This is not a bug.
"a = 5 unless defined? a" is not meant to be equivalent to this:
unless defined? a
a = 5
end
a = unless defined? a
5
end* >> a = 1
=> 1
>> b = 2
=> 2
>> a = 3 unless b==2
=> nil
>> a
=> 1
>> a = unless b==2
>> 2
>> end
=> nil
>> a
=> nil
a =
a = asdfgasdfweradsf
>>> a=b
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'b' is not defined
>>> a
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'a' is not defined
>>>
ra:~$ irb
irb(main):001:0> defined? a
=> nil
irb(main):002:0> defined? b
=> nil
irb(main):003:0> a = b
NameError: undefined local variable or method `b' for main:Object
from (irb):3
irb(main):004:0> a
=> nil
irb(main):005:0> b
NameError: undefined local variable or method `b' for main:Object
from (irb):5
irb(main):006:0> defined? a
=> "local-variable"
irb(main):007:0>
# let a=b;;
Error: Unbound value b
# a;;
Error: Unbound value a
#
Prelude> let a=b
<interactive>:1:6: Not in scope: `b'
Prelude> a
<interactive>:1:0: Not in scope: `a'
Prelude>
guile> (define a b)
Backtrace:
In standard input:
1: 0* (define a b)
standard input:1:1: In expression (define a b):
standard input:1:1: Unbound variable: b
ABORT: (unbound-variable)
guile> a
ERROR: Unbound variable: a
ABORT: (unbound-variable)
Ruby doesn't draw as clear a line as compiled languages do between "compile time" and "run time," but the interpreter does parse your code before running it, and certain decisions are made during that process. An important one is the recognition and allocation of local variables.
When the Ruby parser sees the sequence identifier, equal-sign, value, as in this expression
x = 1
Consider this example:
if false
x = 1
end
p x # Output: nil
p y # Fatal error: y is unknown
The result is that x inhabits a strange kind of variable limbo. It has been brought into being and initialized to nil.
In many cases, a bug by design is a question of trade-offs. In this case, stricter rules for the parser might mean more syntax for the programmer.
if condition
x = true
end
puts "hello" if x