Comments on: Hash Tables and a Wee Bit of Sugar in Common Lisp

By: Mikael Jansson » Blog Archive » Jag, en arg LCHF-människa?

Mikael Jansson » Blog Archive » Jag, en arg LCHF-människa? — Thu, 05 Mar 2009 20:21:21 +0000

[...] Hash Tables and a Wee Bit of Sugar in Common Lisp [...]

By: Mikael Jansson

Mikael Jansson — Fri, 06 Feb 2009 12:38:13 +0000

Property lists are less efficient than hash-tables for larger sizes.

But you’re missing the point. Lisp is the programmable programming language.

You need new syntax? Add it! Try doing the same thing in Python.

In fact, add do...while to your current version of Python as portable Python code so people won’t have to replace their Python with your Python.

In Lisp, that’s just a defmacro away. How much work is it in your language?

By: bearophile

bearophile — Fri, 06 Feb 2009 12:28:57 +0000

If that nice syntax already exists, what’s the point of this whole blog post?

By: Mikael Jansson

Mikael Jansson — Fri, 06 Feb 2009 10:36:27 +0000

The key is to create a readable notation for hash tables. That way, everyone will be able to understand that it is, indeed, a hash table. Adding separators for the sake of readability is trivial, it is just a simple matter of skipping that item when deconstructing the list. Semicolon is not possible, as it's reserved for comments. If you're not picky about the keys, however, there is already built-in syntax for mappings, called property lists:



* (defvar *presidents* (list :abe 25 :lincoln 26))

* (get *presidents* :abe)

25

where (list) would be the empty mapping.

By: bearophile

bearophile — Fri, 06 Feb 2009 10:23:38 +0000

The following syntax is bad, because there’s no guide for the eye to tell apart pairs:
({} 7 5 5 55 2 15 25 5 1 9 5 11 57 9)
{} is ugly as function name, better something like dict or hash or make-hash something else.
Can some separator be added? Is this possible?
(dict 7 5; 5 55; 2 15; 25 5; 1 9; 5 11; 57 9)
A possible syntax for the empty hash:
(dict)
Finally, a really important thing the author here has missed is that such hash syntax has to become standard, so everyone uses it, and you can understand it as soon as you see it. If everyone re-invents something different it’s nearly useless.

By: Mikael Jansson

Mikael Jansson — Fri, 06 Feb 2009 09:34:33 +0000

I forgot about the second version which is indeed less verbose than what I wrote.

The point was that without syntactic sugar, i.e., your first version, you’d have to write it out fully. In Lisp, where you don’t have shortcuts, it would be enough to sprinkle some grains on the basic functionality and come up with something fairly closely resembling the native Python {} syntax. Or, like I concluded, a reader macro.

Which is to say, while Lisp has little sugar, it allows you to invent your own preferred syntax, making the original posts’ argument a moot point in practice.

By: bearophile

bearophile — Fri, 06 Feb 2009 09:26:57 +0000

In Python the syntax for dicts it just:

{'Lincoln': 25, 'Abe': 26}

Or:

dict(Abe=26, Lincoln=25)

You never write:

dict([("Abe", 26), ("Lincoln", 25)])