F#: The 'defaultArg' function
While reading through an old blog post by Matthew Podwysocki about writing F# code in a functional rather than imperative way I came across the 'defaultArg' function which I haven’t seen previously.
It’s quite a simple function that we can use when we want to set a default value if an option type has a value of 'None':
The type signature is as follows:
> defaultArg;;
val it : ('a option -> 'a -> 'a) = <fun:clo@0>And the definition is relatively simple:
let defaultArg x y = match x with None -> y | Some v -> vWe could then use it if we were looking up a key in a dictionary but wanted to return a default value if there wasn’t an entry for that key.
For example:
let myMap = Map.add "key" "value" Map.empty
let result = defaultArg (Map.tryFind "nonExistentKey" myMap) "default"
> val result : string = "default"This is just another of the utility functions we can use in F# to allow us to keep composing functions even when we can get more than one type of result from another function.
About the author
I'm currently working on short form content at ClickHouse. I publish short 5 minute videos showing how to solve data problems on YouTube @LearnDataWithMark. I previously worked on graph analytics at Neo4j, where I also co-authored the O'Reilly Graph Algorithms Book with Amy Hodler.