In Scheme you need to use (let () ...) in both if and cond to
introduce local definitions. That is
(cond [(even? x) (define y 42) (+ x y)]
[else 'huh])
will not work in Scheme, neither will
(cond [(even? x) (begin (define y 42) (+ x y))]
[else 'huh]).
To introduce a new scope (let () ...) is needed.
(cond [(even? x) (let () (define y 42) (+ x y))]
[else 'huh])
will.
In Racket the principle is that, if there is "room", then you define works.
This implies that that
(cond [(even? x) (define y 42) (+ x y)]
[else 'huh])
works. Note that begin is not needed.
2014-03-16 22:06 GMT+01:00 Justin Zamora <jus...@zamora.com>:
> Ok, but why is "cond" defined to behave differently? I expected "cond" to
> behave the same as "if".
>
> Justin
>
> On Mar 16, 2014 4:59 PM, "Jens Axel Søgaard" <jensa...@soegaard.net> wrote:
>>
>> The problem is that begin does not introduce a new scope.
>> You can use (let () ...) or block instead.
>>
>>
>> http://docs.racket-lang.org/reference/block.html?q=block#%28form._%28%28lib._racket%2Fblock..rkt%29._block%29%29
>>
>> /soegaard
>>
>>
>> 2014-03-16 21:38 GMT+01:00 Justin Zamora <jus...@zamora.com>:
>> > What is the reason for not allowing internal definitions in the "then"
>> > and
>> > "else" parts of an "if"?
>> >
>> > This fails with "define: not allowed in an expression context":
>> > (if (< 3 4)
>> > 5
>> > (begin
>> > (define a 7)
>> > a))
>> >
>> > But the equivalent "cond" works fine:
>> > (cond
>> > [(< 3 4) 5]
>> > [else (define a 7)
>> > a])
>> >
>> > I notice that the expansion of "cond" encloses the clauses inside
>> > (let-values () ...). Why doesn't "if" allow this?
>> >
>> > Justin
>> >
>> > ____________________
>> > Racket Users list:
>> > http://lists.racket-lang.org/users
>> >
>>
>>
>>
>> --
>> --
>> Jens Axel Søgaard
--
--
Jens Axel Søgaard
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