How can I use a map as a type in F#?

Question

I have the following:

Map [
        "A", new Dictionary<int64, int64 * float>()
        "B", new Dictionary<int64, int64 * float>()
]

and I would like a dictionary of those, something like:

let a = 
    new Dictionary<
        string,
        Map [
                "A", new Dictionary<int64, int64 * float>()
                "B", new Dictionary<int64, int64 * float>()
        ]>()

How to I get the type for the map to create the dictionary?

Answer 1

It is not entirely clear whether your question is how to write the type of the dictionary you want or whether you are asking how to construct a value of this type. Map is a generic type Map<'K, 'V> , much like the type of the Dictionary, so to write the type you'd use:

Dictionary<string, Map<string, Dictionary<int64, int64 * float>>>

To create a value, you can write the full type, or you can use the fact that F# can often infer the type arguments for you and so you can replace them with _ . For example:

let map = 
  Map [
      "A", new Dictionary<int64, int64 * float>()
      "B", new Dictionary<int64, int64 * float>()
  ]

let a = new Dictionary<_, _>(dict ["first", map])

I would also add that dictionary of maps of dictionaries is a pretty incomprehensible type. It would probably be a good idea to extract some of those into simple named classes that convey some idea about the meaning of the type.