List transformations and laziness

The idea

Enter with lists, functions, and recursion understood. map transforms every demanded element, while filter retains elements satisfying a predicate. These operations describe common list processing without writing the traversal each time. A fold combines elements into another result using a combining function and a base value. Check the combination’s argument order instead of assuming every fold behaves identically.

Haskell is non-strict: expressions are evaluated as their results are needed. An infinite list can be useful when a consumer requests only a finite prefix. take 4 can bound demand, while trying to print or sum the entire infinite list never finishes normally. Laziness is not a promise that every pipeline is efficient; a filter searching for a nonexistent future match can still fail to produce another element.

main :: IO ()
main = print (take 4 (map (*3) [1..]))

[3,6,9,12]

Only the first four mapped results are required. The infinite source does not need to be constructed in its entirety.