From: "David A. Black" <dblack@...>
Date: 2009-01-12T11:35:58+09:00
Subject: Re: functional programming

Hi --

On Mon, 12 Jan 2009, William James wrote:

> Joel VanderWerf wrote:
>
>> Mike Gold wrote:
>>> Brian Candler wrote:
>>>> (2) to understand Mike's assertion
>>>> that Ruby lambdas are not first-class functions (i.e. "The
>> difference >> between lambda { } and a real first-class function is
>> quite profound").
>>>
>>> Ruby:
>>>
>>>   mean = lambda { |*args|
>>>     (args.inject(0) { |acc, x| acc + x })/args.size.to_f
>>>   }
>>>   data = [[1,2,3], [4,5,6,7]]
>>>
>>>   data.map(&mean) # fail
>>>   data.map(&:mean) # fail
>>>   data.map { |*numbers| mean.call(*numbers) } # fail
>>>   data.map { |numbers| mean.call(*numbers) } # phew, found it
>>>
>>> Haskell:
>>>
>>>   map mean data
>>>
>>> On this example alone, I think my characterization of "quite
>>> profound" is justified.
>>
>> Why isn't this the ruby candidate for comparison:
>>
>> mean = lambda { |ary|
>>    (ary.inject { |acc, x| acc + x })/ary.size.to_f
>> }
>> data.map(&mean) # => [2.0, 5.5]
>
> mean = proc{|a| a.inject{|s,x| s+x} / a.size.to_f }
>    ==>#<Proc:0x027949e0@(irb):19>
> data.map &mean
>    ==>[2.0, 5.5]

I believe that's the same as what Joel just posted (give or take some
cosmetics). Or is my eye skipping over something?


David

-- 
David A. Black / Ruby Power and Light, LLC
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