Promises in lapply / R

Question

I am not sure what the promises are doing in R

If one runs

a = lapply(seq_len(2), function(n) { function() {n}})
b = lapply(seq_len(2), function(n)           


        

Answer 1

I find it easier to understand in this form:

f=function(n) {function() {n}}
x=1
a=f(x)
x=2
a()
[1] 2

The key part of the documentation is

When a function is called the arguments are matched and then each of the formal arguments is bound to a promise. The expression that was given for that formal argument and a pointer to the environment the function was called from are stored in the promise.

After the call a=f(x), the function argument n is bound to a promise with the name x and a pointer to the global environment .GlobalEnv.

In your lapply examples, the anonymous function function(n) { function() {n}} is called from the global environment each time. This is why every element of the list a gets the same value of n: it's coming from the global environment. I don't see how it's possible to change this behaviour by rewriting lapply.