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Comment by mjw1007 | Hacker News Reader

I'm sure Norvig is very good, but I think the sudoku webpage is presenting a finished program in an idealised way, not recording the process he used to write it, false starts and all.

(It seems likely to me that there were some false starts, because eliminate() returns either `values` or False, when both of its callers would be just as happy if it returned True or False.)

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Not saying there weren't false starts, but your example of eliminate() is actually a fairly common idiom[0] where, when a function is testing things for "truthiness" or "falsiness", it will return the truthy or falsey value rather than actual True or False. It lets code be more brief because you can do e.g. a check for null and perform assignment of either the value or a default in a single statement, so for example something like this:

  if( maybeNullValue ) {
    myValue = maybeNullValue;
  } else {
    myValue = defaultValue;
  }

can instead be expressed as this:

  myValue = maybeNullValue || defaultValue;

I'm personally a little bit torn about it. On the one hand, now that I'm familiar with it I love how much more concise code can be. On the other, it can have a fairly negative impact on code maintainability: if someone comes along who isn't familiar with that idiom, they might have a bad time trying to figure out what's going on (or worse, they might take offense that all these predicates aren't returning True or False values and rewrite the whole mess--or you could end up with a mix of the two, where some functions follow the idiomatic convention and others return actual boolean values which is arguably the worst)

Also I'm not trying to say that there's not some YAGNI going on here--just that I suspect he did it that way somewhat reflexively, and that in that particular case we probably cannot infer he had originally intended to do something else with those results and then changed his mind later.

[0] at least in the Lisp world, which Norvig has extensive experience with[1]

[1] https://github.com/norvig/paip-lisp

He didn't give an implementation time for the sudoku solver, but he did say he completed the spell checker in a single transcontinental flight.

I don't remember how long it took me to do the spell checker challenge:

https://github.com/dlang/dmd/blob/master/src/dmd/root/spelle...

but I'm sure it was much longer than it took Peter. This checker is used by the D compiler to look for a misspelled identifier when a symbol lookup fails. The identifiers in scope form the dictionary.

It works surprisingly well. I added it to the Digital Mars C++ compiler as well.

https://github.com/DigitalMars/Compiler/blob/master/dm/src/d...

> returns either `values` or False, when both of its callers would be just as happy if it returned True or False.)

That's pythonic.

You don't convert from a data type to Boolean.

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