Struct in F# is a new Custom Value data type created with a combination of multiple Inbuilt native types.

Variables created using struct type hold values of different data types.

How to declare Struct and variables

F# defined two syntaxes for declaring struct type

type [modifier] type-name =  
    struct  
    type-definition-elements-and-members
    end  

or 

[<StructAttribute>]
type [modifier] type-name =
    type-definition-elements-and-members

  • Type is a keyword

  • Modifier is a standard access modifier and visibility to access.

  • type-name is a new type, used for creating a variables

  • struct, end, and StructAttribute are keywords.

  • if struct, and end syntax is used, StructAttribute syntax can be skipped

  • type-definition-elements-and-members contains the following things

    • Member declarations and definitions fields
    • Constructors with zero or parameterized syntax
    • Mutable and Immutable fields
    • Interfaces

  • Contains member declaration using val, does not support let.

  • val variables can have a field name and datatype, not initialized with default, and contain default value null or zero.

  • Explicit Constructor contains logic to initialize fields or members

F# Struct Example

The below Struct code contains the following things

  • Struct definition
  • Members declared with val, field names, and datatypes
  • Constructor defined using new, initialized with all fields
  • Create a Variable using a new struct type
  • Accessed struct members using type. membername syntax.
type Employee = struct  
   val id : int  
   val name: string  
   val salary : int  
   new (id, name, salary) =           // Constructor definition
      {id = id; name = name; salary = salary;}  
end  

let employee1 = new Employee(1,"john", 5000)   // constructor  called
printfn "Id  : %i " employee1.id  
printfn "Name : %s"  employee1.name  
printfn "Salary  : %i"  employee1.salary

Readonly Struct example

F# also defined readonly structs that contain code for readonly properties,

Mark the struct annotation as IsReadOnly. It contains immutable variables only.

Throws an error if mutable variables are used.

[<IsReadOnly; Struct>]
type print (first: int, second: int) =
    member x.first = first
    member x.second = second

Difference between Classes and Struct

It is similar to class to hold data and behavior for a small set of data.

Classes are reference types and Struct are value types that store the data on Stack.

Classes support inheritance. Struts do not support inheritance. Classes contain let or do bindings, Structs do not support let or do binding