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comp.lang.ruby

Point free (pointless) programming in ruby?

John Carter

1/13/2009 11:11:00 PM

I'm very fond of the notion of Concatenative Languages such as Joy,
Factor...
http://en.wikipedia.org...(programming_language)
http://www.latrobe.edu.au/philosophy/phimvt/joy/jp-j...
http://facto...

There is a supreme simplicity about them.

So while reading what the current state of them is, I stumbled across
the notion of Point free (sometimes called Pointless) programming.

The idea of sequences of function compositions that elide the
arguments that they will be applied to.

http://www.haskell.org/haskellwiki...

One can easily define a sequence of methods in Ruby...

fun_seq = [:a, :b, :c]

as a sequence of symbols.

Which one could apply to an arbitrary argument..

irb
> f = [:to_s, :succ]
=> [:to_s, :succ]
> f.inject(4,:send)
=> "5"

Oooh! Looky! That's sneaky of me!
symbol_sequence.inject(arg,:send)

[:a, :b, :c].inject(arg,:send)
that's equivalent to
arg.a.b.c

Not quite function composition. Cute, but let's try...

irb
> def a(a)
> p ["a",a]
> a+"a"
> end
=> nil
> def b(b)
> p ["b",b]
> b+"b"
> end
=> nil
> def c(c)
> p ["c",c]
> c+"c"
> end
=> nil
> f=[:a,:b,:c]
=> [:a, :b, :c]
> f.inject("foo"){|memo,obj|send(obj,memo)}
["a", "foo"]
["b", "fooa"]
["c", "fooab"]
=> "fooabc"

[:a, :b, :c].inject(arg){|memo,obj|send(obj,memo)}
is equivalent to
c(b(a(arg)))

> c(b(a("bah")))
["a", "bah"]
["b", "baha"]
["c", "bahab"]
=> "bahabc"

Or how about working with lambda's or Proc objects...

> a1 = lambda {|x| x+"a"}
=> #<Proc:0xb7d9f744@(irb):41>
> b1 = lambda {|x| x+"b"}
=> #<Proc:0xb7e0feb8@(irb):42>
> c1 = lambda {|x| x+"c"}
=> #<Proc:0xb7dfa5e0@(irb):43>
> [a1, b1, c1]
=> [#<Proc:0xb7d9f744@(irb):41>, #<Proc:0xb7e0feb8@(irb):42>, #<Proc:0xb7dfa5e0@(irb):43>]
> [a1, b1, c1].inject("foo"){|memo,proc| proc.call(memo)}
=> "fooabc"


Anyhoo! Clearly in principle one can do Pointfree programming Ruby.

Questions for the Group:

1) Is there a neater way of expressing a sequence of function
compositions in Ruby?

2) Which Ruby Pointfree sequences are actually useful?



John Carter Phone : (64)(3) 358 6639
Tait Electronics Fax : (64)(3) 359 4632
PO Box 1645 Christchurch Email : john.carter@tait.co.nz
New Zealand
3 Answers

Brian Candler

1/14/2009 10:18:00 AM

0

> 1) Is there a neater way of expressing a sequence of function
> compositions in Ruby?

Maybe you could define an Array#to_proc. e.g.

class Array
def to_proc
lambda { |src| inject(src) { |memo,proc| proc[memo] } }
end
end

a1 = lambda { |x| x+"a1" }
b1 = lambda { |x| x+"b1" }
c1 = lambda { |x| x+"c1" }
p lambda(&[a1,b1,c1])["test"]

src = ["foo","bar","baz"]
p src.map(&[a1, b1, c1])


def a(x); x+[1]; end
def b(x); x+[2]; end

src = [[], [0], [0,0]]

p src.map(&[method(:a), method(:b)])
--
Posted via http://www.ruby-....

John Carter

1/15/2009 11:02:00 PM

0

On Wed, 14 Jan 2009, Brian Candler wrote:

>> 1) Is there a neater way of expressing a sequence of function
>> compositions in Ruby?
>
> Maybe you could define an Array#to_proc. e.g.
>
> class Array
> def to_proc
> lambda { |src| inject(src) { |memo,proc| proc[memo] } }
> end
> end

> p src.map(&[method(:a), method(:b)])

Cool! I had to spend a bit of time rereading the ri docs and playing
with irb to work out what you were doing... :-) ...but very cool
nevertheless.

Ooh.. I bet somebody could...
a) Win an obfuscated programming contest...
b) Do something very very cool and surprisingly useful..
with the stuff.

Basically what I'm seeing here is Ruby is actually a superset of the
concatenative languages. If you stick within a strict subset of Ruby
you can do the almost magical rewrite algebras that you can do in
something like Joy..

http://www.latrobe.edu.au/philosophy/phimvt/joy/j...

John Carter Phone : (64)(3) 358 6639
Tait Electronics Fax : (64)(3) 359 4632
PO Box 1645 Christchurch Email : john.carter@tait.co.nz
New Zealand

Brian Candler

1/16/2009 10:23:00 AM

0

John Carter wrote:
> Cool! I had to spend a bit of time rereading the ri docs and playing
> with irb to work out what you were doing... :-) ...but very cool
> nevertheless.

Sorry, I should have added: this was nothing clever on my behalf. I was
just copying Symbol#to_proc (which you can google for, and has been
incorporated into both Rails and ruby 1.9)
--
Posted via http://www.ruby-....