Discuss types of arguments

functions.Rmd

@@ -335,10 +335,24 @@ Neither `if` not `switch` are vectorised: they work with a single value at a tim
 
 ## Arguments
 
-
 Note that arguments in R are lazily evaluated: they're not computed until they're needed. That means if they're never used, they're never called. This is an important property of R the programming language, but is unlikely to be important to you for a while. You can read more about lazy evaluation at <http://adv-r.had.co.nz/Functions.html#lazy-evaluation>
 
-### Default arguments
+Often the arguments to a function fall into two broad sets: one set supplies the data to compute on, and the other supplies arguments that controls the details of the computation. For example:
+
+* In `log()`, the data is `x`, and the base of the logarithm is `base`.
+
+* In `mean()`, the data is `x`, and `trim` and `na.rm` control the computation.
+
+* In `t.test()`, the data is `x` and `y`, and `alternative`, `mu`, `paired`,
+  `var.equal`, and `conf.level` control the details of the test.
+  
+* In `paste()` you can supply unlimited strings to `...`, and the pasting
+  is controlled by `sep` and `collapse`.
+  
+Generally, the arguments that control computation have default values so you don't need to supply the most commonly used values.
+
+In almost all cases, the default value should be the value that is used most commonly. There are a few exceptions to do with safety. For example, `na.rm` should always have default value `FALSE` even though `TRUE` is what you usually want if you have missing values. The default forces you to confront and deal with the missingness in your data, rather than allowing it to silently propagate.
+
 
 You can choose to supply default values to your arguments for common options. This is useful so that you don't need to repeat yourself all the time.