Blocks and procs

Ruby
1

Just curious, I’m just learning about Procs, and have already looked at
using blocks.
Are they very close in the way they function. It seems that way to me.
Perhaps I’m wrong.
Is one favoured above the other ?

TIA
Stuart

2

Hi –

On Wed, 12 Jul 2006, Dark A. wrote:

Just curious, I’m just learning about Procs, and have already looked at
using blocks.
Are they very close in the way they function. It seems that way to me.
Perhaps I’m wrong.
Is one favoured above the other ?

They’re different things. A block is a syntactic construct, part of a
method call. A Proc object is an object.

They are, however, closely related. Using special argument syntax, a
method can turn the block supplied to it into a Proc:

def meth(&block)
# block is the Proc version of the block
block.call # etc.
end

meth { # this is the block }

Also, you can use Proc objects as block substitutes:

meth &some_proc_object

David

3

On Jul 11, 2006, at 11:55 AM, Dark A. wrote:

Just curious, I’m just learning about Procs, and have already
looked at
using blocks.
Are they very close in the way they function. It seems that way to
me.
Perhaps I’m wrong.
Is one favoured above the other ?

I see David has already answered your question here, but just in case
it helps I wrote about this on my blog a while back:

http://blog.grayproductions.net/articles/2006/01/05/code-as-a-data-type

James Edward G. II

4

Good example. Also because it’s rails centric (something I’d eventually
like to get to)
I am running into some select errors though , via the select statement.
If
your up to reading it through (I don’t find any typos matched against
your
code), if not that is fine as the explanation helped me.

class ClientDB
Record = Struct.new(:client_name, :location, :projects)

def initialize
@records = [ Record.new(“Gray Productions”, “Oklahoma”,
[“Ruby Q.”, “Rails Extensions”] ),
Record.new( “Serenity Crew”, “Deep Space”,
[“Ship Enhancements”] ),
Record.new( “Neo”, “Hollywood”,
[“Rails interface for the Matrix”] ) ]
end # end of method

def select( query )
  # parse query String
  rules = Hash[*query.split(/\s*AND\s*/).map do |rule|
    rule.split(/\s*=\s*/).map { |value| value.sub(/^['"](.+)['"]$/,

‘\1’) }
end.flatten ]

  #match records
  @records.select { |record| block_given? and yield record }
    rules.all? { |field, value| record.send(field) == value }


end # end of method

end # end of class

require “pp”
db = ClientDB.new
pp db.select { |record| record.client_name != “Gray Productions” }
pp db.select { |record| record.client_name +~ /crew/i }
pp db.select { |record| record.projects.size == 1 }
pp db.select { |record| record.projects.include? “Ruby Q.” }

5

oops…this is the error:

rb:32:in `select’: wrong number of arguments (0 for 1) (ArgumentError)

6

On Jul 11, 2006, at 1:18 PM, Dark A. wrote:

I am running into some select errors though , via the select
statement.

Sure I see the trouble…

require “pp”
db = ClientDB.new
pp db.select { |record| record.client_name != “Gray Productions” }
pp db.select { |record| record.client_name +~ /crew/i }
pp db.select { |record| record.projects.size == 1 }
pp db.select { |record| record.projects.include? “Ruby Q.” }

In order to run this example, you need to use the select() code in
the article, just a paragraph above it. I’m slowly making changes
and refining a solution as I go.

Hope that helps.

James Edward G. II

7

On Jul 11, 2006, at 2:31 PM, Dark A. wrote:

     @records.select { |record| block_given? and yield record }
      rules.all? { |field, value| record.send(field) == value }


end # end of method

end # end of class

That’s from higher up in the article. Here’s the one just before the
example you showed:

class ClientDB
def select
@records.select { |record| block_given? and yield record }
end
end

Try that.

James Edward G. II

8

hmmmm…i have that code in there:

end # end of method

end # end of class

9

Got it, had to remove the former query.
Working now, on to lamda.

Stuart