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comp.lang.ruby

Block Passing and &foo Parameters

Brian McCallister

3/23/2005 9:11:00 PM

My understanding of the &parameter syntax is that it just wraps a
passed block in a proc:

irb(main):003:0> def one(&func)
irb(main):004:1> func.call
irb(main):005:1> end
=> nil
irb(main):006:0> one { puts 1 }
1
=> nil
irb(main):007:0>

What would break if we had the &param parameter actually act as a
declared parameter which must still be at the end:

irb(main):008:0> one(lambda {puts 1})
ArgumentError: wrong number of arguments (1 for 0)
from (irb):8:in `one'
from (irb):8
from :0
irb(main):009:0>

You can approximate this now with a default for the argument:

irb(main):011:0> def two(func = lambda { yield })
irb(main):012:1> func.call
irb(main):013:1> end
=> nil
irb(main):014:0> two { puts 1}
1
=> nil
irb(main):015:0> two(lambda { puts 2} )
2
=> nil
irb(main):016:0>

The advantage to something like this is if you have one function you
want to pass to a number of things, such as lists:

irb(main):016:0> a = [1,2,3]
=> [1, 2, 3]
irb(main):017:0> b = [4,5,6]
=> [4, 5, 6]
irb(main):018:0> func = lambda {|i| puts i}
=> #<Proc:0x0034b1b8@(irb):18>
irb(main):019:0> a.each func
ArgumentError: wrong number of arguments (1 for 0)
from (irb):19:in `each'
from (irb):19
from :0
irb(main):020:0> b.each func
ArgumentError: wrong number of arguments (1 for 0)
from (irb):20:in `each'
from (irb):20
from :0
irb(main):021:0>

You can add this now via:

irb(main):031:0> class Array
irb(main):032:1> alias old_each each
irb(main):033:1> def each(func = lambda {|i| yield i})
irb(main):034:2> old_each {|i| func.call i}
irb(main):035:2> end
irb(main):036:1> end
=> nil
irb(main):037:0> [1,2,3].each func
1
2
3
=> [1, 2, 3]
irb(main):038:0>

What would break if a special case allowed for passing of a Proc in the
place of &param in place of passing in a block?

-Brian
2 Answers

Assaph Mehr

3/23/2005 11:05:00 PM

0



> The advantage to something like this is if you have one function you
> want to pass to a number of things, such as lists:
>
> irb(main):016:0> a = [1,2,3]
> => [1, 2, 3]
> irb(main):017:0> b = [4,5,6]
> => [4, 5, 6]
> irb(main):018:0> func = lambda {|i| puts i}
> => #<Proc:0x0034b1b8@(irb):18>
> irb(main):019:0> a.each func
> ArgumentError: wrong number of arguments (1 for 0)
> from (irb):19:in `each'
> from (irb):19
> from :0
> irb(main):020:0> b.each func

You can easily pass a lambda to somethig that expects a block - just
put another & before lambda param at the point of call:

irb(main):001:0> a = [1,2,3]
=> [1, 2, 3]
irb(main):002:0> b = [4,5,6]
=> [4, 5, 6]
irb(main):003:0> func = lambda{|i| puts i}
=> #<Proc:0x02c2d6d0@(irb):3>
irb(main):004:0> a.each &func
1
2
3
=> [1, 2, 3]

Brian McCallister

3/24/2005 1:30:00 AM

0

Ah hah! Thanks, I knew this had to be way too common to not have been
handled. Thank you!

-Brian

On Mar 23, 2005, at 6:09 PM, Assaph Mehr wrote:

>
>
>> The advantage to something like this is if you have one function you
>> want to pass to a number of things, such as lists:
>>
>> irb(main):016:0> a = [1,2,3]
>> => [1, 2, 3]
>> irb(main):017:0> b = [4,5,6]
>> => [4, 5, 6]
>> irb(main):018:0> func = lambda {|i| puts i}
>> => #<Proc:0x0034b1b8@(irb):18>
>> irb(main):019:0> a.each func
>> ArgumentError: wrong number of arguments (1 for 0)
>> from (irb):19:in `each'
>> from (irb):19
>> from :0
>> irb(main):020:0> b.each func
>
> You can easily pass a lambda to somethig that expects a block - just
> put another & before lambda param at the point of call:
>
> irb(main):001:0> a = [1,2,3]
> => [1, 2, 3]
> irb(main):002:0> b = [4,5,6]
> => [4, 5, 6]
> irb(main):003:0> func = lambda{|i| puts i}
> => #<Proc:0x02c2d6d0@(irb):3>
> irb(main):004:0> a.each &func
> 1
> 2
> 3
> => [1, 2, 3]
>
>
>