Inspired by a blog post by Eirik Tsarpalis.
Let's write a small F# function that safely takes the square root of a number. Both the argument and the result should be wrapped in an
Option, and the function should return
None when the argument is
None or negative. Something like this:
let safeSqrt (xOpt : Option<float>) : Option<float> = // implementation?
How would you implement this function? One approach is to use "bare-metal" pattern matching:
let safeSqrt xOpt = match xOpt with | Some x when x >= 0.0 -> sqrt x |> Some | _ -> None
That's easy to understand, but a bit verbose. Personally, I'd get tired quickly reading a lot of functions in that style. Let's try the opposite extreme instead, using a totally "point-free" approach:
let safeSqrt = Option.filter ((<=) 0.0) >> Option.map sqrt
Well, that's certainly shorter, but is it actually better? It's not clear that
safeSqrt is even a function any more, because it doesn't have an argument. Is there perhaps a middle ground?
Option comes with a bevy of composable higher-order functions (like
map) that every F# developer should be comfortable with, so it makes sense to use them instead of low-level pattern matching. However, having an explicit argument to the
safeSqrt function helps a lot with readability, because it gives the function a "protaganist" that we can follow. The function then becomes a story about transformations applied to that protagonist. So with that in mind, here's another version of the function:
let safeSqrt xOpt = xOpt |> Option.filter (fun x -> x >= 0.0) |> Option.map (fun x -> sqrt x)
This approach makes it clear that our function is a good citizen of the
Option monad. We can easily trace the adventures of
xOpt as it moves through the steps of the function via the pipe operator. In particular, I think
fun x -> x >= 0.0 is a lot clearer than
(<=) 0.0. In fact, the latter looks like it could mean "numbers that are not positive" when it is in fact the opposite. On the other hand,
fun x -> sqrt x seems a bit wordy when we could just use
let safeSqrt xOpt = xOpt |> Option.filter (fun x -> x >= 0.0) |> Option.map sqrt
To my eye, this last version is the best because it uses higher-order functions with both lambdas and point-free functions where appropriate.
With this result in mind, here are some guidelines to keep in mind when trying to write clear, idiomatic F# code:
- Consider replacing raw function composition (
<<) with pipe operators (
<|) in order to give the input an explicit name.
- Avoid currying infix operators, such as
>=, especially when it means flipping arguments unnaturally. Use lambdas instead.
- Go ahead and use simple one-argument functions (like
sqrt) without points in order to shorten code.
What do you think? Are there other guidelines you prefer? Let me know in the comments!
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