Arc Forumnew | comments | leaders | submitlogin
2 points by akkartik 2228 days ago | link | parent

Thanks for trying it out, and for the comments! Yeah it's gotten slow :(

I hadn't realized how close to python I've gotten. Seems right given how the whitespace and keyword args are inspired by it. On rosetta code I found a cheap way to get syntax highlighting was to tag my wart snippets with lang python :)

I've been using 1 as the default truth value, and it's not assignable either. I was trying to avoid an extra hard-coded symbol, but now that I've added false perhaps I should also add true.. I'm not averse to going whole-hog on a boolean type, I'd just like to see a concrete use case that would benefit from them. pos seems a reasonable case for keeping 0 truth-y, and the fact that lists include the empty list seems a reasonable case so far to keep nil false-y. But you're right, I might yet make empty strings and tables false-y.

(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.)



2 points by rocketnia 2227 days ago | link

"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.

---

"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.

---

"(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>)

-----

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>)

-----

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.

-----

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.

-----

2 points by rocketnia 2228 days ago | link

"I was trying to avoid an extra hard-coded symbol"

Speaking of which, why are you making false and nil count as symbols at all?

I suppose it gives them an external representation without coining a new syntax like #f.

-----

1 point by akkartik 2227 days ago | link

Yeah, I'm just minimizing how much I need to change, picking my poison between hard-coded symbols and extra cell types.

-----