Unexpected start of structured construct in definition. Expected '=' or other token F#

Question

I am trying to make a vector library with a hidden Vector type but i get the following error:

Unexpected start of structured construct in definition. Expected '=' or other token

I am trying to learn how modules work, and i took this example from a book where I copied exactly as given and still got this error.

Screenshot with code

The given errors are :

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

error FS0010: Unexpected start of structured construct in definition. Expected '=' or other token.

`

Answer 1

F# has 2 sintaxis for module

Top level module without the =

module Vector

type Vector = V of float * float
let (~-.) (V(x, y)) = V(-x, -y)

Which acts as a namespace and module declaration in one. Components can be indented at the same level as the module Vector , like in the book example. F# interactive doesn't recognize this form, so you need to use the other form:

Sub modules with the = :

module Vector =

    type Vector = V of float * float
    let (~-.) (V(x, y)) = V(-x, -y)

Which require its components to be indented further inside.

Here is the documentation: https://docs.microsoft.com/en-us/dotnet/fsharp/language-reference/modules

Answer 2

You need to remove the first type Vector then just indent the module:

module Vector =

    type Vector = V of float*float
    let (~-.) (V(x,y)) = V(-x, -y)
    let make (x, y) = V(x, y)

    let a = make(1.0, 2.0) // V (1.0, 2.0)