A comprehensive survey of the types of things in R; ‘mode’ and ‘class’ and ‘typeof’ are insufficient

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I agree that the type system in R is rather weird. The reason for it being that way is that it has evolved over (a long) time…

Note that you missed one more type-like function, storage.mode, and one more class-like function, oldClass.

So, mode and storage.mode are the old-style types (where storage.mode is more accurate), and typeof is the newer, even more accurate version.

mode(3L)                  # numeric
storage.mode(3L)          # integer
storage.mode(`identical`) # function
storage.mode(`if`)        # function
typeof(`identical`)       # closure
typeof(`if`)              # special

Then class is a whole different story. class is mostly just the class attribute of an object (that’s exactly what oldClass returns). But when the class attribute is not set, the class function makes up a class from the object type and the dim attribute.

oldClass(3L) # NULL
class(3L) # integer
class(structure(3L, dim=1)) # array
class(structure(3L, dim=c(1,1))) # matrix
class(list()) # list
class(structure(list(1), dim=1)) # array
class(structure(list(1), dim=c(1,1))) # matrix
class(structure(list(1), dim=1, class="foo")) # foo

Finally, the class can return more than one string, but only if the class attribute is like that. The first string value is then kind of the main class, and the following ones are what it inherits from. The made-up classes are always of length 1. 

# Here "A" inherits from "B", which inherits from "C"
class(structure(1, class=LETTERS[1:3])) # "A" "B" "C"

# an ordered factor:
class(ordered(3:1)) # "ordered" "factor"

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