My intended point was that comparing lambda: to if: is apples-to-oranges.

Both styles in my example have an outer structure that builds a sequence, wrapped around an inner calculation that says how you build the next value in the sequence. The inner calculation here was simple addition, a one-liner in the for loop and (+) in the Haskell version. However, it could equally have been something more complicated.

If it were, a reader would still have to figure out what the body of the for loop was really doing. In contrast, the (possibly anonymous) function passed into scanl would still have a more constrained purpose, which is immediately known because scanl represents one specific pattern of computation. The implicit algorithm it represents has a hole that is a certain shape, and you can only pass it a function with the right shape to fit into that hole.

Put another way, if you’re programming in this functional style, it’s not writing the word lambda that tells you what the next piece of code is for, it’s passing that lambda expression into a higher-order function like scanl. Passing an anonymous function is like writing the nested block for a branch of an if statement or the body of a for loop. It’s the call to scanl that is roughly analogous to writing the if or for statement itself.