How to choose between private member and let binding?

Question

When writing a private method that has no need to access other members of the same class, how do you choose between private member and let binding?

• I tend to use private members because it's easier to change accessibility if required, but are there other aspects to keep in mind when making the choice?
• Are let bindings compiled as private members (making this only a style choice)?

Answer 1

let bindings cannot be accessed through the class instance but private method can. For examples:

type A() =
    let someUtil() = "util code"

    member private this.AnotherUtil() = "anotherUtil"

    member private this.DoSomething() =
       let anotherA = A()
       A.someUtil()  // compilation failed
       A.AnotherUtil() // ok

Answer 2

The relevant portion of the spec is section 8.6.2. It states:

The compiled representation used for values declared in “let” bindings in classes is either:

• A value that is local to the object constructor (if the value is not a syntactic function, is not mutable and is not used in any function or member).

• An instance field in the corresponding CLI type (if the value is not a syntactic function, but is used in some function or member).

• A member of the corresponding CLI type (if the value is a syntactic function).

Also:

Non-function let-bindings that are not used in either the members of a type or a function binding are optimized away and become values that are local to the resulting CLI constructor. Likewise, function bindings are represented as instance members.

I prefer let bindings to private members because they're more "functional," i.e., they emphasize "what" over "how." The compiler takes care of the optimal compiled form.

Answer 3

let bindings in a class are private. The main difference I think of between let and a private member are that let bindings cannot be overloaded, and are called with name() rather than this.Name(). As such, I think it's a mostly stylistic choice.