Errata for Seven Languages in Seven Weeks

#48024: You say "... class will be much more difficult to debug because Ruby can no longer tell when a method is missing. We would definitely want strong error checking to make sure it was accepting valid Roman numerals."

But, adding a line such as

return super if (!name.to_s.match(/[IVXLC]+$/))

first thing in the method_missing definition, Ruby will be able to handle the missing methods just fine.

The problem of finding the definition of the methods remains, of course.

I'd personally strip the quoted lines from that paragraph and add them as an introduction to an exercise to find the return super if cannot_handle_method idiom.--Vinko

#47508: Figure 2.1 is correct but to make it consistent with the code you show, the three blue boxes on the left should be moved down with the left arrow coming from Fixnum, not Numeric. Then add an Object box above Numeric with an up arrow showing the inheritance.--art gittleman

#47649: In the chapters about Prolog, tuples and list are mentioned.

1. Prolog does not have a real tuple type, and especially not a constant access one.
2. It is not true that lists and tuples are "containers" respectively of variable and fixed size.

In Prolog there is a binary left associative term constructor ,/2 which is what is used to build things which look as tuples:

?- (1, (2, (3, 4))) = (1, 2, 3, 4).
true.

?- (((1, 2), 3), 4) = (1, 2, 3, 4).
false.

In fact, what looks like a tuple is simply a nested term, not different from lists. Things are slightly more complicated as there is a ,/2 predicate which represents the logical conjunction of two predicates:

?- (true, false).
false.

?- (true, true).
true.

The list functor is '.'.

?- .(3, []) = [3].
true.

The usual [...] syntax is just syntactic sugar.

Essentially a list is not a "mutable size container" (lists are essentially immutable[0]).
It is just a cons list, like in Scheme. Simply the pair constructor is . and not cons.

?- ','(3, 4) = (3, 4).
true.

The 'term' syntax can be used essentially to bypass usual precedence rules which would make the desugared version ,(3, 4) a syntax error.

Since in Prolog it is possible to use any functor (including ',') to build terms of arbitrary arity, it is correct to write:

?- X = ','(3, 4, 5).
X = ','(3, 4, 5).

However, that is *not* a tuple type:

?- ','(3, 4, 5) = (3, 4, 5).
false.

Because (1, (2, (3, 4))) = (1, 2, 3, 4).

----
[0] In some sense it can be "modified" because if we have variables we can "extend" the list through unification if the last term is a variable
(this is also the basis for difference lists). Same applies to tuples, though.

?- (1, A) = (1, 2, 3).
A = (2, 3).

--Enrico Franchi

#47401: Immediately after mentioning Erlang is tail recursion optimized, two recursive examples are given - factorial and fibonacci. But neither of them are implemented in a tail recursive fashion. This could be confusing. --Martin Leech

#48034: In Hackers and Painters, Graham chronicles a start-up that leveraged produc- tivity with Lisp to achieve productivity that no other vendors could match.

should probably be:

In Hackers and Painters, Graham chronicles a start-up that leveraged Lisp to achieve productivity that no other vendors could match.--Julian Ceipek

#48060: The Fibonacci sequence is defined with the 0th element as 0. The definition on page 250, as fib, starts with the 0th element as 1. This gives different results from the alternative definition on page 251 where the 0th element is 0.--Tyler Bindon