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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 arm1=>1, arm2=>lambda { puts "lambda" } do
puts "In bar block"
end

# fails syntax error
foo arm1=>lambda { puts "lambda" }, arm2=>1 do
puts "In bar block"
end

# fails syntax error
foo {arm1=>1, arm2=>lambda { puts "lambda" }} do
puts "In bar block"
end

# succeeds???
foo arm1=>1, arm2=>lambda { puts "lambda"; } do
puts "In bar block"
end

# succeeds???
foo (arm1=>1, arm2=>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
--
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Hi --

On Sat, 6 Dec 2008, Charlton Wang wrote:

> 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

--
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Charlton Wang wrote:

> 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 arm1=>1, arm2=>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:0x02b14a5c@(irb):6>
in block
==>nil

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

William James wrote:
> Charlton Wang wrote:
>
>>
>>
>> # fails syntax error
>> foo arm1=>1, arm2=>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:0x02b14a5c@(irb):6>
> in block
> ==>nil

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Charlton Wang wrote:
>
> # fails syntax error
> foo arm1=>1, arm2=>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 arm1=>1, arm2=>lambda { puts("lambda") } do
puts "In bar block"
end

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The Higgs bozo wrote:
> Charlton Wang wrote:
>>
>> # fails syntax error
>> foo arm1=>1, arm2=>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 arm1=>1, arm2=>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!

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