Create a F# class with a sequence property

Question

Here's the code:

type Advertisement() =
    member val Photos = seq<images> with get,set

but this returns a function signature for the property Photos as

seq<images> -> seq<images>

I just want the property to represent a sequence of images

seq<images>

What am I missing?

Answer 1

The issue here is that you want to use seq<images> as a type annotation, but you're using it as an expression. These have different meanings; also, it's not clear what sequence you want to assign to Photos.

For example, if you want to initialize Photos to an empty sequence, you could use Seq.empty and have seq<images> as a type annotation:

    member val Photos = Seq.empty : seq<images> with get, set

Or, as ildjarn suggested, use an explicit type parameter for Seq.empty<'T>:

    member val Photos = Seq.empty<images> with get, set

If the type of Photos is clear from its usage in the same code file, the compiler will infer the type and you can even shorten it to:

    member val Photos = Seq.empty with get, set

Why did the original code return a fuction?

When you wrote seq<images> where an expression, not a type is expected (i.e. not behind a : or as a type parameter), the compiler resolved it to Operators.seq<'T>, which is the function of signature seq<images> -> seq<images> you are getting.

The implementation of seq is a little weird; it is supposed to behave like a computation expression but is actually a special case for the compiler. It is used both to cast to seq<'T> via the function you've seen, and to signify sequence expressions, as in seq { yield 1 }.

Answer 2

seq<images> is referring to the seq builder, like the one that you use in the following way

let foo = seq { yield "bar" }

You can actually see this if you hit F12 on seq. To instantiate an empty sequence you need to use Seq.empty<images>.