How to destructure a constructor argument?

Question

In F# I can use pattern matching in a variety of places within the syntax.

For example:

// Given this type...
type SingleCaseUnion = | SingleCaseUnion of int

/// ...I can do this:
let destructureInFunc (SingleCaseUnion n) =
    printfn "%d" n

// ...and this:
type DestructureInMethod() =
    member t.M(SingleCaseUnion n) =
        printfn "%d" n

But I can't work out how to do this:

type DestructureInCtor(SingleCaseUnion n) =
    do printfn "%d" n

//  type DestructureInCtor(SingleCaseUnion n) =
//  ---------------------------------------^
//
// stdin(3,40): error FS0010: Unexpected identifier in type definition. Expected ')' or other token.

Do I have the syntax wrong, or does F# not support pattern matching in constructor parameters?

Answer 1

No, the language spec explicitly says no:

primary-constr-args : attributesopt accessopt (simple-pat, ... , simplepat)
simple-pat :
 | ident
 | simple-pat : type

As has been pointed out, secondary constructors are allowed pattern-matched parameters, but the difference with the primary constructor is each of the primary's parameters is both a function parameter and a private field declaration.

If F# were to allow pattern matching here, there would be some patterns that would break this one-parameter-one-field relationship.

type DestructureInCtor(SingleCaseUnion _) = 
    // doesn't declare a private field

or:

type DestructureInCtor((a:int, b:int)) = 
    // declares two private fields? 

It's not inconceivable that this could work but I'm guessing the complexity of allowing pattern matching to be extended to providing field declarations outweighs the benefits.