usually much less than that. It takes unusual effort to design a program that uses all 7 of those values as distinct cases of a single variable.

This means if we have a variant of Arc's (and ...) or (all ...) that short-circuits when it finds a truthy value, we don't usually have to worry about skipping over valuable information in the falsy values.

If every mutable table is falsy as long as it's empty, then a falsy value can encode some valuable information that a practical program would care about, namely the reference to a particular mutable table.

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"(True, False = 0, 1 :( That's the ugliest thing I've ever seen python allow. At least throw a warning, python! Better no booleans than this monstrosity.)"

There's some rationale here:

http://www.python.org/dev/peps/pep-0285/

http://docs.python.org/2/whatsnew/2.3.html

http://www.python.org/download/releases/2.2.1/NEWS

The PEP describes the design and rationale of introducing booleans to Python this way. Version 2.3 implements this. Version 2.2.1 preemptively implements bool(), True, and False to simplify backporting from 2.3.

Notably, the variable names "True" and "False" were chosen to be similar to the variable name "None", and all three of these are just variables, not reserved words.

Later, version 2.4 made it an error to assign to None:

http://docs.python.org/2/whatsnew/2.4.html

From what fallintothis says, apparently the same change hasn't been made for True and False.



2 points by akkartik 2227 days ago | link

Hmm, so how is one expected to check for list membership in arc? Ah, this would seem to be the canonical idiom:

  (aif (mem f seq)
    <operate on car.it>)

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2 points by rocketnia 2227 days ago | link

Oh, very nice! ^_^ I remember using this a few times, but yours looks much better:

  (aif (pos f seq)
    <operate on seq.it>)

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1 point by akkartik 2227 days ago | link

Thanks a bunch for the python links, especially the last one. They were most illuminating.

I think the error is that None, False and True were ever 'constants' rather than literals.

Update: ah, this is fixed in python 3.

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2 points by akkartik 2225 days ago | link

So I emailed Guido van Rossum with this question and he was nice enough to respond :)

http://python-history.blogspot.com/2013/11/story-of-none-tru...

Couldn't have done it without you, rocketnia.

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