F# Incomplete structured construct

Question

I recently started learning F# and today I got error that i can't get rid of. I have following code:

open System

[<EntryPoint>]
let main argv =

    type BinaryTree =
        | Node of int * BinaryTree * BinaryTree
        | Empty

    let rec printInOrder tree = 
        match tree with
        | Node (data, left, right)
            -> printInOrder left
               printfn "Node %d" data
               printInOrder right
        | Empty
            -> () 

    let binTree = 
         Node(2, 
            Node(1, Empty, Empty),
            Node(4, 
                Node(3, Empty, Empty),
                Node(5, Empty, Empty)
            )
    )

printInOrder binTree
0

With this code I'm getting following error:

Incomplete structured construct at or before this point in binding

Unfortunately I have no idea how to fix it. This is code example from book Programming F# 3.0.

I would very much appreciate any kind of answer that could help me understand how to avoid these kind of mistakes in future.

Answer 1

You need to define the types and functions in the proper context (outside of the function).

open System 

type BinaryTree =
    | Node of int * BinaryTree * BinaryTree
    | Empty  

let rec printInOrder tree = 
    match tree with
    | Node (data, left, right)
        ->  printInOrder left
            printfn "Node %d" data
            printInOrder right
    | Empty
        -> () 

let binTree = 
    Node(2, 
        Node(1, Empty, Empty),
            Node(4, 
                Node(3, Empty, Empty),
                Node(5, Empty, Empty)
            )
    )

[<EntryPoint>]
let main argv =  
    printInOrder binTree
    0