From: Jim Weirich <jim@...>
Date: 2006-05-18T23:59:00+09:00
Subject: Re: Partial functions?

Berger, Daniel wrote:
> Hi all,
> 
> I'm trying to understand a new feature of Python 2.5 (PEP 309) called
> "partial functions".  Here's the Python example I pulled from Vincent
> Foley's blog:

[... curry example elided ...]

> I understand what it's doing, I just don't understand what purpose of
> partial functions would be.  Can someone provide a use case for me?

Suppose you have a collection of integers, say a = [1,2,3].  And you 
wanted to create a new array consisting of all the integers in a, but 
multiplied by 3.  We have an array method (collect) that constructs a 
new array by calling a function of one argument on each element of the 
array.  Now we just need to create a function of one argument to pass to 
collect:

  # Convert a 2 arg operator (*)
  # into a 1 arg function (ie. a lambda).
  multiply_by_3 = lambda { |n| n * 3 }

  a = [1,2,3]
  a_times_3 = a.collect(&multiply_by_3)

> Also, can/should we do this in Ruby?

We do it all the time.  We just don't generally bother to give the newly 
created function a name:

   a_times_3 = a.collect { |n| n * 3 }

-- Jim Weirich

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