Functions IV

This section will teach you how to define functions with fun.

fun

fun takes two arguments: a list of sets and an expression. It uses the sets to construct the domain of the function, and uses the expression to construct the range. Internally, fun calls bind on the expression to bind it to its arguments. For more on bind, see Percent.

w> anything-or-0 = fun [_ int] (? (> %k 5) 0 %%k)
w> anything-or-0 'hello 6
'hello
w> anything-or-0 'hello 5
0
w> hello-or-0 = anything-or-0 'hello
w> hello-or-0 6
'hello

Recalling from the Percent section, %k refers to one level up in a function heirarchy and %%k represents to two levels up. In pseudocode, we could say that anything-or-0 is constructed like so:

anything-or-0 = {
  <element of _>: {
    <element of int>: bind (? (> %k 5) 0 %%k)
  }
}

Looking at the above pseudocode, and remembering the syntax presented in Percent, it is more clear that %k represents <element of int> and %%k represents <element of _>.