Function Pattern Shorthand

Match Shorthand

There is a shorthand for a function that immediately passes the last argument (See Currying and Applying Arguments). We can use this shorthand to rewrite our greeting function to be:

let greeting = function
    | {Name = "Dave"; Year = 2001 } 
        -> "I'm sorry Dave, I'm afraid I can't do that."
    | {Name = "Neil"}
        -> "The first person to set foot on the moon."
    | {Name = name}
        -> sprintf "Hello %s." name

Our shorthand above will still have a type signature of Astronaut -> string. The difference is the argument is implied. Loading the Astronaut type and the shorthand greeting function in Visual Studio Code with Ionide-FSharp you will see a result of val greeting : _arg1:Astronaut -> string. Using the shorthand is a very common practice when your function is immediately doing a pattern match.

This shorthand is a matter of preference by the developer. There are multiple styles to use for a function which goes directly into a pattern match.

If a pattern match has only 2 values, some people prefer to use an if then else block. We can still use a match expression or a function pattern. The functions below have the same result and behaviour. It becomes a matter of readability and preference.

let boolYesNo b = if b then "Yes"
                  else "No"

let boolYesNo' b = 
    match b with
    | true  -> "Yes"
    | false -> "No"

let boolYesNo'' = function
    | true  -> "Yes"
    | false -> "No" 

Summary

Using a Function Pattern Shorthand can remove some overhead of the developer by indicating an immediate match. The function pattern shorthand will immediately match on the last argument supplied to the function. Some prefer to use if then else if a match has only two outcomes. It is the preference of the developer as to which style of matching they will use.