# Instance\_eval, yield, C extensions

**URL:** <https://www.ruby-forum.com/t/instance-eval-yield-c-extensions/66718>\
**Category:** Ruby\
**Created:** [August 28, 2006, 6:43pm UTC](https://www.ruby-forum.com/t/instance-eval-yield-c-extensions/66718 "2006-08-28T18:43:30Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![danielb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/c57346/32.png) [@danielb](https://www.ruby-forum.com/u/danielb)\
**Post date:** [August 28, 2006, 6:43pm UTC](https://www.ruby-forum.com/t/instance-eval-yield-c-extensions/66718/1 "2006-08-28T18:43:30Z")

</div>

Hi all,

How do I write the code below within a C extension? I’ve seen a few  
threads  
about it (including ruby-talk: 86105) but I’m just not “getting it”.

# Assume Foo is a subclass of String

module Kernel  
def my\_meth  
instance\_eval{ Foo.new(yield) }  
end  
end

With that I can do my\_meth{ “some\_string” } and get back a String  
object.

I tried this:

static VALUE foo\_block(VALUE self){  
return rb\_funcall(cFoo, rb\_intern(“new”), 0, rb\_yield(self));  
}

static VALUE kernel\_foo(VALUE self){  
VALUE v\_proc = rb\_proc\_new(foo\_block, self); /\* Also tried Qnil \*/  
return rb\_funcall(rb\_cObject, rb\_intern(“instance\_eval”), 0,  
v\_proc);  
}

rb\_define\_method(rb\_mKernel, “my\_meth”, kernel\_foo, 0);

But this doesn’t seem to work. When I try I do call my\_meth{  
“some\_string” } I  
get `instance\_eval’: block not supplied (ArgumentError)

What am I doing wrong here?

Thanks,

Dan

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**Author:** ![danielb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/c57346/32.png) [@danielb](https://www.ruby-forum.com/u/danielb)\
**Post date:** [August 28, 2006, 8:58pm UTC](https://www.ruby-forum.com/t/instance-eval-yield-c-extensions/66718/2 "2006-08-28T20:58:11Z")

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Daniel B. [2006-08-29 01:41]:

Hi Daniel,  
I never worked with blocks in a C extension like you do, but I noticed a  
general problem in your code:

> static VALUE foo\_block(VALUE self){  
> return rb\_funcall(cFoo, rb\_intern(“new”), 0, rb\_yield(self));  
> ^  
> You’re passing one argument (the result of rb\_yield()), yet you tell  
> ruby to expect zero arguments. So make that 0 a 1.

> static VALUE kernel\_foo(VALUE self){  
> VALUE v\_proc = rb\_proc\_new(foo\_block, self); /\* Also tried Qnil \*/  
> return rb\_funcall(rb\_cObject, rb\_intern(“instance\_eval”), 0, v\_proc);

Some problem here.

HTH,  
Tilman

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**Author:** ![danielb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/c57346/32.png) [@danielb](https://www.ruby-forum.com/u/danielb)\
**Post date:** [August 28, 2006, 9:40pm UTC](https://www.ruby-forum.com/t/instance-eval-yield-c-extensions/66718/3 "2006-08-28T21:40:19Z")

</div>

Tilman S. wrote:

> ruby to expect zero arguments. So make that 0 a 1.
> 
> > static VALUE kernel\_foo(VALUE self){  
> > VALUE v\_proc = rb\_proc\_new(foo\_block, self); /\* Also tried Qnil \*/  
> > return rb\_funcall(rb\_cObject, rb\_intern(“instance\_eval”), 0, v\_proc);
> 
> Some problem here.
> 
> HTH,  
> Tilman

Ok, fixing that I get “can’t convert Proc into String (TypeError)”.

Maybe I’m supposed to be using rb\_block\_proc, but I’m not entirely  
certain how  
it works.

Thanks,

Dan

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**Author:** ![danielb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/c57346/32.png) [@danielb](https://www.ruby-forum.com/u/danielb)\
**Post date:** [August 28, 2006, 9:50pm UTC](https://www.ruby-forum.com/t/instance-eval-yield-c-extensions/66718/4 "2006-08-28T21:50:21Z")

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Daniel B. wrote:

> > You’re passing one argument (the result of rb\_yield()), yet you tell  
> > Tilman
> 
> Ok, fixing that I get “can’t convert Proc into String (TypeError)”.
> 
> Maybe I’m supposed to be using rb\_block\_proc, but I’m not entirely  
> certain how it works.
> 
> Thanks,
> 
> Dan

I guess I was over thinking it. This function works by itself:

static VALUE kernel\_foo(VALUE self){  
return rb\_funcall(cFoo, rb\_intern(“new”), 1,  
rb\_yield(rb\_block\_proc()));  
}

Regards,

Dan

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**Author:** ![danielb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/c57346/32.png) [@danielb](https://www.ruby-forum.com/u/danielb)\
**Post date:** [August 28, 2006, 9:52pm UTC](https://www.ruby-forum.com/t/instance-eval-yield-c-extensions/66718/5 "2006-08-28T21:52:44Z")

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Daniel B. wrote:

> Tilman S. wrote:
> 
> > ruby to expect zero arguments. So make that 0 a 1.
> > 
> > > static VALUE kernel\_foo(VALUE self){  
> > > VALUE v\_proc = rb\_proc\_new(foo\_block, self); /\* Also tried Qnil \*/  
> > > return rb\_funcall(rb\_cObject, rb\_intern(“instance\_eval”), 0, v\_proc);
> > 
> > Some problem here.
> > 
> > HTH,  
> > Tilman
> 
> Ok, fixing that I get “can’t convert Proc into String (TypeError)”.

Right now that code is equivalent to

obj.instance\_eval block

Where it should be

obj.instance\_eval &block  
obj.instance\_eval {some\_code}  
obj.instance\_eval ‘some\_code’

> Maybe I’m supposed to be using rb\_block\_proc, but I’m not entirely  
> certain how  
> it works.
> 
> Thanks,
> 
> Dan

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<div class="post-metadata">

**Author:** ![danielb](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/c57346/32.png) [@danielb](https://www.ruby-forum.com/u/danielb)\
**Post date:** [August 29, 2006, 5:07pm UTC](https://www.ruby-forum.com/t/instance-eval-yield-c-extensions/66718/6 "2006-08-29T17:07:29Z")

</div>

> > > > > “D” == Daniel B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes:

D\> static VALUE kernel\_foo(VALUE self){  
D\> return rb\_funcall(cFoo, rb\_intern(“new”), 1,  
rb\_yield(rb\_block\_proc()));  
^^^^^^^^^^^^^^^

You are giving the proc to the block, you have writing something like  
thid

def xxx(&block)  
Foo.new(yield(block))  
end

–

Guy Decoux