how to declare a f# method with list of tuple as parameter?

c#, c#-to-f#, f#, tuples

Solution

If you want to use idiomatic functional lists, then you can write:

let func (param:(int64 * string) list) = 
  [ for n, s in param -> n, s, "new val" ]

I added the annotation `(int64 * string) list` to make sure that you get the same type as the one in your C#. If you didn't add it, then the F# compiler would infer the function to be generic - because it actually does not matter what is the type of the first two elements of the tuple. The annotation can be also written in a C# style notation using `param:list<int64 * string>` which might be easier to read.

If you wanted to use .NET `List` type (which is not usually recommended in F#), you can use the F# alias `ResizeArray` and write:

let func (param:(int64 * string) ResizeArray) = 
  ResizeArray [ for n, s in param -> n, s, "new val" ]

This creates an F# list and then converts it to .NET `List<T>` type. This should give you exactly the same public IL signature as the C# version, but I would recommend using F# specific types.

As a side-note, the second example could be implemented using imperative F# code (using `for` to iterate over the elements just like in C#). This generates exactly the same IL as C#, but this is mainly useful if you need to optimize some F# code later on. So I do not recommend this version (it is also longer :-)):

let func (param:(int64 * string) ResizeArray) = 
  let res = new ResizeArray<_>()
  for n, s in param do
    res.Add(n, s, "new val")
  res

You could also use higher-order functions like `List.map` and write:

let func (param:(int64 * string) list) = 
  params |> List.map (fun (n, s) -> n, s, "new val")

Problem

Original source