let vs. member assignments

What’s the difference between writing let in a type vs. writing member?

For starters, there’s not a huge difference.

type Test () =
    let mutable lastTime = DateTime.MinValue

is equivalent to:

public class Test
{
	internal DateTime lastTime = DateTime.MinValue;
}

let mutable resolves to an internal field.

As you may already know,

type Test () =
    let firstTime = DateTime.MinValue

always stays in the scope of the constructor, unless referenced outside. Equivalent to:

	public class Test
	{
		public Test()
		{
			var firstTime = DateTime.MinValue;
		}
	}

Referencing it in a member lifts it to be a field.

type Test () =
    let firstTime = DateTime.MinValue
    member this.getFirst() = firstTime

is equivalent to:

public class Test
{
	internal DateTime firstTime = DateTime.MinValue;
	public DateTime getFirst() => firstTime;
}

let vs. member functions

For functions, the behavior is similar, except using member makes it public by default.

let getFirst() = firstTime

becomes

internal DateTime getFirst() => firstTime;

and

member this.getFirst() = firstTime

becomes

public DateTime getFirst() => firstTime;

Using member private changes visibility back to internal (not private).

These two statements are generated IL-wise, identical:

let firstTime = DateTime.MinValue
member private this.firstTime = DateTime.MinValue

There is no protected

If you have a protected virtual method, the IL method attributes would be:

PrivateScope, Family, Virtual, HideBySig

F# has no concept of family or protected. The CLI compiled form of all non-public entities is internal.

If you override a member which was protecte, it becomes public.

In the F# 4.1 Spec, section 10.5:

Accessibility Annotations

The table lists the accessibilities that can appear in user code:

public

No restrictions on access.

private

Access is permitted only from the enclosing type, module, or namespace declaration group.

internal

Access is permitted only from within the enclosing assembly, or from assemblies whose name is listed using the InternalsVisibleTo attribute in the current assembly.

…

Note: The family and protected specifications are not supported in this version of the F# language.

member or let?

For using F# in an idiomatic way, let is the way to go. If cross-language compatibility is important to you, use member.

member also lets you use functions out of order, if that is important to you.

type Test () =
    let mutable firstTime = DateTime.MinValue
    member this.isValidDate () = 
    	this.getFirst() <> DateTime.MinValue //getFirst isn't declared yet
    member this.getFirst() = firstTime

Last modified on 2019-02-22