What is the difference between `static let` and `static member` in F#?

Paraphrasing the F# language guide's let Bindings in Classes article:

A let binding in a class creates a private field or function; to expose data or functions publicly, declare a property or a member method.

• An instance let binding is a let binding that is not preceded by the static keyword.

  Instance let bindings execute when objects are created.

• Static let bindings are part of the static initializer for the class.

  Static let bindings are guaranteed to execute before the type is first used.

If one tried to refactor the Vector2D type by removing the static keywords before zero, onex, and oney private functions,

type Vector2D(dx : float, dy : float) =
     let zero = Vector2D(0.0, 0.0)
     let onex = Vector2D(1.0, 0.0)
     let oney = Vector2D(0.0, 1.0)
     /// Get the zero vector
     static member Zero = zero
     /// Get a constant vector along the X axis of length one
     static member OneX = onex
     /// Get a constant vector along the Y axis of length one
     static member OneY = oney

then they would have received

error FS0039: The value or constructor 'zero' is not defined.

as zero and co. are only guaranteed to evaluate when the class is instantiated.

posted 4 months ago

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toraritte‭

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