Ah, ok. Is there any kind of speed advantage to this approach? Or is
it more of a design/DRY technique?
Dan
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-----Original Message-----
From: list-bounce@example.com
[mailto:list-bounce@example.com] On Behalf Of Jim Weirich
Sent: Thursday, May 18, 2006 8:59 AM
To: ruby-talk ML
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
> >
> > 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 think partial functions is unapropriate name. The first thing I
thought of when I read the subject of this email was the mathematical
concept of it[0].
> 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 }
Ah, ok. Is there any kind of speed advantage to this approach? Or is
it more of a design/DRY technique?
I think giving it a name allows you to reuse it multiple times without
rewriting it. Everytime you introduce a new name function you have a
new abstraction. You could later redefine it (e.g. for speed/space
optimization) and that change would propagate to all its uses.
Having high order functions is a nice thing to have because you can
use them to build complex functions out of simpler ones. I asked a
similar question sometime ago[1].
Cheers,
Ed
[0] Partial Function -- from Wolfram MathWorld
[1] http://tinyurl.com/rtuuz
···
On 5/18/06, Berger, Daniel <Daniel.Berger@qwest.com> wrote:
--
Encontrá a "Tu psicópata favorito" http://tuxmaniac.blogspot.com
Thou shalt study thy libraries and strive not to reinvent them without cause,
that thy code may be short and readable and thy days pleasant and productive.
-- Seventh commandment for C programmers
In functional languages, the compiler can reduce all the computation
involving the curried args, storing the computation.
This can make things faster.
f x y = (do_something_expensive_with x) + y
f_15 = f 15 # takes a while because reduces
# (do_something_expensive_with 15) to a value
(1..10000).map(f_15) # executes very fast
In ruby, it makes things slower 
(Two calls instead of one, no caching of results.)
···
On 5/18/06, Berger, Daniel <Daniel.Berger@qwest.com> wrote:
Ah, ok. Is there any kind of speed advantage to this approach? Or is
it more of a design/DRY technique?
Exactly. Partial functions are another term for what some references
call 'recursively enumerable' functions, meaning a function that can
be computed by some Turing machine. I believe the appropriate term is
'Curried functions', from the logician who first proposed them,
someone called Haskell Curry (yes, *that* Haskell).
I've been programming in Ocaml too, and there it seems that curried
functions are extremely useful. In Ocaml most often I've used it to
call a function with several arguments multiple times with only a few
arguments changing between calls, or for passing a function with say 4
arguments to an HOF that expects a function with only one, I create a
curried function that keeps 3 of the 4 arguments fixed and pass that.
Since it is indeed possible to program in the functional style in
Ruby, I could imagine that the technique would be just as usable.
···
On 5/18/06, Edgardo Hames <ehames@gmail.com> wrote:
I think partial functions is unapropriate name. The first thing I
thought of when I read the subject of this email was the mathematical
concept of it[0].