How can a time function exist in functional programming?

Question

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 retu

Answer 1

Your question conflates two related measures of a computer language: functional/imperative and pure/impure.

A functional language defines relationships between inputs and outputs of functions, and an imperative language describes specific operations in a specific order to perform.

A pure language does not create or depend on side effects, and an impure language uses them throughout.

One-hundred percent pure programs are basically useless. They may perform an interesting calculation, but because they cannot have side effects they have no input or output so you would never know what they calculated.

To be useful at all, a program has to be at least a smidge impure. One way to make a pure program useful is to put it inside a thin impure wrapper. Like this untested Haskell program:

-- this is a pure function, written in functional style.
fib 0 = 0
fib 1 = 1
fib n = fib (n-1) + fib (n-2)

-- This is an impure wrapper around the pure function, written in imperative style
-- It depends on inputs and produces outputs.
main = do
    putStrLn "Please enter the input parameter"
    inputStr <- readLine
    putStrLn "Starting time:"
    getCurrentTime >>= print
    let inputInt = read inputStr    -- this line is pure
    let result = fib inputInt       -- this is also pure
    putStrLn "Result:"
    print result
    putStrLn "Ending time:"
    getCurrentTime >>= print