Advanced Ruby: Eval over multiple Bindings?

Ruby
1

This is an extremely advanced Ruby question. Given:

def binding1
a = 1
binding
end

def binding2
b = 2
binding
end

eval(‘a’, binding1) #=> 1
eval(‘b’, binding2) #=> 2

Is there a way to “merge” the binding to do something like:

eval(‘a + b’, binding1 + binding2) #=> 3

2

The only method directly on binding is eval, so you’d have to do it in
some sort of manual fashion.

3

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Am 12.11.2011 21:19, schrieb Intransition:

Is there a way to “merge” the binding to do something like:

eval(‘a + b’, binding1 + binding2) #=> 3

Not sure if that’s exactly what you want, but this works:

binding1.eval(“a + binding2.eval(‘b’)”) #=> 3

This is not ‘merging’ the two bindings, it rather opens the second
binding inside the first one.

Regarding Binding#eval, is there a specific reason why it can’t take a
block instead of a string, the same way e.g. Module#instance_eval does?

Vale,
Marvin
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4

On Saturday, November 12, 2011 4:10:09 PM UTC-5, Quintus wrote:

binding1.eval(“a + binding2.eval(‘b’)”) #=> 3

This is not ‘merging’ the two bindings, it rather opens the second
binding inside the first one.

My usecase is for template generation. For example ERB, it’s render
method
takes a Binding.

Regarding Binding#eval, is there a specific reason why it can’t take a
block instead of a string, the same way e.g. Module#instance_eval does?

Actually I suggested that feature on ruby issue tracker and I believe
the
anwser given was YAGNI.

If it were possible could it solve this?

5

There doesn’t seem to be a way to ‘merge’ the bindings, but there
are ways around it, I bet. This is kinda hacky but it works for
this simple case at least:

class Binding

def +(other)
raise “Other has to be a binding” unless Binding === other
values = []
instance_variables.each do |iv|
values << "#{iv[1…-1]} = #{instance_variable_get(iv)}; "
end
other.instance_variables.each do |iv|
values << "#{iv[1…-1]} = #{other.instance_variable_get(iv)}; "
end
binding.tap do |bind|
bind.eval values.join
end
end

end

class Test
def binding1
a = 1
binding.tap do |_b|
local_variables.each do |lv|
_b.instance_variable_set("@#{lv}", _b.eval(lv.to_s)) unless lv
=~ /_b/
end
end
end

def binding2
b = 2
binding.tap do |_b|
local_variables.each do |lv|
_b.instance_variable_set("@#{lv}", _b.eval(lv.to_s)) unless lv
=~ /_b/
end
end
end

end

t = Test.new

puts eval(‘a + b’, t.binding1 + t.binding2) # => 3

hahah, at least I tried.

-Luke

6

On Sat, Nov 12, 2011 at 4:17 PM, Intransition [email protected]
wrote:

Not sure if thats exactly what you want, but this works:

I needed it once, while trying to write a tool for acceptance tests. Gave
up on it b/c this feature didn’t exist.

7

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Am 12.11.2011 23:17, schrieb Intransition:

If it were possible could it solve this?

I don’t think so. I just wondered.

Vale,
Marvin

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8

Da: Intransition [mailto:[email protected]]
Inviato: sabato 12 novembre 2011 21:20
A: ruby-talk ML; [email protected]
Oggetto: Advanced Ruby: Eval over multiple Bindings?

This is an extremely advanced Ruby question. Given:

def binding1
a = 1
binding
end

def binding2
b = 2
binding
end

eval(‘a’, binding1) #=> 1
eval(‘b’, binding2) #=> 2

Is there a way to “merge” the binding to do something like:

eval(‘a + b’, binding1 + binding2) #=> 3

–
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9

-----Messaggio originale-----
Da: Quintus [mailto:[email protected]]
Inviato: domenica 13 novembre 2011 09:56
A: ruby-talk ML
Oggetto: Re: Advanced Ruby: Eval over multiple Bindings?

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Am 12.11.2011 23:17, schrieb Intransition:

If it were possible could it solve this?

I don’t think so. I just wondered.

Vale,
Marvin

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