"Turning your code inside out" is a great piece of advice, as it often opens up abstractions and refactorings that you didn't realize where there to begin with. The same idea is behind several common object-oriented design patterns (Command, Mediator, Strategy, Visitor) but it's baked into to many functional programming languages.
For example, say we want to write a function to compute square roots. A common approach to computing sqrt(n) is to start with a guess of x = 1.0, and keep replacing x with (0.5 * (x + n/x) until the relative difference between subsequent guesses is small enough.
sqrt n = loop x0 x1
where
loop x y = if converged x y
then y
else loop y (0.5 * (y + n/y))
converged x y = abs (x/y - 1) < 1e-10
x0 = 1.0;
x1 = 0.5 * (1.0 + 1.0/n)
That's good, but it has the test for convergence all mixed up with the logic for generating the guesses. What if we could factor out the code that generates an infinite sequence of guesses?
sqrtGuesses n = go 1.0
where
go x = x : go (0.5 * (x + n/x))
Note that this works in Haskell because of laziness, but it's simple in any language that has a mechanism for delaying computations. Now we've decoupled the method for generating a sequence of guesses, we can write a function that checks for relative convergence
converge (x:y:rest) = if abs (x/y - 1) < 1e-10
then y
else converge (y:rest)
and define the square root function in terms of these
sqrt n = converge (sqrtGuesses n)
The logic of the program is now much cleaner, and we've got a useful function 'converge' which can be re-used in other parts of the program.
This kind of 'turning inside out' is often possible in functional languages, often leads to more compact and more compositional code, and is one of the reasons that I enjoy programming functionally so much.
For example, say we want to write a function to compute square roots. A common approach to computing sqrt(n) is to start with a guess of x = 1.0, and keep replacing x with (0.5 * (x + n/x) until the relative difference between subsequent guesses is small enough.
That's good, but it has the test for convergence all mixed up with the logic for generating the guesses. What if we could factor out the code that generates an infinite sequence of guesses? Note that this works in Haskell because of laziness, but it's simple in any language that has a mechanism for delaying computations. Now we've decoupled the method for generating a sequence of guesses, we can write a function that checks for relative convergence and define the square root function in terms of these The logic of the program is now much cleaner, and we've got a useful function 'converge' which can be re-used in other parts of the program.This kind of 'turning inside out' is often possible in functional languages, often leads to more compact and more compositional code, and is one of the reasons that I enjoy programming functionally so much.