Foo || 0 will cause exception but not for himself

Ruby
1

if foo is undefined, it will throw exception when referred to… as in
try 1, 2, and 3 below. However, it won’t throw exception when assigned
to himself… how is that interpreted?

irb(main):001:0> foo
NameError: undefined local variable or method foo' for main:Object from (irb):1:inKernel#binding’

irb(main):002:0> foo || 0
NameError: undefined local variable or method foo' for main:Object from (irb):2:inKernel#binding’

irb(main):003:0> bar = foo || 0
NameError: undefined local variable or method foo' for main:Object from (irb):3:inKernel#binding’

irb(main):004:0> foo = foo || 0
=> 0

irb(main):005:0> foo
=> 0

2

On Sep 21, 8:37 pm, SpringFlowers AutumnMoon
[email protected] wrote:

    from (irb):2:in `Kernel#binding'

irb(main):003:0> bar = foo || 0
NameError: undefined local variable or method foo' for main:Object from (irb):3:in Kernel#binding’

irb(main):004:0> foo = foo || 0
=> 0

irb(main):005:0> foo
=> 0

When Ruby says “undefined local variable or method”, what it means is
“Hey, I don’t know what this is, and I don’t know if it’s supposed to
be a variable or a method.”

When you write “foo = …” Ruby says, “Ah ha! Foo is supposed to be a
variable. I’ll treat it as one.”

That’s my (somewhat informed) guess anyhow. The determination of the
method/variableness of ‘foo’ is determined before runtime, when the
lexer/parser is analyzing the program. This is similar to why this
code…
if foo=42
p foo
end
works just fine, but this code:
p foo if foo=42
throws an error. When it first see’s foo in the first code sample, it
knows that it is a variable. When it first sees foo in the second code
sample, it doesn’t know what it is yet, and throws an error to
indicate its confusion.

3

SpringFlowers AutumnMoon wrote:

if foo is undefined, it will throw exception when referred to… as in
try 1, 2, and 3 below. However, it won’t throw exception when assigned
to himself… how is that interpreted?

irb(main):004:0> foo = foo || 0
=> 0

irb(main):005:0> foo
=> 0

Maybe your Variable/Method Ambiguity thread applies here?

if false
foo = 10
end

puts foo #nil

4

7stud – wrote:

if false
foo = 10
end

puts foo #nil

i see…

so after

if false
foo = 10
end

now, bar = foo || 0 will run.

Since for instance methods, the setter method can be foo=()

so how about just a method not inside a class, can it be named “foo=” as
well? if so, how does Ruby know foo=1 is really an assignment or a
method call foo=(1)?

Ruby doesn’t seem to like “foo=” as a method:

def foo=(i)
p “ha”, i+1
end

def bar?(i)
i % 10
end

p foo=(1)

p foo

p bar?(22)

it will print out

1
1
2

5

On Sep 22, 7:26 am, SpringFlowers AutumnMoon
[email protected] wrote:

so how about just a method not inside a class, can it be named “foo=” as
well? if so, how does Ruby know foo=1 is really an assignment or a
method call foo=(1)?

Ruby doesn’t seem to like “foo=” as a method:

self.foo = 10

6

Gavin K. wrote:

On Sep 22, 7:26 am, SpringFlowers AutumnMoon
[email protected] wrote:

so how about just a method not inside a class, can it be named “foo=” as
well? if so, how does Ruby know foo=1 is really an assignment or a
method call foo=(1)?

Ruby doesn’t seem to like “foo=” as a method:

self.foo = 10

def foo=(val)
“hello”
end

puts self.foo = 10 #10

7

On Sep 22, 2:51 pm, 7stud – [email protected] wrote:

def foo=(val)
“hello”
end

puts self.foo = 10 #10

Er, what’s your point? Methods whose names end with a ‘=’ character
always return the value ‘assigned’ to them, not the last expression in
the method. How is that relevant to this discussion of how to invoke
them, and the scope and determinatino of variables versus methods?

8

On 9/23/07, Phrogz [email protected] wrote:

self.foo = 10
them, and the scope and determinatino of variables versus methods?
Actually, it’s not that, it’s the way assignments get parsed/compiled:

irb(main):001:0> class Foo
irb(main):002:1> def x
irb(main):003:2> @x
irb(main):004:2> end
irb(main):005:1> def x=(val)
irb(main):006:2> @x = val
irb(main):007:2> “Surprise”
irb(main):008:2> end
irb(main):009:1> end
=> nil
irb(main):010:0> f = Foo.new
=> #Foo:0x7a1fc
irb(main):011:0> f.x=2
=> 2
irb(main):012:0> f.send(:x=, 3)
=> “Surprise”
irb(main):013:0> f.x
=> 3

–
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/