Re: begin at top-level?

[Per Bothner <per at bothner dot com>]

Chris Dean wrote:

> How come begin returns a different value at the top level?  For example:
>
>     #|kawa:2|# (begin 'a 'b)
>     ab
>     #|kawa:3|# (define (foo) (begin 'a 'b))
>     #|kawa:4|# (foo)
>     b

Because (begin FORM1 FORM2) is the equivalent to FORM1 followed by 
FORM2.  In a function body, evaluating multiple forms returns the last 
value.  At top level, evaluating multiple forms prints them all - after 
all it's a read-eval-print loop.

At least that's the logic for how it works in Kawa.  Other Scheme 
implementations are different.

The begin form is kind of funny.  It's a pure syntactic grouping form, 
and is neither a scoping form nor a control form.  It is mainly useful 
for macros, and a few syntax forms like (if ...).

E.g. in:

(begin
  (define a 1)
  (begin
    (begin
      (define b 2)
      (define c 3))
    (define d 4)))

all the variable a, b, c, d are in the same scope.  If they were 
functions they could mutually reference each other.

In the (experimental) KRL and (even more experimenal) Q2, evaluating 
multiple forms even in a function body returns multiple values.
--
	--Per Bothner
per@bothner.com   http://per.bothner.com/