[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: macros vs. blocks
On Friday, November 22, 2002, at 02:26 AM, Trevor Blackwell wrote:
> John Clements <firstname.lastname@example.org> writes:
>> Hmm. If you consider the three largest categories of macros [*] to be
>> 1) those that affect evaluation order (if),
>> 2) those that introduce literal data (quote), and
>> 3) those that introduce bindings (let),
>> I don't see how block notation solves any other than category (1).
> I would refine my macros-are-useless claim a bit: languages that
> have syntax for things such as assignment, variable declaration, and
> quoted data (Python, Perl, Smalltalk, ...) don't benefit much from
> Using macros for #2 is probably a bad idea. I think it's generally
> clearer to include the ' symbol (or whatever your favorite language
> uses) than to have some arguments be magically quoted.
Your use of ' indicates to me that you're speaking of Scheme here. You
may be surprised to discover that ' is (in some scheme systems)
translated into (quote ...). That is, the ' _becomes_ an instance of
the thing you're protesting.
> Using macros for #3 is also usually a bad idea. Having macros like CL
> (do) or (while), which magically introduce a variable with non-obvious
> scoping, is hazardous. I'd rather just have one way of declaring
> control-structure variables, like Smalltalk:
> list do: [:x |
> x print.
> where it's totally obvious what the scope of the variable is. Scheme
> succeeds here too, because it uses lambda semantics everywhere.
Again, you like what Scheme does, but the typical implementation for
'let' is in fact exactly the sort of macro form I mention.
The difference between 'let' and CL's 'while' seems to be one of
hygiene. I agree that macros should be hygienic.
Ultimately, what you seem to be saying (if I can put words in your
mouth) is that macros are okay, but only if they're part of the