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comp.lang.ruby

Unless Expression Parsing

Gary Wright

4/20/2007 1:54:00 AM

I just came across an unexpected parsing problem:

x = 3 unless true # parses just fine, x = nil

p(3 unless true) # parser chokes on unless

p((3 unless true)) # OK, 'nil' is printed


This seems like a bug in the parser rather than a real
syntax problem, but maybe I'm missing something?


Gary Wright
2 Answers

Bernard Kenik

4/20/2007 3:48:00 AM

0

On Apr 19, 9:53 pm, Gary Wright <gwtm...@mac.com> wrote:
> I just came across an unexpected parsing problem:
>
> x = 3 unless true # parses just fine, x = nil
>
> p(3 unless true) # parser chokes on unless
>
> p((3 unless true)) # OK, 'nil' is printed
>
> This seems like a bug in the parser rather than a real
> syntax problem, but maybe I'm missing something?
>
> Gary Wright

in irb:

x = 3 unless true => nil

x => nil # x declared as a nil
object since it was never assigned an object
p 3 unless true => nil

p (( 3 unless true)) => nil # you're asking for the result
of '3 unless true', )



p nil => # no result since p did
nothing

3 unless true => # the result is nil but not
captured
so
p (3 unless true) => # no result since you are asking
p to print nothing

Note:

the above is equivalent to
unless true so 'p 3' never gets
interpreted and nothing happens
p 3
end

Gary Wright

4/20/2007 5:06:00 AM

0


On Apr 19, 2007, at 11:50 PM, bbiker wrote:
> p (( 3 unless true)) => nil # you're asking for the result
> of '3 unless true', )


if 'x unless y' is a valid expression then why is the *extra* set
of parenthesis needed when that expression is used as a method argument?

foo(x unless y) # syntax error
foo((x unless y)) # not a syntax error

This seems to be a problem with any of the statement modifier forms:
if, rescue, while, until.

Looking at parse.y it seems that the grammar differentiates between
arguments and statements. Adding the parens forces the statement to
become an expression.