How to evaluate file as Ruby code?

Ruby
1

How do I evaluate a text file as a sequence of Ruby statements?
(For those who happen to know Perl: I would like to have an
equivalent of the Perl ‘do FILENAME’ feature).

For example, I have a file “foo” which contains the lines

abc=4
def=5

I would like to “evaluate” the file such that after this, the
variable abc is set to 4 and def is set to 5.

I tried the following:

eval(File.new(“foo”).read)

But after this, abc is still undefined. What am I doing wrong here?

Ronald

2

Ronald F. wrote:

variable abc is set to 4 and def is set to 5.

I tried the following:

eval(File.new(“foo”).read)

But after this, abc is still undefined. What am I doing wrong here?

Ronald

require will load and evaluate the file once, but will not load
a file that has already been 'require’d

load will load and evaluate the file, it will reload and
re-evaluate.

Cheers
Chris

3

On Wed, 2006-02-22 at 23:08 +0900, Ronald F. wrote:

variable abc is set to 4 and def is set to 5.

I tried the following:

eval(File.new(“foo”).read)

But after this, abc is still undefined. What am I doing wrong here?

It’s just the way Ruby works with respect to deciding whether ‘a’ is a
variable or a method. Your assignment didn’t get ‘seen’ except inside an
eval. You can use eval again to get back there (changed the var names,
def is a keyword):

eval File.read('foo.rb')
p eval('a')
# => 4
p eval('b')
# => 5

But that’s a hack, and really I think you should consider an alternate
design - local variables are supposed to be, well, local. Maybe
constants or (though it pains me to say it) globals would be more
suitable?

Another technique I’ve used with great success is having a class that
acts as context for an external script, providing methods and holding
data in instance variables. You supply a script similar to yours above,
except using instance variables, which is then instance_eval’d in an
instance of that class, after which you have a self-contained
configuration, or whatever else it is you made.

4

For example, I have a file “foo” which contains the lines

abc=4
def=5

I would like to “evaluate” the file such that after this, the
variable abc is set to 4 and def is set to 5.

load will load and evaluate the file, it will reload and
re-evaluate.

Keep in mind that using load keeps local variables local to their scope
within the file:

tc@tc8 ~ $ cat foo.txt
a = 5
@b = 10

tc@tc8 ~ $ irb
irb(main):001:0> load ‘foo.txt’
=> true
irb(main):002:0> @b
=> 10
irb(main):003:0> a
NameError: undefined local variable or method `a’ for
#<Object:0xb7d37970
@b=10>

5

Ross B. wrote:

Another technique I’ve used with great success is having a class that
acts as context for an external script, providing methods and holding
data in instance variables. You supply a script similar to yours above,
except using instance variables, which is then instance_eval’d in an
instance of that class, after which you have a self-contained
configuration, or whatever else it is you made.

For example, http://raa.ruby-lang.org/project/script

6

On Wed, 22 Feb 2006, Ronald F. wrote:

variable abc is set to 4 and def is set to 5.

I tried the following:

eval(File.new(“foo”).read)

But after this, abc is still undefined. What am I doing wrong here?

first, def=5 is a problem because def is a reserved word
So make ‘foo’ to state
a=4
b=5

and then try:

a = b = nil
eval(File.new(‘foo’).read)
puts “a=#{a} b=#{b}”

7

load will load and evaluate the file, it will reload and
re-evaluate.

This is exactly what I’m looking for (and for my problem it is
OK that I revert to local variables).

But now I have a new problem: Exceptions thrown by ‘load’ can not
be caught!

For example,

begin
load “.defaultpar”
rescue
puts “file not found”
end

If the file does not exist, I get the error message

in `load’: no such file to load – .defaultpar (LoadError)

Of course I can circumvent it by testing before for the existence
of the file, but I wonder how I can find (from the Ruby specification),
which exceptions can be caught by “rescue” and which ones can not.

Ronald

8

Hi –

On Thu, 23 Feb 2006, Ronald F. wrote:

Of course I can circumvent it by testing before for the existence
of the file, but I wonder how I can find (from the Ruby specification),
which exceptions can be caught by “rescue” and which ones can not.

I believe rescue on its own catches RuntimeError and its descendants.
If you want to rescue something else, you can do:

rescue LoadError

David

–
David A. Black ([email protected])
Ruby Power and Light (http://www.rubypowerandlight.com)

“Ruby for Rails” chapters now available
from Manning Early Access Program! Ruby for Rails

9

I think rescue by default only catches StandardError (and descendants).

Maybe:

rescue Exception => ex

Will catch this one?

-Harold