I agree that (f ∘ g)(x) is g(f(x)) is more intuitive from a purely
programmatic point of view.  It is "natural" for the operations to be
applied left to right, exactly like method chaining.


On 10 November 2012 13:06, duerst (Martin Dürst) <duerst@it.aoyama.ac.jp>wrote:

>
> Issue #6284 has been updated by duerst (Martin Dürst).
>
>
> marcandre (Marc-Andre Lafortune) wrote:
> > +1 for #*
> >
> > The symbol used in mathematics for function composition is a circle (∘);
> the arrows are for the definitions of functions (like lambdas) only, so #<-
> or whatever make no sense to me.
>
> Very good point.
>
> > Finally, the f ∘ g(x) is defined as f(g(x)), so there is no argument
> there either.
>
> Not true. Depending on which field of mathematics you look at, either (f ∘
> g)(x) is either f(g(x)), or it is g(f(x)). The later is in particular true
> in work involving relations, see e.g.
> http://en.wikipedia.org/wiki/Composition_of_relations#Definition.
>
> Speaking from a more programming-related viewpoint, f(g(x)) is what is
> used e.g. in Haskell, and probably in many other functional languages, and
> so may be familiar with many programmers.
>
> However, we should take into account that a functional language writes
> e.g. reverse(sort(array)), so it makes sense to define revsort = reverse *
> sort (i.e. (f ∘ g)(x) is f(g(x))). But in Ruby, it would be
> array.sort.reverse, so revsort = sort * reverseve may feel much more
> natural (i.e. (f ∘ g)(x) is g(f(x))).
>
> ----------------------------------------
> Feature #6284: Add composition for procs
> https://bugs.ruby-lang.org/issues/6284#change-32728
>
> Author: pabloh (Pablo Herrero)
> Status: Feedback
> Priority: Normal
> Assignee: matz (Yukihiro Matsumoto)
> Category:
> Target version: 2.0.0
>
>
> =begin
> It would be nice to be able to compose procs like functions in functional
> programming languages:
>
>     to_camel = :capitalize.to_proc
>     add_header = ->val {"Title: " + val}
>
>     format_as_title = add_header << to_camel << :strip
>
> instead of:
>
>     format_as_title = lambda {|val| "Title: " + val.strip.capitalize }
>
>
> It's pretty easy to implement in pure ruby:
>
>   class Proc
>     def << block
>       proc { |*args| self.call( block.to_proc.call(*args) ) }
>     end
>   end
> =end
>
>
> --
> http://bugs.ruby-lang.org/
>
>


-- 
  Matthew Kerwin, B.Sc (CompSci) (Hons)
  http://matthew.kerwin.net.au/
  ABN: 59-013-727-651

  "You'll never find a programming language that frees
  you from the burden of clarifying your ideas." - xkcd