R: How default argument is passed to a function?

Question

In the .GlobalEnv, I defined the following variable and function

x = 0; 
foo <- function(t=x) {x=1; t}

When I called the function in the following ways

foo() # gives 1
foo(t=x) # gives 0

Can anyone help explain this? Thanks!!!

Answer 1

To make the comment into an answer:

In the first case, the function knows it needs to pass x to t. It is looking for x in its lexical scope and finds x=1. In the second case, you pass x=0 from the global environment, thus it doesn't look for it in the lexical scope again and passes it straight to t.