Question

I have a method accepting a lambda which will be called or a string which will be evaluated.
Why does the following method fail:

def test(expr)
  expr = expr.respond_to?(:call) ? expr : ->{ eval(expr) }
  expr.call
end

test 'puts 1'
# => TypeError: can't convert Proc into String

But this one work:

def test(expr)
  foo = expr
  expr = expr.respond_to?(:call) ? expr : ->{ eval(foo) }
  expr.call
end

test 'puts 1'
# => 1

Answer 1

In here:

expr = expr.respond_to?(:call) ? expr : ->{ eval(expr) }

You end up with a Proc in expr that is a closure over expr so when the Proc is evaluated, it tries to eval itself because the expr inside the lambda is the lambda itself.

In here:

foo  = expr
expr = expr.respond_to?(:call) ? expr : ->{ eval(foo) }

the closure is over foo which refers to the original String value of expr so there is no funky self-referential confusion and you’ll end up using eval on a String.