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comp.lang.ruby

define_method with a &block?

Erik Veenstra

2/5/2006 1:16:00 AM

In the code below, I want test2 to receive a block, like test1,
so they do the same. Is that possible?

(I use this "define_method" in meta-programming code. Using
"def" is not an option.)

Thanks.

gegroet,
Erik V. - http://www.erikve...

----------------------------------------------------------------

class Test
def test1(*args, &block)
p [:test1, args, block_given?]
end

define_method(:test2) do |*args|
p [:test2, args, block_given?]
end
end

Test.new.test1(1, 2, 3){}
Test.new.test2(1, 2, 3){}

----------------------------------------------------------------
6 Answers

Ara.T.Howard

2/5/2006 3:46:00 AM

0

Joel VanderWerf

2/5/2006 5:09:00 AM

0

ara.t.howard@noaa.gov wrote:
> module_eval <<-def
> def test2 *args
> p [:test2, args, block_given?]
> end
> def

Or:

module_eval do
def test2 *args
p [:test2, args, block_given?]
end
end

if strings make you queasy.

But define_method won't define a method that takes a block until 1.9, IIRC.

--
vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407

Erik Veenstra

2/5/2006 12:03:00 PM

0

> module_eval do
> def test2 *args
> p [:test2, args, block_given?]
> end
> end

The problem is that, in my situation, the name of the method
("test2") is not fixed, but stored in a string, like this:

method_name = "test2"
module_eval do
def #{method_name}(*args, &block) # Won't work!
p [method_name, args, block_given?]
end
end

One way to overcome this, without putting the whole method in
one big string, is using a temporary method, like this:

method_name = "test2"
module_eval do
def temp_method(*args, &block)
p [method_name, args, block_given?]
end
eval("alias :#{method_name} :temp_method")
undef :temp_method
end

See code below, Test#test4.

Comments? Suggestions?

Thanks.

gegroet,
Erik V. - http://www.erikve...

----------------------------------------------------------------

class Test
def test1(*args, &block)
p [:test1, self, args, block_given?]
end

define_method(:test2) do |*args|
p [:test2, self, args, block_given?]
end

module_eval do
def test3(*args, &block)
p [:test3, self, args, block_given?]
end
end

method_name = "test4"
module_eval do
def temp_method(*args, &block)
p [:test4, self, args, block_given?]
end
eval("alias :#{method_name} :temp_method")
undef :temp_method
end
end

t = Test.new

t.test1(1, 2, 3){}
t.test2(1, 2, 3){}
t.test3(1, 2, 3){}
t.test4(1, 2, 3){}

----------------------------------------------------------------

Ara.T.Howard

2/5/2006 4:31:00 PM

0

Erik Veenstra

2/5/2006 7:15:00 PM

0

> i'm having a hard time imagining a case where only the name
> of the method would need to be added and not the body -
> doesn't really seem like meta-programming - for instance the
> method above generates something which will always use
> 'method_name' as 'test2'.

I've got a couple of cases where I want to wrap a method in a
dynamically generated method. Something like this:

class Test
def do_something(a, b, &block)
# Do something with a, b and block.
end

monitor :do_something
end

This Module::monitor wraps the original Test#do_something in a
block of code which "monitors" (just an example...) the
invocation or the execution of the original method. Adding or
removing this monitor statement may not affect the behavior of
the original method. The arguments to the dynamically generated
method are passed to the original method. So is &block...
(damn...)

This Module::monitor might be implemented like this:

class Module
def monitor(method_name, *types)
org_method = instance_method(method_name)

define_method(method_name) do |*args|
block = nil # ??? &block

# Do a lot of checking on args.
# A lot of lines.
# You don't want to do that in a string.

org_method.bind(self).call(*args, &block)
end
end
end

I'm playing with a couple of monitor functions, like
statistics, benchmarking, role-validation and type-checking. In
each situation, the impact on the code must be reduced to a
minimum. The idiom (right term?) of (temporarily) wrapping a
method, as described above, doesn't affect the code of the
original method at all. That's nice.

See the code below for a full example, in which the arguments
of a method call are checked on class/behavior.

> sure would be easier ruby had some way to declare blocks that
> take blocks! ;-)

It's introduced in Ruby 1.9... :)

Thanks.

gegroet,
Erik V. - http://www.erikve...

----------------------------------------------------------------

# LIBRARY

class Module
def typed(method_name, *types)
org_method = instance_method(method_name)

define_method(method_name) do |*args|
block = nil # ??? &block

if args.length > types.length
raise ArgumentError, "Wrong number of arguments (#{args.length} instead of #{types.length})."
end

args.length.times do |n|
arg = args[n]

[types[n]].flatten.each do |typ|
if typ.kind_of?(Module)
unless arg.kind_of?(typ)
raise ArgumentError, "Wrong argument type (#{arg.class} instead of #{typ}, argument #{n+1})."
end
elsif typ.kind_of?(Symbol)
unless arg.respond_to?(typ)
raise ArgumentError, "#{arg} doesn't respond to :#{typ} (argument #{n+1})."
end
else
raise ArgumentError, "Wrong type in types (#{typ}, argument #{n+1})"
end
end
end

org_method.bind(self).call(*args, &block)
end
end
end

----------------------------------------------------------------

# TEST SCRIPT

class Thing
def go(x, y, z)
# x should be Numeric
# y should be a String
# z should respond to :gsub and :to_s
:good
end

typed :go, Numeric, String, [:gsub, :to_s]
end

def test(*args)
begin
puts "#{args.inspect} : OK : #{Thing.new.go(*args).inspect}"
rescue Exception => e
puts "#{args.inspect} : NOK : #{e.message}"
end
end

test(7)
test(7, 8, 9)
test(7, 8, "9")
test(7, "8", 9)
test(7, "8", "9")

----------------------------------------------------------------

Joel VanderWerf

2/5/2006 9:03:00 PM

0

Erik Veenstra wrote:
> eval("alias :#{method_name} :temp_method")

This is a bit cleaner using alias_method:

class A
def temp_method; "foo"; end
method_name = "bar"
alias_method method_name, :temp_method
end

p A.new.bar # ==> "foo"

--
vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407