ERB as a macro pre-processor?

Ruby
1

“The Ruby Way” (pp. 43-45) shows a couple of different ways
to generate methods dynamically (using eval). Looking at the
examples, I began to wonder about different Ways To Do It.

One possibility that occurred to me is to use ERB as a macro
pre-processor. The book suggests the following code:

if platform == Windows
def my_action
action1
end
if platform == Linux
def my_action
action2
end
else
def my_action
default_action
end
end

Although this is approach is direct and simple, it seems like it
wouldn’t scale very well, given multiple variations. However, I
think that the following approach might:

% cat erb_ml
#!/usr/bin/env ruby

require ‘erb’

def action_D; print “default action\n”; end
def action_L; print “Linux action\n”; end
def action_W; print “Windows action\n”; end

action = { :Linux => ‘action_L’,
:Windows => ‘action_W’ }

template = ERB.new <<-EOF
def my_action
<%= action[platform] || ‘action_D’ %>
end
EOF

for platform in [ :Linux, :Windows, :Plan9 ] do
eval template.result(binding)
print "#{platform.to_s}: "
my_action
end

% erb_ml
Linux: Linux action
Windows: Windows action
Plan9: default action

Obviously, the “action” hash could be replaced by other
code-generation and/or -retrieval methods. Is anyone
here using ERB in this manner? Are there any caveats
or alternative approaches that I should be aware of?

-r

http://www.cfcl.com/rdm Rich M.
http://www.cfcl.com/rdm/resume [email protected]
http://www.cfcl.com/rdm/weblog +1 650-873-7841

Technical editing and writing, programming, and web development

2

On Wed, Oct 11, 2006 at 02:40:58PM +0900, Rich M. wrote:

end

Although this is approach is direct and simple, it seems like it
def action_L; print “Linux action\n”; end

Obviously, the “action” hash could be replaced by other
code-generation and/or -retrieval methods. Is anyone
here using ERB in this manner? Are there any caveats
or alternative approaches that I should be aware of?
Bah I say! How does that saying go, “Replace conditionals with
polymorphism”?

class Platform
end

class Linux < Platform
def define_methods
def a
puts “linux”
end
end
end

class Windows < Platform
def define_methods
def a
puts “windows”
end
end
end

class Platform
def self.get_platform
case RUBY_PLATFORM
when /linux/
Linux
when /win32/
Windows
end
end
end

platform = Platform.get_platform.new
platform.define_methods

3

On Wed, 11 Oct 2006, Rich M. wrote:

“The Ruby Way” (pp. 43-45) shows a couple of different ways
to generate methods dynamically (using eval). Looking at the
examples, I began to wonder about different Ways To Do It.

Obviously, the “action” hash could be replaced by other
code-generation and/or -retrieval methods. Is anyone
here using ERB in this manner? Are there any caveats
or alternative approaches that I should be aware of?

http://groups.google.com/group/comp.lang.ruby/browse_thread/thread/4e2f0f29308c9647/d985a8bf050d88d5?lnk=gst&q=r4&rnum=1#d985a8bf050d88d5

i did quite a bit more work on it but never released…

cheers.

-a

4

At 11:36 PM +0900 10/11/06, [email protected] wrote:

i did quite a bit more work on it but never released…

interesting stuff; why not put out a WIP snapshot?

Both r4 and ERB allow the use of arbitrary Ruby code. It
would be interesting to see which features of each work
better for what kinds of problems.

-r

http://www.cfcl.com/rdm Rich M.
http://www.cfcl.com/rdm/resume [email protected]
http://www.cfcl.com/rdm/weblog +1 650-873-7841

Technical editing and writing, programming, and web development