Warnings on assignments in conditionals?

ruby-talk
1

Apologies in advance if you already understand this distinction, but
it is one that has tripped many a programmer, even those who should
know better :wink: If you indeed mean to assign the value to ‘a’ (as in
the first example), you can probably avoid the warning from Ruby by
breaking this into two lines, e.g.

a = 1
if a then true else false end

I did understand the distinction, I was just puzzled that Ruby would
throw a warning on something that worked as intended and as advertised
although it would probably be considered ambigious.

But in this real-life example, it doesn’t that seem ambigious (and
hence useful):

def apply_coupon(code)
if @coupon = Coupon.find_by_code(code) then true else false end
end

I guess I’m questioning when, or even if, the parser should throw
warnings on something that it considers to be bad style.

/ David

2

David Heinemeier Hansson wrote:

Apologies in advance if you already understand this distinction, but
it is one that has tripped many a programmer, even those who should
know better :wink: If you indeed mean to assign the value to ‘a’ (as in
the first example), you can probably avoid the warning from Ruby by
breaking this into two lines, e.g.

a = 1
if a then true else false end

I did understand the distinction, I was just puzzled that Ruby would
throw a warning on something that worked as intended and as advertised
although it would probably be considered ambigious.

But in this real-life example, it doesn’t that seem ambigious (and
hence useful):

def apply_coupon(code)
if @coupon = Coupon.find_by_code(code) then true else false end
end

I guess I’m questioning when, or even if, the parser should throw
warnings on something that it considers to be bad style.

/ David

For what its worth, you can get the same effect, without the warning,
like this:

@coupon = Coupon.find_by_code(code) and true or false

Not nearly as readable, but…

Alternatively, you can take advantage of the fact that Ruby treats all
values as ‘true’ EXCEPT ‘nil’ and ‘false’ (which are always false), you
could just say

def apply_coupon( code )
@couple = Coupon.find_by_code(code)
end

This would then return the value of @coupon, which (if null or false)
would be false, and otherwise may be treated as a ‘true’ value.

···

–
Jamis Buck
jgb3@email.byu.edu

ruby -h | ruby -e ‘a=;readlines.join.scan(/-(.)[e|Kk(\S*)|le.l(…)e|#!(\S*)/) {|r| a << r.compact.first };puts “\n>#{a.join(%q/ /)}<\n\n”’

3

In Message-Id: 7EC8AB04-45F2-11D8-A416-000A958E6254@loudthinking.com
David Heinemeier Hansson david@loudthinking.com writes:

def apply_coupon(code)
if @coupon = Coupon.find_by_code(code) then true else false end
end

I guess I’m questioning when, or even if, the parser should throw
warnings on something that it considers to be bad style.

> ruby -wc -e 'if str = ""; end'
-e:1: warning: found = in conditional, should be ==
Syntax OK
> ruby -wc -e 'if str = gets; end'
Syntax OK
> ruby -v
ruby 1.8.0 (2003-09-09) [i386-freebsd4]

So I’m not sure but at least 1.8.0, only assigning a literal in a
condition is warned for my understanding.

That can be pointed out as a mistake on high probability, isn’t it?

···

–
kjana@dm4lab.to January 14, 2004
What can’t be cured must be endured.