2015-09-30 19:35 GMT+02:00 Lyle Kopnicky <lylew...@gmail.com>:
> Hi folks,
>
> It seems that if I (incorrectly) quote a symbol in a 'define' form, it
> breaks all symbols from that point on. I would expect instead it would give
> me an error saying that I passed the wrong sort of thing to 'define', and
> not break future use of symbols. Is this a bug?
>
> Welcome to DrRacket, version 6.2.1 [3m].
> Language: racket; memory limit: 128 MB.
> > 'foo
> 'foo
>
Note that 'foo is short for (quote foo).
> > 'bar
> 'bar
> > (define 'foo 5)
>
This is the same as
(define (quote foo) 5)
which means that you are defining a function named quote that when called
with one argument returns the number 5.
> > 'foo
> . . foo: undefined;
>
This means
(quote foo)
i.e. you are now calling the (newly defined) quote function - but the
argument foo is not defined - hence the error.
> cannot reference an identifier before its definition
> > 'bar
> . . bar: undefined;
> cannot reference an identifier before its definition
> >
>
Try
> '3
5
The lesson here is that at the top-level (in the REPL) it is possible to
redefine builtin syntax.
Here the shorthand ' makes it hard to spot.
/Jens Axel
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