F# Incomplete Structured Construct Caused By Type Specification

Question

So, I have a function, corners, that I want to take a 2D array (with abbreviated type HieghtMap) and return a list of the record type of Coordinates. Initially, I wasn't specifying matrixLocal's type, which led to System.Exception: Operation could not be completed due to earlier error The type 'matrixLocal' is not defined. at 30,28

Now that I am specifying the type, I get this new error

 Syntax error in labelled type argument at 30,39 
 Incomplete structured construct at or before this point in
 interaction. Expected incomplete structured construct at or before
 this point, ';', ';;' or other token.

I believe this is due to farside (as it does not function even on its own), but I do not know why, hence I am here. The other questions that I found regarding the latter error do not seem to apply in this case (one was about indenting, and the other was about trying to redefine a variable in a loop).

The Code:

module DiamondSquare =

//create type for defining shapes
///Defined by length of the side of a square that the ovject is inscribed in
type Shape =
    | Square of int
    | Diamond of int

///the X and Y position
type Coordinates = {X: int; Y: int}

///The Hieghtmap of a given chunk of region as a series of floats that are the offset from the base hieght
//was HieghtMap = HieghtMap of float[,], but was changed so that any 2D float array would be accepted
type HieghtMap = float[,]

//Create matrix of zeroes of chunk size to initilize this variable
let matrix = Array2D.zeroCreate<float> 9 9

//locate center of shape
//  since each shape is a square, or can be inscribed within one, pass it a matrix and find the
//  coordinate of the center (same value for i and j)
///Finds center of shape inscribed within a square. Takes a matrix, returns coordinates for within the matrix
let locateCenterpoint matrixLocal = 
    let coord = int ((Array2D.length1 matrixLocal) - 1) / 2
    {X = coord; Y = coord;}

//locate corners of a shape that is inscribed in a square
///Returns list of corner values for a given shape. Takes a matrix and returns a list of Coordinates
let corners shape:Shape matrixLocal:HieghtMap =
    let farSide = Array2D.length1 matrixLocal - 1
    let getSquareCorners = 
        {X = 0; Y = 0}::{X = farSide; Y = 0}::{X = 0; Y = farSide}::{X = farSide; Y = farSide}::[]
    let getDiamondCorners =
        {X = farSide / 2; Y = 0}::{X = farSide; Y = farSide / 2}::{X = farSide / 2; Y = farSide}::{X = 0; Y = farSide / 2}::[]
    match shape with
    | Square -> getSquareCorners
    | Diamond -> getDiamondCorners
    | _ -> None

Fyodor Soikin Fyodor Soikin · Accepted Answer · 2017-10-10T17:38:22

When defining a value in F#, the first colon signifies the end of value names and start of the type declaration. For example:

let f x y : string = ...

In this declaration, string is the return type of the function, and not type of the y parameter. In order to apply type declaration to a single value in the list, use parentheses:

let f x (y: string) = ...

This way, string is the type of y.

For your specific case, look at this line:

let corners shape:Shape matrixLocal:HieghtMap =

See what the problem is? Shape is being parsed as the return type of the corners function, and this makes the subsequent matrixLocal:HeightMap nonsensical. To fix, apply parentheses:

let corners (shape:Shape) (matrixLocal:HieghtMap) =

F# Incomplete Structured Construct Caused By Type Specification

1 Answers