F# Active Patterns Active Patterns can be used to validate and transform function arguments
Example
An interesting but rather unknown usage of Active Patterns in F# is that they can be used to validate and transform function arguments.
Consider the classic way to do argument validation:
// val f : string option -> string option -> string
let f v u =
let v = defaultArg v "Hello"
let u = defaultArg u "There"
v + " " + u
// val g : 'T -> 'T (requires 'T null)
let g v =
match v with
| null -> raise (System.NullReferenceException ())
| _ -> v.ToString ()
Typically we add code in the method to verify that arguments are correct.
Using Active Patterns in F# we can generalize this and declare the intent in the argument declaration.
The following code is equivalent to the code above:
let inline (|DefaultArg|) dv ov = defaultArg ov dv
let inline (|NotNull|) v =
match v with
| null -> raise (System.NullReferenceException ())
| _ -> v
// val f : string option -> string option -> string
let f (DefaultArg "Hello" v) (DefaultArg "There" u) = v + " " + u
// val g : 'T -> string (requires 'T null)
let g (NotNull v) = v.ToString ()
For the user of function f and g there's no difference between the two different versions.
printfn "%A" <| f (Some "Test") None // Prints "Test There"
printfn "%A" <| g "Test" // Prints "Test"
printfn "%A" <| g null // Will throw
A concern is if Active Patterns adds performance overhead. Let's use ILSpy to decompile f and g to see if that is the case.
public static string f(FSharpOption<string> _arg2, FSharpOption<string> _arg1)
{
return Operators.DefaultArg<string>(_arg2, "Hello") + " " + Operators.DefaultArg<string>(_arg1, "There");
}
public static string g<a>(a _arg1) where a : class
{
if (_arg1 != null)
{
a a = _arg1;
return a.ToString();
}
throw new NullReferenceException();
}
Thanks to inline the Active Patterns adds no extra overhead compared to the classic way of the doing argument validation.
