Parsing = operator in R does not yield a language object

Question

I am trying to manipulate some parsed R code and have run into difficulty with the = operator. As this snippet shows, I can get an object that says its type is "language", but it then returns false on the "is" test that R uses when assigning a value to a slot of an S4 class.

Here is some sample code:

parsed <- parse(text = "cylinders = c(4, 6, 8)")
print (typeof(parsed))  # Prints "expression"

langObj <- parsed[[1]]
print (typeof(langObj))  # Prints "language"
print (is(langObj, "language"))  # Prints FALSE

setClass("Foo",
  slots = list(
    s1 = "language")
)

setMethod ("initialize",
  "Foo",
  function(.Object, obj){
    .Object@s1 <- obj
    return (.Object)
  }
)

new (Class = "Foo", langObj)

This last line produces the error:

Error in (function (cl, name, valueClass)  : 
  assignment of an object of class “=” is not valid for @‘s1’ in an object of class “Foo”; is(value, "language") is not TRUE

Note that if the <- operator is used in place of =, the code works as expected.

What is the difference between "typeof" and "is"? Why does the = operator not yield a "language" value, while <- does?

Answer 1

You need to understand that typeof returns a fairly low level characterization and that is( ... , "language") tests a somewhat higher level of abstraction. There is not much use for typeof. It's generally more useful to ask for the class of an object:

> class(parsed)
[1] "expression"
> class(parsed[[1]])
[1] "="

This second one might seem a bit odd, and I would have thought it to be eitehr a call or and Ops result, but if you look at:

parsed[[1]]
#cylinders = c(4, 6, 8)

You see that the call object is represent internally, i.e. the parse-tree, as:

`=`( cylinders, c(4, 6, 8) )

... noting that:

 parsed[[1]][[1]]
`=`    # note the backticks signifying a function, a language object

... and that this is really a call-object:

  is.call( parsed[[1]] )
 #[1] TRUE

See ?parse where it is explained that the function returns an unevaluated call-object. I'm more of an S3 guy so trying to explain what's going wrong with your S4 stuff is above my pay grade. Notice that the error message from your failed S4 efforts referred to a mismatch of 'class' rather than 'typeof'