F# collection initializer syntax

Question

What is the collection initializer syntax in F#? In C# you can write something like:

new Dictionary() {
    {\"One\", 1},
    {\"two\", 2}}
         


        

Answer 1

The lack of a collection initializer is annoying for some XAML-centric APIs like Workflow 4.0 which rely on collection initializers instead of ctors, e.g.

new Sequence { Activities = { WriteLine { Text = "In the sequence!" } } };

In such cases, imperative .Add() is awkward because the value is conceptually declarative even though it's technically mutable/imperative. However, there's no common base class for the set of all activities which declare an Activities child: the "Activities" member is a pattern and not an interface, so you can't just write a normal helper function which adds children to any activity. Fortunately, F# member constraints come to the rescue.

In order to write this:

Sequence() |> add [Sequence(DisplayName="InnerSeq"); WriteLine(Text = InArgument<_>("In the sequence!"))]

You first need to define an inline helper function called "add":

let inline add (children: Activity seq) =
    let inline doAdd (activity: ^Activity) : ^Activity when ^Activity : (member get_Activities : unit -> Activity Collection) =
        let collection = (^Activity : (member get_Activities : unit -> Activity Collection) (activity))
        for child in children do
            collection.Add(child)
        activity
    doAdd

This still isn't quite as nice as the C# syntax but at least it's still declarative. IMHO this is not so much as a fault with F# as with collection-initializer-centric APIs, but at least F# allows a workaround.