From: Dan Doel <djd15@...>
Date: 2003-08-27T16:45:59+09:00
Subject: Blocks, eval and named parameters

Hello,

So I was reading the brief discussion on named parameters in the 
gigantic troll-fueled thread (forward
note, the hash option wasn't discussed, and I do know about it, so I 
don't need to be lectured :) ), and I
saw the example of named parameters in a constructor like so:

class Foo
    attr_accessor :foo, :bar
    def initialize
       ...
       yield self if block_given?
    end
end

Foo.new { |obj| obj.foo = 5; obj.bar = 6 }

And someone liked it.  However, this isn't really any better than:

obj = Foo.new
obj.foo = 5
obj.foo = 6

Unless you make the accessors private, in which case I think you need to 
instance_eval the block.

So, I got to thinking of a way to possibly use a block to assign to 
local variables.  For example,
thinking of the up-and-coming scoping rule changes, maybe one could do:

class Foo
    def initialize(&blk)
       if blk
          instance_eval &blk
       end

       @height = height
    end
end

Foo.new do height = 5 end

However, then I thought, perhaps this already works like so:

class Foo
    def initialize(height=nil, &blk)
       if blk then instance_eval &blk end
       @height = height
    end
end

Since height will be defined as a local variable, and should propogate 
out.  However, instance_eval
doesn't work this way, so I doubt that it will work with the new scoping 
rules.

So, finally for my question:

instance_eval, class_eval and module_eval all optionally take code 
blocks to evaluate in a certain
context. However, eval itself only takes a string.  Is there, or will 
there ever be a way to evaluate
a code block with an arbitrary binding object/in an arbitrary context, 
so that something like the
above will work?  I suppose a block used in such a way is no longer a 
closure, but it could be
an interesting addition to Proc's (note, I'm not sure how much utility 
it would have outside this
example).

Until then, I think the closest I can come to named parameters using 
blocks is:

class Foo
    def initialize(&blk)
       if blk
          bnd = blk.call
          @height = eval "height", bnd
       end
    end
end

Foo.new do height = 6; binding end

Then again, writing to allow:

Foo.new(:height => 6)

Isn't terribly hard, looks fine, and allows you to still pass a block 
for other purposes if you need it,
so I don't think things should be changed merely for this example's sake.

Anyhow, any thoughts on all this? Thanks in advance for the stuff I'll 
probably learn from any discussion
that ensues.

- Dan