Eval needs explicit self for accessor methods?


#1

hi all!

i encountered a strange problem with eval. please see the following
test case:

require ‘test/unit’

class TestContext

def eval_in_context( ruby_code )
instance_eval ruby_code
end

def toast
@toast
end

def toast=( val )
@toast = val
end

end

class TestMetaEval < Test::Unit::TestCase

def test_toast
tc = TestContext.new
tc.toast = ‘yes’

assert_equal 'yes', tc.toast
assert_equal 'yes', tc.eval_in_context( "toast" )

# These two lines - using explicit self - work as expected:
assert_equal 'maybe', tc.eval_in_context( "self.toast = 'maybe'" )
assert_equal 'maybe', tc.toast

# What is happening here:
assert_equal 'no', tc.eval_in_context( "toast = 'no'" )

# Why does the next test fail?
# What did the previous line set? A local variable?

# tc.toast returns 'maybe' here instead of the expected 'no':
assert_equal 'no', tc.toast

end

end

why is the ‘toast=(val)’ method not visible in eval unless i
explicitly use “self.toast = …”?

what is it i don’t understand about ruby scoping? :confused:

any help really appreciated!

cheers,

daniel


#2

It really has nothing to do with eval, your just setting a local
variable.

An unfortuate side effect of the setter notation is that it conflicts
with local var setting. Local var setting wins out, so you have to use
self as the reciever in order to tell ruby you mean the setter method.

T.


#3

Of course! Thank you! Sorry for the dumb question. Makes perfect sense
now…

dl


#4

Trans wrote:

It really has nothing to do with eval, your just setting a local
variable.

An unfortuate side effect of the setter notation is that it conflicts
with local var setting. Local var setting wins out, so you have to use
self as the reciever in order to tell ruby you mean the setter method.

T.

Since you bring this up, there are a few more cases where implicit
locals
declaration causes unexpected behavior (if new to ruby that is):

x = 10
(1…10).each { |i| x = x + i } # x == 65
x = 10
(1…10).each { |i| x = i } # x == 10

So if you’re assigning x to itself in some way, it recognizes x in the
enclosing scope, else it creates a new local. That’s kind of a
confusing rule.
Also, what if I wanted the first example to create a local? How do
I refer
to the toplevel x in the second?

I’d really like to see explicit locals declarations in Ruby, and this
would all
be solved by simply introducing var or local, as in var x = 10.
Even if
it was only optional, and invoked with -strict-locals, I think many
people
would be happy. Maybe I’ll download the source and hack away to test it
:slight_smile:

Mike


#5

On Apr 1, 2006, at 2:13 PM, Mike A. wrote:

So if you’re assigning x to itself in some way, it recognizes x in
the enclosing scope, else it creates a new local. That’s kind of a
confusing rule. Also, what if I wanted the first example to
create a local? How do I refer to the toplevel x in the second?

The ‘x’ in each of those blocks references the local variable ‘x’
defined outside the block.
In neither case is a new local variable ‘x’ created within the
block. In both cases a new
local variable ‘i’ is created within the blocks and is only visible
within the individual block.

Here are the rules I use to understand local variable scope and blocks:

  1. Local variables created outside a block are visible inside the
    block.

  2. Local variables created inside a block are not visible outside
    the block.

  3. Block arguments behave like local variables not formal
    (method) arguments.

If you combine 1 and 2 you see that block scope is sort of a one-way
barrier. Code inside the block can see variables created outside the
block but not the other way around.

The third rule is the one that throws everyone because most
programmers tend to think of block arguments as formal arguments that
shadow any similarly named variables outside the block but this is
incorrect (in Ruby). Block arguments behave like local variables
so if a block argument has the same name as a variable in the
enclosing scope then a new local variable is not created. On the
other hand if there is no local variable in the enclosing scope with
the same name then a new local variable is created and is only
visible within the block (rule 2).

I believe it is rule 3) that Matz is considering changing for Ruby 2.0

Gary W.


#6

On Apr 1, 2006, at 2:53 PM, removed_email_address@domain.invalid wrote:

(method) arguments.
Good list!


#7

removed_email_address@domain.invalid wrote:

within the individual block.

new local variable is not created. On the other hand if there is no
local variable in the enclosing scope with the same name then a new
local variable is created and is only visible within the block (rule 2).

I believe it is rule 3) that Matz is considering changing for Ruby 2.0

Changing the hiding characteristics of formal arguments sounds like a
good
idea, but there’s still a big problem of not being able to declare
locals. For
example, given the following:

def test()
a = 2
Proc.new { Proc.new { a = 10 }.call() }.call()
puts a
end

test() => 10

Lexical closure are a very nice thing, but if I think I’m creating a
local when
in fact I’m modifying some value in the enclosing scope, then that’s
bad. I
think shadowing a parameters is a lot less dangerous, and a warning can
be
printed when that happens. So the above would look like this:

Proc.new { Proc.new { var a = 10 }.call() }.call()

Mike


#8

Mike A. schrieb:

confusing rule. Also, what if I wanted the first example to create a
local? How do I refer to the toplevel x in the second?

Mike, you’re accessing the toplevel “x” in both cases:

x = nil
(1…10).each { |i| x = i } # x == 10

Regards,
Pit