Ruby 1.8.7 Lambas: syntax and scoping issue (are these bugs?


C

Charlton Wang

I'm seeing two odd behaviours with using lambdas:

1. It seems as though having a lambda as a function argument with a
block causes a syntax error. It can be mitigated by adding a semicolon
or by using parentheses

def foo(rest)
puts rest.inspect
yield
end


# fails syntax error
foo :parm1=>1, :parm2=>lambda { puts "lambda" } do
puts "In bar block"
end

# fails syntax error
foo :parm1=>lambda { puts "lambda" }, :parm2=>1 do
puts "In bar block"
end

# fails syntax error
foo {:parm1=>1, :parm2=>lambda { puts "lambda" }} do
puts "In bar block"
end

# succeeds???
foo :parm1=>1, :parm2=>lambda { puts "lambda"; } do
puts "In bar block"
end

# succeeds???
foo :)parm1=>1, :parm2=>lambda { puts "lambda" }) do
puts "In bar block"
end

2. I'm not understanding the scoping rules for lambda with
instance_evals inside a class:

# If these statements are added at the beginning the
# the lambda will evaluate them first
# a = 4 # stmt 1
# b = 5 # stmt 2
class Bar
attr_accessor :a, :b
def initialize
@a = 1
@b = 2
end
def foo(&block)
instance_eval(&block)
end
end

m = lambda{puts a + b}
Bar.new.foo(&m)

Without stmt1 and stmt2 above, the code correctly outputs 3. But if
stmt1 and stmt2 are incommented, the output is 9 which seems to be the
global scope rather than the scope of the instance of Bar expected
with instance_eval.

Am I missing something?

Thanks,
Charlton
 
Ad

Advertisements

D

David A. Black

Hi --

I'm seeing two odd behaviours with using lambdas:

I'm skipping to #2 if that's OK.
2. I'm not understanding the scoping rules for lambda with
instance_evals inside a class:

# If these statements are added at the beginning the
# the lambda will evaluate them first
# a = 4 # stmt 1
# b = 5 # stmt 2
class Bar
attr_accessor :a, :b
def initialize
@a = 1
@b = 2
end
def foo(&block)
instance_eval(&block)
end
end

m = lambda{puts a + b}
Bar.new.foo(&m)

Without stmt1 and stmt2 above, the code correctly outputs 3. But if
stmt1 and stmt2 are incommented, the output is 9 which seems to be the
global scope rather than the scope of the instance of Bar expected
with instance_eval.

Am I missing something?

When Ruby sees a and b, it favors the interpretation that they are
local variables. If you want to ensure that they're interpreted as
method calls, you can do:

puts a() + b()


David
 
W

William James

Charlton said:
I'm seeing two odd behaviours with using lambdas:

1. It seems as though having a lambda as a function argument with a
block causes a syntax error. It can be mitigated by adding a semicolon
or by using parentheses

def foo(rest)
puts rest.inspect
yield
end


# fails syntax error
foo :parm1=>1, :parm2=>lambda { puts "lambda" } do
puts "In bar block"
end

The syntax is wrong whether or not you are passing
a Proc object as an argument.

def foo(rest)
puts rest.inspect
yield
end
==>nil
foo 'hello' { puts 'in block' }
SyntaxError: compile error
(irb):5: syntax error
foo 'hello' { puts 'in block' }
^
(irb):5: syntax error
from (irb):5
from :0
foo( 'hello' ){ puts 'in block' }
"hello"
in block
==>nil
foo( proc{p "I'm neither sheep nor lambda."} ){ puts 'in block' }
#<Proc:[email protected](irb):6>
in block
==>nil
 
C

Charlton Wang

Sorry, you lost me with your example.

My block is being bound with do/end rather than {}. In your examples,
you're using {} which has tighter binding and the syntax error is
expected.

foo 'hello' do puts 'in block'; end

works just find but this isn't the same as what I'm asking. I'm binding
the lambda function as a hash value.

Or...maybe I'm missing something obvious.

Charlton
 
T

The Higgs bozo

Charlton said:
# fails syntax error
foo :parm1=>1, :parm2=>lambda { puts "lambda" } do
puts "In bar block"
end

Curiously, that works in 1.9.

I also noticed this succeeds in 1.8 (and 1.9):

foo :parm1=>1, :parm2=>lambda { puts("lambda") } do
puts "In bar block"
end
 
Ad

Advertisements

C

Charlton Wang

The said:
Curiously, that works in 1.9.

I also noticed this succeeds in 1.8 (and 1.9):

foo :parm1=>1, :parm2=>lambda { puts("lambda") } do
puts "In bar block"
end

That's really interesting. So the parentheses as arguments to the puts
make it work. How odd!
 
Ad

Advertisements


Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments. After that, you can post your question and our members will help you out.

Ask a Question

Top