> by running it in your head This is probably related to a factor named 'local reasoning', procedural programing tried to encourage this through procedure, OO

> by running it in your head This is probably related to a factor named 'local reasoning', procedural programing tried to encourage this through procedure, OO tried to encourage this through encapsulation, and FP encourage this through purity. Basically the goal is when anyone look into a function, the reader can easily make sense of the code without moving around. For pure functional programming, to make sense of a function is to make sense of the branch under the acyclic calling tree of the function. The caller and sibling branches are always completely irrelevant. So that it will be much easier to run in people's head.