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Why do one have to leave the first argument to struct constructor unnamed?
f#compiler

I defined a struct:
[<Struct>]
type Rect =
val x_tl: int
val y_tl: int
val x_br: int
val y_br: int
val score: int
val mutable score_sum: int
val mutable parent: Rect option
new (x_tl,y_tl,x_br, y_br,score,score_sum, parent) =
{x_tl = x_tl;y_tl = y_tl;x_br = x_br;y_br = y_br;score=score;score_sum=score_sum;parent = parent}

Now I try to instantiate the Rect first naming all arguments, it does not work, than leaving the first argument unnamed, which works. Why? Is it legal to have "Rect option" inside of struct? (At least the compiler accepted this.)

> let max3 = new Rect (x_tl = -1 , y_tl = -1 , x_br = -1 , y_br = -1 , score = 0 , score_sum = 0 , parent = None);;

let max3 = new Rect (x_tl = -1 , y_tl = -1 , x_br = -1 , y_br = -1 , score = 0 , score_sum = 0 , parent = None);;
-----------^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

C:\Users\AppData\Local\Temp\stdin(19,12): error FS0041: The method 'Rect' is overloaded. Possible matches are shown below (or in the Error List window)

Possible overload: 'Rect()'.
Possible overload: 'new : x_tl:int * y_tl:int * x_br:int * y_br:int * score:int * score_sum:int * parent:Rect option -> Rect'.

> let max3 = new Rect (-1 , y_tl = -1 , x_br = -1 , y_br = -1 , score = 0 , score_sum = 0 , parent = None);;
val max3 : Rect = FSI_0002+Rect

.

Good question is why do you need this constructor?

By on 5/29/2009 12:15 AM ()

You must have a constructor for struct, or am I wrong(see error below)?
Initially I used a record for Rect, but had to convert it to a struct so that arrays of them are more space efficient. Would be happy to be able to do this:

[<Struct>]
type MyClass() =
let mutable x = 0;;

let mc = new MyClass (x=4);;
---------------------------------------------------------------------
> [<Struct>]
type MyClass() =
let mutable x = 0
member this.X with get() = x
and set(n) = x <- n
;;

type MyClass() =
-------^^^^^^^

stdin(33,8): error FS0081: Implicit object constructors for structs must take at least one argument

By on 5/29/2009 1:26 AM ()

You must have a constructor for struct, or am I wrong(see error below)?

The diagnostic here is misleading. Structs cannot use the implicit constructor syntax and 'let's, they must use explicit constructor syntax and 'val's, see also [link:msdn.microsoft.com] . But there's no reason you need to define any constructor at all (you can have just the autogenerated default constructor be the only one):

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[<Struct>]

type MyStruct =
    val mutable x : int

let mc = new MyStruct(x=4) // calls MyStruct() and then does .x <- 4
By on 5/29/2009 7:09 AM ()

Default constructor for structs is always autogenerated.

By on 5/29/2009 6:10 AM ()

Thanks. I understand the better way to instantiate this struct would be by just setting the properties and not having the constructor at all.

By on 5/29/2009 1:06 AM ()

The same thing is true with a class if the class has a default constructor. In both cases the call becomes ambiguous. Note that F# also allows you to 'assign properties' in a constructor call, a la

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type MyClass() =
    let mutable x = 0
    member this.X with get() = x
                  and set(n) = x <- n

let mc = new MyClass(X=4)

and so in this case that 'property assignment' shorthand is an ambiguous conflict with the 'named parameter' shorthand.

By on 5/28/2009 11:52 PM ()

"In both cases the call becomes ambiguous. Note that F# also allows you to 'assign properties' in a constructor call"

Using named argument here should not be ambiguous since x_tl field is immutable, is not it?

By on 5/29/2009 2:10 PM ()

"In both cases the call becomes ambiguous. Note that F# also allows you to 'assign properties' in a constructor call"

Using named argument here should not be ambiguous since x_tl field is immutable, is not it?

You're right. I'll file a low-priority bug (I don't know if we'll fix, since there are ample workarounds).

By on 5/29/2009 2:28 PM ()

You can always work around named arguments :-)

By on 5/29/2009 2:58 PM ()

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