More on S3

OO-essentials.Rmd

@@ -46,6 +46,24 @@ The following chapters describe each system in turn, starting with base types. Y
 
 You'll need the pryr package, `install.packages("pryr")`, to access useful functions for examining OO properties.
 
+## Why OO?
+
+The primary use of OO programming in R is for print, summary and plot methods. These methods allow us to have one generic function, e.g. `print()`, that displays the object differently depending on its type: printing a linear model is very different to printing a data frame.
+
+## Why generic functions?
+
+Compare pipe with method chaining.
+
+This is a different school of though to most popular programming languages, but is a good fit to the problem of data analysis. Knowing this fact probably won't help you much in your day-to-day programming, but it will avoid some fundamental confusing if you're coming from another OO programmming language. \index{functions!generics|see{generics}} \index{S3!generics} \index{generics!S3}
+
+```{r, eval = FALSE}
+class.method(arg1, arg2)
+class$method(arg1, arg2)
+method(arg1, arg2, arg3)
+```
+
+(In fact this message is so powerful that I've talked to programmers who moved to R from javascript and it took them a while to figure out that they're not calling the `frame` method of the `data` object.)
+
 ## S4 {#s4}
 
 S4 works in a similar way to S3, but it adds formality and rigour. Methods still belong to functions, not classes, but: \index{objects!S4|see{S4}} \index{S4}