From: Victor Shepelev <vshepelev@...>
Date: 2006-05-09T15:42:07+09:00
Subject: Re: Why not just create a new proc with the desired parameters?

From: Juergen Strobel [mailto:strobel@secure.at]

> >l = lambda{|a,b,c| puts "a=#{a}, b=#{b}, c=#{c}"}
> >
> >puts l.arity	#=>3
> >
> >l.bind(2, 'b')
> >
> >puts l.arity	#=>2
> >
> >l.call('a', 'c')  #a=a, b=b, c=c
> 
> >Two questions:
> >1. Does somebody think it is useful?
> >2. Does somebody already done this in some library?
> 
> 1. Binding parameters is useful sometimes.
> 2. Do you really need this, considering how simple it is to create a new
> proc:
> 
> adder = proc { |a,b| a+b }
> add_3 = proc { |a| adder.call(a,3) }  # adder bound with b=3
> 

Unfortunally, this wouldn't work in statements like:

adder = proc { |a,b| self.output(a+b) }
add_3 = proc { |a| adder.call(a,3) }  # adder bound with b=3

class A
  def output(val)
     puts val
  end 
end

A.new.instance_exec(4, &add_3) #want to output 4+3, but receive
#undefined method `output' for main:Object (NoMethodError)

Also, this bounding is less clear (somebody who would read my code, would
may to think "WTF? Why this strange new proc?"

So, library approach is better.

> That said, there are situations a library based approach is useful.
> I'd prefer solutions that create new proc objects without the bound
> parameters, much like I did above man�ally:
> 
> add_7 = bind_2nd(adder, 7)
> add_7.call(1) ==> 8
> 
> add_11 = bind_nth(adder, 1, 11)
> add_11.call(1) ==> 12

I dislike the code above. Why? Because it is not very object-oriented. I
think this way: @I do the operation with Proc, so, the most natural way
should be Proc#some_operation". Also, bind_1st, bind_2nd are C++-like, but I
don't understand why we need such specialized methods in Ruby.

At the moment, RubyMurray's approach seems to me as the most natural:

adder = proc { |a,b| self.output(a+b) }
add_3 = adder.curry(Curry::HOLE, 3) 

Here is no need to think about argument's numbers. I just provide bounding
in the same order as arguments was.

> 
> J�rgen
> 

Victor.