[R] Is function(x){x}(5) a valid expression?

[Duncan Murdoch]

On 13/11/2012 1:33 PM, Jamie Olson wrote:
> I was surprised to notice that statements like:
>
> h = function(...){list(...)}(x=4)
>
> do not throw syntax errors.  R claims that 'h' is now a function, but I
> can't seem to call it.
>
> > h = function(x){list(x)}(4)
> > is(h)
> [1] "function"         "OptionalFunction" "PossibleMethod"
> > h()
> Error in list(x) : 'x' is missing
> > h(4)
> Error in h(4) : attempt to apply non-function
> >
>
> What's going on?

The body of your function is

{list(x)}(4)

The problem is,

{list(x)}

does not return a function, so you can't call it with the argument 4.  
If you had

h <- function(x) { function(y) y }(4)

it would return 4 every time, because the anonymous function does that.

Duncan Murdoch