Constants, Bindings and instance_eval

Ruby
1

Dear rubyists,

i’m fairly new to ruby, so please apologize if i’m missing something
obvious. I’m trying to figure out how constant lookup works and find
myself confused. Here we go:

class A; C=3 end

$a = A.new
$a.instance_eval "C"  # => 3
$a.instance_eval {C}
    # ~> NameError: uninitialized constant C [...]

This is ruby 1.8.6. My naive mental model of what’s happening: constants
are evaluated statically before the block is passed along, and wrapping
it up as a string prevents the interpreter from looking inside too
early. :slight_smile:
That seems to have changed with Ruby 1.9 (yesterday’s trunk), but there
are a few surprises still:

def a_eval(&block) $a.instance_eval(&block) end

$a.instance_eval {C}  # => 3
a_eval {C}
    # ~> NameError: uninitialized constant C [...]
a_eval {instance_eval {C}}  # => 3  -- ???

I gather it’s a binding issue, but why should the binding of constants
(of anything, really) differ between inside “self.instance_eval { … }”
and outside?

a_eval {binding}.eval("C")
    # ~> NameError: uninitialized constant C [...]
a_eval {instance_eval{binding}}.eval("C")  # => 3
$a.instance_eval {binding}.eval("C")  # => 3

Can anybody shed some light on this? Thanks a lot!

Kind regards,
achim

2

Achim Passen wrote:

Dear rubyists,

i’m fairly new to ruby, so please apologize if i’m missing something
obvious. I’m trying to figure out how constant lookup works and find
myself confused. Here we go:

class A; C=3 end

$a = A.new
$a.instance_eval "C"  # => 3
$a.instance_eval {C}
    # ~> NameError: uninitialized constant C [...]

Can anybody shed some light on this? Thanks a lot!

Kind regards,
achim

class A
C=3
end

$a = A.new
$a.instance_eval “C”
#=> 3

$a.instance_eval {C}

~> NameError: uninitialized constant C […]

Its not being called properly for 1.8

instead use this…

$a.instance_eval {A::C}

A being the class, and C being the constant.

Regards,

  • Mac
3

Achim Passen wrote:

I’m trying to figure out how constant lookup works […]

class A; C=3 end

$a = A.new
$a.instance_eval “C” # => 3
$a.instance_eval {C}
# ~> NameError: uninitialized constant C […]

That’s an interesting question!

Michael L. already gave an “answer”, but I think I have another
one.

Look here:

class A
C=3
def prt_C
p C
end
end

a = A.new

def a.another_prt_C
p C
end

a.prt_C # OK!
a.another_prt_C # fails!

the ‘another_prt_C’ method fails to see the constant. I have the book
‘The Ruby P.ming Language’ and it has a page about ‘Constant
Lookup’. That page explains that “ruby attempts to resolve a constant
reference in the lexical scope of the reference. This means that it
first checks the class or module that encloses the constant reference.”
In other words, since our class A doesn’t enclose the ‘another_prt_C’
method, this method can’t reference the constant (directly). “Lexical”
means textual. That page also continues to explain that if the lookup
fails, ruby checks the class hierarchy as well. But the wording of the
book here is vague and there’s roon for interpretation that
‘another_prt_C’ indeed shouldn’t work and that it’s not a bug.

That’s why you shouldn’t expect instance_eval to see ‘C’: because this
constant isn’t defined in a class or module lexically enclosing your
block.

You say it’s fixed in 1.9. It probably means that ‘another_prt_C’ too
works in 1.9, or else my nice theory should be garbage collected! If it
does work in 1.9, I guess what they fixed is the section I said was
vague.