Pure computation and IO sequencing

The idea

Enter with function types and list transformations understood. A pure function computes a result from its inputs without performing external effects. An IO a value describes an action that can produce an a when executed as part of the program’s IO sequence. It is not an a that can be extracted anywhere. The type keeps interactions such as reading input and printing output visible.

Inside do notation, <- binds an action’s result for later actions, while let introduces a pure local definition. Statements in that IO sequence occur in order, even though pure computations remain non-strict. A useful design keeps validation and transformation in pure helpers and lets main handle interaction. This makes the helper easier to reason about independently of terminals, files, and user timing.

double :: Integer -> Integer
double n = n * 2
main :: IO ()
main = do
  let result = double 6
  putStrLn "Result:"
  print result

Result:
12

The helper produces a pure Integer. Main sequences a text output action and then an action that prints the result.