Hello all,
Does Ruby have something like “#define” for C/C++? Especially with
arguments like “#define foo(x) (something_done_with_x(x))”.
Please don’t say that “def” do the same. It doesn’t.
Thanks,
Michael
Michael S. wrote:
Hello all,
Does Ruby have something like “#define” for C/C++? Especially with
arguments like “#define foo(x) (something_done_with_x(x))”.Please don’t say that “def” do the same. It doesn’t.
The closest analogue is to conditionally define a constant, then used
defined? on that constant.
For example, I do something like this in the windows-pr project, where
some methods are not supported by Win2k or earlier:
module Windows
module Console
begin
AttachConsole = Win32API.new(‘kernel32’,‘AttachConsole’,‘L’, ‘I’)
rescue LoadError
# Requires Windows XP or later
end
end
end
And then, while using the above module:
class MyConsoleClass
include Windows::Console
def test
if defined? AttachConsole
# do something
else
# do something else
end
end
end
Regards,
Dan
Use the P4 preprocessor on your scripts 
You’ll get #define EXACTLY.
No, but seriously - text substitute macros are awful. There’s a good
reason that Matz didn’t offer them in Ruby.
If you want real macros, there’s a sort of a way to do it with
ParseTree, but it’s kind of complicated AFAIK (never tried).
Tell us what you want to do and we’ll probably find a way to do it
without the awful #define.
Aur
On Mar 12, 1:52 am, Daniel B. [email protected] wrote:
end
else
# do something else
end
end
endRegards,
Dan
I was talking more about macros rather then constants.
But I agree that constant are convenient way to substitute “#define
myconst 15”. But can you substitute “#define myfunc(x)
(sin(x)*cos(x)*exp(x))”?
You should know that “x” could be as much other functions as a
constant/variable.
On 3/11/07, SonOfLilit [email protected] wrote:
Use the P4 preprocessor on your scripts
No, but seriously - text substitute macros are awful. There’s a good
reason that Matz didn’t offer them in Ruby.
There’s also the interesting problem of what the semantics of changing
a text macro in a dynamically compiled language like ruby. Let’s say
you use a macro expansion in a method, and then later change the
macro. Do the methods which use that macro then need to be
re-compiled? What if you introduce a new macro that has the same name
as a non-macro name.
–
Rick DeNatale
My blog on Ruby
http://talklikeaduck.denhaven2.com/
On Mar 12, 9:03 am, “Rick DeNatale” [email protected] wrote:
macro. Do the methods which use that macro then need to be
re-compiled? What if you introduce a new macro that has the same name
as a non-macro name.–
Rick DeNataleMy blog on Rubyhttp://talklikeaduck.denhaven2.com/
I agree there are some semantic problems. But there are huge
advantages as well.
I’d prefer for have some warning about redefinitions.
Mike wrote:
rescue LoadError def testDan
I was talking more about macros rather then constants.
But I agree that constant are convenient way to substitute “#define
myconst 15”. But can you substitute “#define myfunc(x)
(sin(x)*cos(x)*exp(x))”?
You should know that “x” could be as much other functions as a
constant/variable.
I would be interested in hearing what you want to do with text
substitution that you cannot already do with standard Ruby
metaprogramming techniques.
On 3/12/07, Timothy H. [email protected] wrote:
beginclass MyConsoleClass
constant/variable.
I would be interested in hearing what you want to do with text
substitution that you cannot already do with standard Ruby
metaprogramming techniques.
Easy.
Let’s we have following function:
- set_default(var, val)
– if variable “var” is not defined (not existed) then assign value
“val”
else do nothing.
The variable could be ANY type including “Fixnum”.
Try to implement it using standard techniques.
On 3/11/07, Michael S. [email protected] wrote:
Let’s we have following function:
- set_default(var, val)
– if variable “var” is not defined (not existed) then assign value “val”
else do nothing.The variable could be ANY type including “Fixnum”.
Try to implement it using standard techniques.
var ||= val
On 3/11/07, Alexey V. [email protected] wrote:
Try to implement it using standard techniques.
var ||= val
Although this will only work with @var
On Mon, 12 Mar 2007, Michael S. wrote:
The variable could be ANY type including “Fixnum”.
Try to implement it using standard techniques.
harp:~ > cat a.rb
unless defined? var
var = 42
end
p var
harp:~ > ruby a.rb
42
-a
On Mon, 12 Mar 2007, Mike wrote:
I was talking more about macros rather then constants. But I agree that
constant are convenient way to substitute “#define myconst 15”. But can you
substitute “#define myfunc(x) (sin(x)*cos(x)*exp(x))”? You should know that
“x” could be as much other functions as a constant/variable.
perhaps i’m being dense, but i fail to see how the above example is none
other
than a simple function in ruby:
harp:~ > cat a.rb
def myfunc x
Math.sin(x) * Math.cos(x) * Math.exp(x)
end
# x as function
def x() 42 end
p myfunc(x)
# x as constant
X = 42
p myfunc(X)
# x as variable
x = 42
p myfunc(x)
harp:~ > ruby a.rb
6.37609775534436e+17
6.37609775534436e+17
6.37609775534436e+17
text macros like the above are really only useful in static/strong typed
languages. in ruby, they are rather meaningless.
regards.
-a
[email protected] wrote:
On Mon, 12 Mar 2007, Mike wrote:
I was talking more about macros rather then constants. But I agree that
constant are convenient way to substitute “#define myconst 15”. But can
you substitute “#define myfunc(x) (sin(x)*cos(x)*exp(x))”? You should
know that “x” could be as much other functions as a constant/variable.perhaps i’m being dense, but i fail to see how the above example is none
other than a simple function in ruby:
Imagine you would call the function with rand() as parameter. With
#define
rand would be called three times, with def it would be called once and
its
value would be used three times.
On Mon, Mar 12, 2007 at 05:16:08PM +0900, Sebastian H. wrote:
Imagine you would call the function with rand() as parameter. With #define
rand would be called three times, with def it would be called once and its
value would be used three times.
???
irb(main):001:0> def myfunc(x); Math.sin(x)*Math.cos(x)*Math.exp(x); end
=> nil
irb(main):002:0> p myfunc(rand)
0.44720293351854
=> nil
irb(main):003:0> p myfunc(rand)
1.07121692170638
=> nil
irb(main):004:0> p myfunc(rand)
1.01827292445777
=> nil
Now, if you store the value of rand within the function, that’s a
different case. But Ruby has different idioms for deferred execution:
one of
those is
class Foo
def initialize(&blk)
@gen = blk
end
def value
@gen.call
end
end
a = Foo.new { rand }
puts a.value
puts a.value
puts a.value
On Mon, Mar 12, 2007 at 12:56:26PM +0900, Michael S. wrote:
The variable could be ANY type including “Fixnum”.
Try to implement it using standard techniques.
Do you really need to abstract out “var ||= val” into its own function?
If for some reason you do, then local variables are not the right
abstraction. Local variables are statically created when code is parsed
(*);
local variables are not objects; and you cannot take references to them.
Use instance variables on an object instead.
Regards,
Brian.
(*) which means you can create them using “eval”, but I’m pretty sure
you
don’t want to do that.
On Mar 12, 4:07 pm, [email protected] wrote:
The variable could be ANY type including “Fixnum”.
harp:~ > ruby a.rb
42-a
be kind whenever possible… it is always possible.
- the dalai lama
I know it but imagine you must do it 100 times?
I’d rather write:
set_default(var1, 15)
set_default(myvar1, “string”)
…
set_default(lastvar, myhash)
In your case I must use “var” twice. Don’t you think it is too much?
On Mar 12, 4:21 pm, [email protected] wrote:
harp:~ > cat a.rb p myfunc(X)text macros like the above are really only useful in static/strong typed
languages. in ruby, they are rather meaningless.regards.
-a
be kind whenever possible… it is always possible.
- the dalai lama
Just try to implement:
set_default(var, val)
– if variable “var” is not defined (not existed) then assign value
“val”
On Mar 12, 7:16 pm, Sebastian H. [email protected]
wrote:
Imagine you would call the function with rand() as parameter. With #define
rand would be called three times, with def it would be called once and its
value would be used three times.–
Ist so, weil ist so
Bleibt so, weil war so
Just try to implement:
set_default(var, val)
– if variable “var” is not defined (not existed) then assign value
“val”
On Mar 12, 7:47 pm, Brian C. [email protected] wrote:
=> nil
puts a.value
puts a.value
puts a.value
Just try to implement:
set_default(var, val)
– if variable “var” is not defined (not existed) then assign value
“val”
On Mon, Mar 12, 2007 at 06:05:25PM +0900, Mike wrote:
irb(main):003:0> p myfunc(rand)
class Foo
puts a.value
puts a.valueJust try to implement:
set_default(var, val)
– if variable “var” is not defined (not existed) then assign value
“val”
I think we have a stuck record here 
For this particular example, the code you are looking for is:
var ||= val
with no method call at all. This is a tiny pattern and not worth
abstracting.
Local variables are static. Local variables are not intended to be
handled
in the way you are asking. The language provides other facilities (i.e.
not
local variables) when you want to be more dynamic.
“Doctor, it hurts when I do this.”
“Then don’t do that!”
Try the following if you don’t understand what I mean by local variables
being “static”.
def x
“This method is never called”
end
if false
x = “Local variable”
end
puts x
This code prints “nil”. Reflect on why this is the case.
Regards,
Brian.