I see a lot of functional programming related topics mention destructive updates. I understand that it is something similar to mutation, so I understand the update part. But what is the destructive part? Or am I just over-thinking it?
2 Answers
The imperative programming languages allow variables to be redefined, e.g
x = 1
x = 2
So x first has the value 1 then, later, it has the value 2. The second operation is an destructive update, because x looses its initial definition as being equal to 1.
This is not how definition is handled in common mathematics. Once defined, a variable keeps its value. The above, seen as system of equations, would allow to subtract the first from the second equation, which would give
x - x = 2 - 1 <=> 0 = 1
which is a false statement. It is assumed that once introduced, x is the same.
A familiar statement like
x = x + 1
would lead to the same conclusion.
The functional languages have the same use of variables, once they are defined it is not possible to reassign them. The above statement would turn into
x2 = x + 1
and we would have no for or while loop but rather recursion or some higher order function.
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