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Re: another take on hackers and painters
On Thu, 22 May 2003, Michael Vanier wrote:
> Of course, this isn't autoconversion, but something like this could be used
> for autoconversion.
Indeed. Here's a complete autoconverting + implemented in PLT Scheme:
(define real+ +)
(define (+ . args)
(define (?->num x)
((string? x) (or (string->number x)
(error '+ "expected a number string, got non-number
string ~s" x)))
((number? x) x)
(else (error '+ "expected number or string, got ~s" x))))
(apply real+ (map ?->num args)))
> (+ 3 "12")
> (+ "3" "12")
> (+ 3 12)
> (+ 3 "oops!")
. +: expected a number string, got non-number string "oops!"
So the argument becomes, what implementation of + is best for a language?
It could be:
* "Normal" numbers-only +
Seems like the best choice if you don't want to process text often and
want to be able to statically detect type errors arising from incorrect
use of +.
* an autoconverting, error-raising + as above
Seems like the best choice if you want to write lots of programs that deal
with data files that have simple ad-hoc structure: you don't need a lexer
and parser to deal with a file full of three-column text where column two
is a number, but if it turns out after you finish the program and
somebody's relying on it that in fact column two is only *usually* a
number, you can detect and fix your bug easily.
* an autoconverting, silent-failing +
I really can't think of a group for whom this is the best choice. Perhaps
someone can enlighten me.