How to a time function exist in functional programming

functional-programming

I've to admit that I don't know much about functional programming. I read about it from here and there, and so came to know that in functional programming, a function returns the same output, for same input, no matter how many times the function is called. It's exactly like a mathematical function which evaluates to the same output for the same value of the input parameters which involves in the function expression.

For example, consider this:

f(x,y) = x*x + y; // It is a mathematical function

No matter how many times you use f(10,4), its value will always be 104. As such, wherever you've written f(10,4), you can replace it with 104, without altering the value of the whole expression. This property is referred to as referential transparency of an expression.

As Wikipedia says (link),

Conversely, in functional code, the output value of a function depends only on the arguments that are input to the function, so calling a function f twice with the same value for an argument x will produce the same result f(x) both times.

Can a time function (which returns the current time) exist in functional programming?

  • If yes, then how can it exist? Does it not violate the principle of functional programming? It particularly violates referential transparency which is one of the property of functional programming (if I correctly understand it).

  • Or if no, then how can one know the current time in functional programming?

Best Solution

Yes and no.

Different functional programming languages solve them differently.

In Haskell (a very pure one) all this stuff has to happen in something called the I/O Monad - see here.

You can think of it as getting another input (and output) into your function (the world-state) or easier as a place where "impureness" like getting the changing time happens.

Other languages like F# just have some impureness built in and so you can have a function that returns different values for the same input - just like normal imperative languages.

As Jeffrey Burka mentioned in his comment: Here is the nice introduction to the I/O Monad straight from the Haskell wiki.