Why does `puts(nil or 4)` fail in Ruby?

Question

When I do:

puts(nil or 4)

Ruby complains:

SyntaxError: syntax error, unexpected ke         


        

Answer 1

Short answer

Because that's how ruby syntax is.

Longer answer

and/or keywords were designed to be used in control flow constructs. Consider this example:

def die(msg)
  puts "Exited with: #{msg}"
end

def do_something_with(arg)
  puts arg
end

do_something_with 'foo' or die 'unknown error'
# >> foo
# >> Exited with: unknown error

Here or works nicely with ruby's optional parentheses, because of ruby parsing rules (pseudo-BNF).

In short, an argument list (CALL_ARGS) is a list of ARG, separated by comma. Now, most anything is an ARG (class definitions, for example, through being a PRIMARY), but not an unadorned EXPR. If you surround an expression with parentheses, then it'll match a rule for "compound statement" and, therefore, will be a PRIMARY, which is an ARG. What this means is that

puts( (nil or 4) ) # will work, compound statement as first argument
puts (nil or 4)  # same as above, omitted optional method call parentheses
puts(nil or 4) # will not work, EXPR can't be an argument
puts nil or 4 # will work as `puts(nil) or 4`

You can read the grammar referenced above to understand exactly how it works.

BONUS: Example of class definition being a valid ARG

puts class Foo
       def bar
         puts "hello"
       end
     end, 'second argument'

# >> bar # this is the "value" of the class definition
# >> second argument