Functional Programming Languages
function: a mathematical correspondence that assigns exactly one element of one set to each element of the same or another set
The mathematical notion of function is that if f (x) = a “this time,” then f (x) = a “next time”; there is no other value equal to f (x). This allows the use of equational reasoning familiar from algebra: that if a = f (x) then g (f (x), f (x )) is equivalent to g (a , a). Pure functional programming languages encourage a kind of programming in which equational reasoning ...