From: Joey <rubytalk@...>
Date: 2006-05-09T04:40:56+09:00
Subject: Re: ml style 'let' impl - fun with metaprogramming...

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http://chneukirchen.org/blog/archive/2005/04/dynamic-variables-in-ruby.html

On 5/8/06, ara.t.howard@noaa.gov <ara.t.howard@noaa.gov> wrote:
>
>
> i was just playing with this - an ml style let for ruby:
>
>
>      harp:~ > cat a.rb
>      require 'let'
>      #
>      # let allows you to setup local variables that will only be in scope
> for the
>      # following block in an ml style
>      #
>        let :list =3D> [] do
>          list << 42
>          p list # =3D> [42]
>        end
>      #
>      # you can, of course, nest blocks
>      #
>        let :list =3D> [] do
>          list << 'forty-two'
>
>          let :list =3D> [] do
>            list << 42
>            p list # =3D> [42]
>          end
>
>          p list # =3D> ['forty-two']
>        end
>      #
>      # the return value of let allows one to get a handle on the context
> to
>      # retrieve values later if needed.
>      #
>        context =3D
>          let :a =3D> nil, :b =3D> nil do
>            self.a =3D 4
>            self.b =3D 2
>          end
>        p [context.a, context.b] # [4, 2]
>      #
>      # note that the special variable '_' is a shorthand for 'self' insid=
e
> the
>      # block.
>      #
>        context =3D
>          let :a =3D> nil, :b =3D> nil do
>            _.a =3D 4
>            _.b =3D 2
>          end
>        p [context.a, context.b] # [4, 2]
>      #
>      # 'open' getter/setters are allowed
>      #
>        context =3D
>          let :a =3D> nil, :b =3D> nil do
>            a 4
>            b 2
>          end
>        p [context.a, context.b] # [4, 2]
>      #
>      # 'let' params may be a list or hash.  iff list values default to
> nil.
>      #
>        context =3D let(:a, :b =3D> 2){ a 4 }
>        p [context.a, context.b] # [4, 2]
>
>
>      harp:~ > ruby a.rb
>      [42]
>      [42]
>      ["forty-two"]
>      [4, 2]
>      [4, 2]
>      [4, 2]
>      [4, 2]
>
>
>      harp:~ > cat let.rb
>      def let *argv, &b
>      #--{{{
>        hashes, list =3D argv.partition{|arg| Hash =3D=3D=3D arg}
>        hashes.inject(h=3D{}){|h,h2| h.update h2}
>        list.each{|elem| h[elem] =3D nil}
>
>        blank_slate =3D Class.new{
>          %w( inspect instance_eval instance_variable_get
> instance_variable_set ).each do |m|
>            alias_method "__#{ m }__", "#{ m }"
>          end
>
>          instance_methods.each{|m| m !~ %r/^__/ and undef_method m}
>
>          def _(); @_ ||=3D self; end
>
>          h.each do |k,v|
>            get, set, iff, var =3D "#{ k }", "#{ k }=3D", "#{ k }?", "@#{ =
k }"
>            define_method(get) do |*argv|
>              case argv.size
>                when 0
>                  unless eval("defined? #{ var }")
>                    __instance_variable_set__ var, v
>                  end
>                  __instance_variable_get__ var
>                when 1
>                  __send__ set, *argv
>                else
>                  raise ArgumentError, "#{ argv.size } instead of 0 or 1"
>              end
>            end
>            define_method(set) do |arg|
>              __instance_variable_set__ var, arg
>            end
>            alias_method iff, get
>          end
>
>        }.new
>
>        blank_slate.__instance_eval__ &b
>        blank_slate
>      #--}}}
>      end
>
>
> it's kind of fun but perhaps not incredibly useful.  thoughts?
>
> -a
> --
> be kind whenever possible... it is always possible.
> - h.h. the 14th dali lama
>
>

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