passing an anonymous function as argument

Question

Why does the following work:

foo <- function(x) {x}
curve(foo)
# plots the identity function between 0 and 1

And this does not:

Answer 1

Actually the help page for curve does not say that 'expr' argument can be a function-object. The three types of accepted argument are "name of a function, or a call or an expression written as a function of x which will evaluate to an object of the same length as x." (Emphasis added.)

All of the following succeed:

curve( (function(x) {x})(x) )
curve( local(x)  )
curve( eval(x)  )

When you saw that ...

all.equal(foo, function(x) {x})
# TRUE

... it was saying that the language object attached to the name foo was the same as function(x) {x}. (The all.equal.language-function deparses the object(s) or object-names and compares the character results.)