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comp.lang.ruby

Is this possible in Ruby

Damjan Rems

2/2/2009 10:49:00 PM


Just an idea which may come in handy. Can this be done and how in Ruby.

def calc(what)
some_eval(what)
end

a=10
b=12
calc('a+b') # expression as parameters
=> 12 # would return

a='aa'
b='bb'
calc('a+b')
=> 'aabb'

This is of course strictly hypothetical. I would like the expression to
be evaluated inside the method using variables which are local to the
calling part.


by
TheR
--
Posted via http://www.ruby-....
4 Answers

Sean O'Halpin

2/2/2009 11:33:00 PM

0

On Mon, Feb 2, 2009 at 10:49 PM, Damjan Rems <d_rems@yahoo.com> wrote:
>
> Just an idea which may come in handy. Can this be done and how in Ruby.
>
> def calc(what)
> some_eval(what)
> end
>
> a=10
> b=12
> calc('a+b') # expression as parameters
> => 12 # would return
>
> a='aa'
> b='bb'
> calc('a+b')
> => 'aabb'
>
> This is of course strictly hypothetical. I would like the expression to
> be evaluated inside the method using variables which are local to the
> calling part.
>
>
> by
> TheR

ruby 1.9 comes with Binding#eval. In 1.8.6 you can do this:

class Binding
def eval(str)
Kernel.eval(str, self)
end
end

a = 10
b = 12
binding.eval('a + b') # => 22

def another(c, d)
binding.eval('c + d')
end
another(20, 22) # => 42

Regards,
Sean

list. rb

2/3/2009 12:12:00 AM

0


On Feb 2, 2009, at 6:33 PM, "Sean O'Halpin" <sean.ohalpin@gmail.com>
wrote:

> On Mon, Feb 2, 2009 at 10:49 PM, Damjan Rems <d_rems@yahoo.com> wrote:
>>
>> Just an idea which may come in handy. Can this be done and how in
>> Ruby.
>>
>> def calc(what)
>> some_eval(what)
>> end
>>
>> a=10
>> b=12
>> calc('a+b') # expression as parameters
>> => 12 # would return
>>
>> a='aa'
>> b='bb'
>> calc('a+b')
>> => 'aabb'
>>
>> This is of course strictly hypothetical. I would like the
>> expression to
>> be evaluated inside the method using variables which are local to the
>> calling part.
>>
>>
>> by
>> TheR
>
> ruby 1.9 comes with Binding#eval. In 1.8.6 you can do this:
>
> class Binding
> def eval(str)
> Kernel.eval(str, self)
> end
> end
>
> a = 10
> b = 12
> binding.eval('a + b') # => 22
>
> def another(c, d)
> binding.eval('c + d')
> end
> another(20, 22) # => 42
>
> Regards,
> Sean
>

I thought in 1.8.6 it could be done.. Not positive though

a, b = 3, 5
formula="a*b"
p eval(formula)

Sean O'Halpin

2/3/2009 12:40:00 AM

0

On Tue, Feb 3, 2009 at 12:12 AM, List.rb <list.rb@gmail.com> wrote:
>
> I thought in 1.8.6 it could be done.. Not positive though
>
> a, b = 3, 5
> formula="a*b"
> p eval(formula)
>
>
D'oh. You're absolutely right. Maybe I was thinking of this:

a = 1
b = 2
def some_eval(context)
eval('a + b', context)
end

some_eval(binding) # => 3

Late night. Off to bed :)

Regards,
Sean

Robert Klemme

2/3/2009 10:06:00 AM

0

2009/2/2 Damjan Rems <d_rems@yahoo.com>:
>
> Just an idea which may come in handy. Can this be done and how in Ruby.
>
> def calc(what)
> some_eval(what)
> end
>
> a=10
> b=12
> calc('a+b') # expression as parameters
> => 12 # would return
>
> a='aa'
> b='bb'
> calc('a+b')
> => 'aabb'
>
> This is of course strictly hypothetical. I would like the expression to
> be evaluated inside the method using variables which are local to the
> calling part.

As has been demonstrated you need the binding of the context in which
you want to evaluate your expression string. I see two options.

1. explicitly, i.e.

def calc(what, bind)
eval(what, bind)
end

but this is pointless as "calc" is just an alias for "eval".

2. implicitly

You can keep track of bindings by implementing a trace function (see
set_trace_func). Argument 4 is the current binding and you can place
them in a thread local stack, e.g.

# untested
set_trace_func lambda {|*a|
case a.first
when /call$/
(Thread.current[:bindings] ||= []).push(a[4])
when /return$/
Thread.current[:bindings].pop
end
}

def calc(s)
eval(s, Thread.current[:bindings][-2])
end

Kind regards

robert

--
remember.guy do |as, often| as.you_can - without end