how to do instance_eval with arguments

[Jim Weirich]

I'm prototyping a DSL and came across a situtation where I have a lambda
that I would like to give to instance_eval, but the lambda takes
arguments. Instance_eval will not supply any arguments when evaluating
the lambda and I don't see a straightforward way around this.

Any suggestions?

Here's a test case ...

class Dummy
def f
:dummy_value
end
end

def instance_eval_with_args(obj, *args, &block)
# Magic goes here to evaluate +block+ in the scope of
# +obj+, yet pass a list of argument values to the block.
end

class TestInstanceEvalWithArgs < Test::Unit::TestCase
def test_instance_eval_with_args
# Create a block that returns the value of an argument and a value
# of a method call to +self+. This is the basic functionality I
need.
block = lambda { |a| [a, f] }

assert_equal [:arg_value, :dummy_value],
instance_eval_with_args(Dummy.new, :arg_value, &block)
end
end

 

[Yukihiro Matsumoto]

Hi,

In message "Re: how to do instance_eval with arguments"
|
|I'm prototyping a DSL and came across a situtation where I have a lambda
|that I would like to give to instance_eval, but the lambda takes
|arguments. Instance_eval will not supply any arguments when evaluating
|the lambda and I don't see a straightforward way around this.
|
|Any suggestions?

CVS HEAD (1.9) has instance_exec method that works exactly what you
want.

matz.

 

[Jim Weirich]

|
|I'm prototyping a DSL and came across a situtation where I have a lambda
|that I would like to give to instance_eval, but the lambda takes
|arguments. Instance_eval will not supply any arguments when evaluating
|the lambda and I don't see a straightforward way around this.
|
|Any suggestions?

CVS HEAD (1.9) has instance_exec method that works exactly what you
want.

Excellent! I think Matz's time machine is every bit as good as
Guido's[1].

 

[Jim Weirich]

Any suggestions?

[...]

One way to implement it in 1.8:

[... solution elided ...]

Thanks! That should be good enough for my prototype ... at least until
1.9 features become standard.