# Working around "single-pass" constraint

**URL:** <https://www.ruby-forum.com/t/working-around-single-pass-constraint/65809>\
**Category:** Ruby\
**Created:** [August 21, 2006, 4:31pm UTC](https://www.ruby-forum.com/t/working-around-single-pass-constraint/65809 "2006-08-21T16:31:33Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![iceburgh](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/c4cdca/32.png) [@iceburgh](https://www.ruby-forum.com/u/iceburgh)\
**Post date:** [August 21, 2006, 4:31pm UTC](https://www.ruby-forum.com/t/working-around-single-pass-constraint/65809/1 "2006-08-21T16:31:33Z")

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I like to order my program code files as follows:

Initial comment header

require statements, etc.

“main” code

method definitions

Unfortunately, Ruby has a “single pass” execution flow  
(a bit reminiscent of Pascal’s single-pass compiler :-/)  
that doesn’t allow this order to be used, by default.

My current hack is to wrap the “main” code in a method  
(e.g., main 🙂 and invoke it at the end of the file, I  
can get around this issue. I also like the fact that it  
(a) puts the code at the same level of indentation as the  
code in my methods and (b) limits the scope of variables:

#!/usr/bin/env ruby -w

# 

# foo - code formatting demo

def main  
bar  
end

def bar  
puts ‘Hi!’  
end

main

However, I have two concerns with this approach:

- I might be missing a common Ruby idiom (at least,  
common within the small group of folks who like to  
order their code as I do).

- I might be in danger of running into some sort of  
“dynamic language issue”, in which some method has  
to be defined early, because it gets used in main’s  
_definition_.

Comments? Clues? Suggestions?

## -r

[http://www.cfcl.com/rdm](http://www.cfcl.com/rdm) Rich M.  
[http://www.cfcl.com/rdm/resume](http://www.cfcl.com/rdm/resume) removed\_email\_address@domain.invalid  
[http://www.cfcl.com/rdm/weblog](http://www.cfcl.com/rdm/weblog) +1 650-873-7841

Technical editing and writing, programming, and web development

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**Author:** ![iceburgh](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/c4cdca/32.png) [@iceburgh](https://www.ruby-forum.com/u/iceburgh)\
**Post date:** [August 21, 2006, 4:41pm UTC](https://www.ruby-forum.com/t/working-around-single-pass-constraint/65809/2 "2006-08-21T16:41:35Z")

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On 21.08.2006 16:29, Rich M. wrote:

> Unfortunately, Ruby has a “single pass” execution flow  
> #!/usr/bin/env ruby -w
> 
> main
> 
> However, I have two concerns with this approach:
> 
> - I might be missing a common Ruby idiom (at least,  
> common within the small group of folks who like to  
> order their code as I do).

As I don’t belong to that group I don’t have any insight there. 🙂

Personally I don’t mind having the order Ruby imposes.

> - I might be in danger of running into some sort of  
> “dynamic language issue”, in which some method has  
> to be defined early, because it gets used in main’s  
> _definition_.

This is not an issue at all:

16:34:42 [~]: ruby -e ‘def main() foo() end; def foo() puts “icks” end;  
main()’  
icks

IOW, the method needs to be there when it’s called - not earlier.

Kind regards

```
robert
```

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<div class="post-metadata">

**Author:** ![iceburgh](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/c4cdca/32.png) [@iceburgh](https://www.ruby-forum.com/u/iceburgh)\
**Post date:** [August 21, 2006, 4:53pm UTC](https://www.ruby-forum.com/t/working-around-single-pass-constraint/65809/3 "2006-08-21T16:53:36Z")

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On Mon, 21 Aug 2006, Rich M. wrote:

> I like to order my program code files as follows:
> 
> Initial comment header

require statements, etc.

“main” code

BEGIN{  
method definitions  
}

note that there are also END{} blocks

> bar  
> end
> 
> def bar  
> puts ‘Hi!’  
> end
> 
> main

if you take tis to the next step it’s really useful

class Main  
def foo  
end  
def bar  
end  
def run  
foo and bar  
end  
end

Main.new(ENV, ARGV).run if $0 == **FILE**

the reason is that it allows your program to be used as a library  
without  
polluting the global namespace.

regards.

-a

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<div class="post-metadata">

**Author:** ![iceburgh](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/c4cdca/32.png) [@iceburgh](https://www.ruby-forum.com/u/iceburgh)\
**Post date:** [August 22, 2006, 3:06am UTC](https://www.ruby-forum.com/t/working-around-single-pass-constraint/65809/4 "2006-08-22T03:06:53Z")

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On 8/22/06, removed\_email\_address@domain.invalid [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> ```
> end
> 
> ```
> 
> end
> 
> Main.new(ENV, ARGV).run if $0 == **FILE**
> 
> the reason is that it allows your program to be used as a library without  
> polluting the global namespace.

I agree, but the OP wants to see the main algorithm at the top of the  
file (at least, I think that’s the idea), so “run” should be at the  
top:  
class Main  
def run  
foo and bar  
end  
def foo  
end  
def bar  
end  
end

Main.new(ENV, ARGV).run if $0 == **FILE**

Neat and Tidy… Tidy and Neat (  
[http://pages.eidosnet.co.uk/pobotrol/whomessy.htm](http://pages.eidosnet.co.uk/pobotrol/whomessy.htm) )

---

<div class="post-metadata">

**Author:** ![iceburgh](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/c4cdca/32.png) [@iceburgh](https://www.ruby-forum.com/u/iceburgh)\
**Post date:** [August 22, 2006, 4:20am UTC](https://www.ruby-forum.com/t/working-around-single-pass-constraint/65809/5 "2006-08-22T04:20:56Z")

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At 11:52 PM +0900 8/21/06, removed\_email\_address@domain.invalid wrote:

> Main.new(ENV, ARGV).run if $0 == **FILE**
> 
> the reason is that it allows your program to be used as a  
> library without polluting the global namespace.

Assuming that my code contains generally useful functions,  
this makes sense (and why bet the other way? :-). However,  
I’m curious whether I need to worry about the possibility  
of bringing in multiple “Main” classes and “run” methods,  
if I use this approach a lot. Do you have any relevant  
experience to offer?

## -r

[http://www.cfcl.com/rdm](http://www.cfcl.com/rdm) Rich M.  
[http://www.cfcl.com/rdm/resume](http://www.cfcl.com/rdm/resume) removed\_email\_address@domain.invalid  
[http://www.cfcl.com/rdm/weblog](http://www.cfcl.com/rdm/weblog) +1 650-873-7841

Technical editing and writing, programming, and web development