EverythingIsACall.md

(Almost) Everything is a Call

We saw that we have symbols/names such as pi and x and that R evaluates this by finding the symbol in an environment and then returning the value bound to that.

We also saw literal values such as 1, 2, TRUE, FALSE, "a string". R evaluates these as themselves.

Everything else is a function call, e.g., numeric(), list(), plot(x, y), 1 + 2, x[1], x[2] = 10.

What about :, the sequence creation operator. This is a function.

`:`
.Primitive(":")

Similar to +, all the math and logic operators are functions, e.g.,

`&`
function (e1, e2)  .Primitive("&")

What about if(), for() and while(), i.e. the control flow operators. Indeed, these are also functions. Consider the simple function

z = function(n = 10)  
{ 
  ctr = 0 
  while(ctr < n) 
     ctr = ctr + 1
  ctr
}

If we call this, we get the count up to 10.

However, if we redefine while, we can change how this works.

while =
function(...)
 101

Now if we call z(), we get back 0, i.e. the value we assigned to ctr in the first line. The second expression in the body of z calls our new while function and that just returns 101. We ignore this and so return 0.

Similarly, we can redefine if() and for().