@@ -165,12+165,14 @@ Function composition, which mathematicians write as `f` ○ `g`, is defined as
We've already come across the `id` function, namely λ `x. x`.
We've already come across the `id` function, namely λ `x. x`.
-Other common functions are `fst`, which takes two arguments and returns the first of them; `snd`, which takes two arguments and returns the second of them; and `swap`, which takes two arguments and returns them both but with their positions swapped. These functions can be defined like this:
+Other common functions are `fst`, which takes two arguments and returns the first of them; `snd`, which takes two arguments and returns the second of them; and `swap`, which takes two arguments and returns them both but with their positions swapped. A fourth function is `dup`, which takes one argument and returns it twice.