F# Interview Questions

A list of top frequently asked F# interview questions and answers are given below.

1) What is F#?

F# is a Computer programming language. It was designed and developed by Microsoft. It supports functional, object-oriented and imperative programming approaches. You can create an application by using this programming language.

2) What are the features of F#?

F# has many features. Following are the main features of F#:

Type inference
Type extension
Less code
Immutable data
Pattern matching
Assertion
Lambda expression
Function composition and pipelining
Object expression
Lazy computation and many more

3) What are the available data types in F#?

F# provides a rich set of data types. It helps to deal with any data whether it is scientific data, data of business analysis, etc. You can see the table of data types here.

Data Types in F#

Types	Data Types
Primitive data types	char, byte, bool, int, float
Derived data types	class, array, list, records, sequence
Enumeration	enum
Unit type	It is used if other data types are not specified.

4) What is the unit type in F#?

The unit type is a type which indicates the absence of specific value. The unit type has only a single value. This value acts as a placeholder when no other value exist.

Example:

let function1 x y = x + y               

function1 10 20      // this line results a compiler warning
    
// changing the code to the following and eliminates the warning
let result = function1 10 20

// Use this if you are calling the function and don't want the return value
function1 10 20 |> ignore

5) What is upcasting and downcasting in F#?

Casting is a process of converting one type to another type. F# provides mainly two operators to deal with upcasting and downcasting. The :> operator is used to upcast object and :?> operator is used to downcast object.

Example:

type BaseClass() =
 class
  member this.ShowClassName()=
    printfn "BaseClass"
 end

type DerivedClass() = 
 class
  inherit BaseClass()
  member this.ShowClassName()=
   printfn "DerivedClass"
 end

let baseClass = new BaseClass()            
let derivedClass : DerivedClass = new DerivedClass()
baseClass.ShowClassName()    
derivedClass.ShowClassName()
let castIntoBaseClass = derivedClass :> BaseClass        // upcasting 
castIntoBaseClass.ShowClassName()
let castIntoDerivedClass = baseClass :?> DerivedClass   // downcasting
castIntoDerivedClass.ShowClassName()

6) What are available operators in F#?

An operator is simply a symbol that is used to perform operations. There can be many types of operations like arithmetic, bitwise, logical, etc.

There are following types of operators to perform different types of operations in F# language.

Arithmetic operators: The Arithmetic operators take numerical values as their operands and return a single numerical value.
Boolean operators: The Boolean operators are used to check conditional expressions. It returns true if expression satisfies the condition otherwise it returns false.
Bitwise operators: In F#, the bitwise operator works on individual bits and return a result after evaluation.
Nullable operators: The Nullable operators are used to work with database queries. It handles null values which are stored in tables in place of data.

7) Which tokens are available in F#?

Keywords: It contains the links to information about all F# language keywords.
Symbol and operators: It contains a table of symbols and operators that are used in the F# language.

8) What is function composition and pipelining in F#?

In F#, functions can be composed from other functions. It is a process of composing in which a function represents the application of two composed functions. F# function pipelining allows us to call functions in the chain. Pipelining operator takes a function and an argument as operands and returns a value.

F# Function Composition Example

let function1 name= 
  name + " FSharp"
let function2 name = 
   name + " Programming"

let programmingName = function1 >> function2
let result = programmingName "Hello"
printf "%s" result

F# Function Pipelining Example

let function1 name= 
  name + " FSharp"
let function2 name = 
   name + " Programming"

let result = "Hello" |> function1 |> function2
printf "%s" result

9) What is lambda expression in F#?

Lambda expression is an unnamed or anonymous function. Sometimes instead of defining a full name function, you may create a lambda expression. It optimizes the code. You must use a fun keyword to define lambda expression.

Example

let result = (fun a -> a+1) 100
printf "%d" result

10) What is an inline function in F#?

The F# inline function is a function that is integrated directly into the calling code. It helps to optimize code and sometimes can improve performance too.

Example

type InlineClass() =
  class
    member inline this.inlineFunction(a) = a*a
  end
 
let obj = new InlineClass()
let result = obj.inlineFunction 2
printf "%d" result

11) What is let bindings in F#?

Binding is a process of associating of identifier or function to a value. Let keyword is used to bind an identifier to a value. In F#, We use let keyword to declare variable, function and private class members.

F# Let binding in function

We use let keyword to define a function in F#.

let ShowName() = 
 printf "Hello FSharp"
ShowName()

12) What is "do bindings" in F#?

Do binding is used to execute code without defining a function or any type. You can write independent code by using do binding in F#.

Example

type DoBindingClass() = 
 class
  do printf "Hello FSharp"   
 end
new DoBindingClass()

13) What is type annotation in F#?

F# allows type annotation so that you can explicitly mention the type of identifier or parameter or return type of a function. You must use: (colon) to apply annotation in F#.

Example

let a:int = 10
printf "%d" a

14) What is Type Inference in F#?

Type inference means when you are writing code then you don't need to specify the type of values or variables. F# compiler is strong enough to infer the type of value.

Example

let add a b =      //Here, all parameters are inferred to int because of passing values during calling
 a+b               // Return type of this function is int because the type of this expression is int.
printf "%d" (add 10 20)

15) What is Automatic Generalization in F#?

When code does not specify any type explicitly, then the compiler considers generic type. It is called an automatic generalization. It helps to write generic code without increasing complexity.

16) What are the Tuples in F#?

In F#, tuples are a collection of anonymous values. Values may be the same or different types. It allows us to put expression as a value also.

Example

let (id,name) = (1, "Fsharp")
printfn "%d" id
printfn "%s" name  

17) Can a function return multiple values in F#?

Yes, by using a tuple, you can return multiple values in a function.

Example

let TupleExample a b = 
 let c = a+b
 let d = a-b
 (c, d)
 
let tupleValues = TupleExample 50 10 
printf "%A" tupleValues

18) What is a list in F#?

It is an immutable collection of same type elements. It maintains the order of elements.

F# List Example

let list = [1;2;3;4;5;6;7;8;9]
for i  in list do
 printfn "%d" i

19) What is Array in F#?

Arrays are mutable collections of data of the same type. It starts from index 0 and goes to n-1 where n is the length of arrays.

Example

let arr = [| 1; 2; 3; 4; 5; |]               // Creating and initializing array
for i = 0 to arr.Length-1 do           		// Traversing of array
  printfn "%d" arr.[i]

20) What is Sequence in F#?

The Sequence is a series of the same type of elements. It provides better performance than list.

Example

You can create sequence expression like following. Here, we have used Seq.iter () function to iterate sequence. We can also use for loop or array format specifier to iterate sequence elements.

let seq1 =
 seq { 0 .. 10 }
Seq.iter(fun a->printf " %d" a)seq1

21) What are the options in F#?

Options type is used when there is no value present for function or variable. It provides an expression Some and a value None for handling the empty values or variables.

Example

let sub a b =
 if(a>b) then
  Some(a-b)
  else None
printf "%A" (sub 20 10)

22) What is Generics in F#?

F# allows you to write a generic function, method, type, variable, etc. It helps to avoid repeating of code for each type. By writing generic code, you can apply it for any type or value.

Example

let genericFunctionExample<'T> x y =
    printfn "%A %A" x y

genericFunctionExample<int> 1 2

23) What are the Records in F#?

Records are used to store elements in the form of label and value. It can store any data. You are not bound to store same type values as a list. Records are immutable by default so you can't modify original records.

Example

type RecordExample = { x : float; y: float; z: float; }
let getRecordValues = { x = 2.0; y = 1.0; z = 0.0; }
printf "%A\n" getRecordValues         	// Access all values of record
printf "%f" getRecordValues.x	// Access individual values of record

24) What is Enumeration in F#?

Enumeration is popularly known as enums. It is a combination of label and value pair. Labels are assigned to a subset of the values. You can use them in place of literals to make the code more readable and maintainable.

Example

type Year =
   | January = 0
   | Fabruary = 1
   | March = 2
   | April = 3

// Use of an enumeration.
let monthName = enum<Year>(3)
printf "%A" monthName
let monthLiteral : Year = Year.January
let n = int monthLiteral
printf "\n%d" n

25) What are Reference cells in F#?

Reference cells refer to memory locations. It allows you to create mutable values. F# uses immutable data structure by default.

Example

let refVariable = ref 50
printf "%d" refVariable.Value

26) What is a structure in F#?

The F# structure is a data structure which is used to organize data, and it is value types and efficient than class. It does not allow let binding, so you must declare fields by using val keyword.

Example

type Book = struct
   val title : string
   val author: string
   val price : float
   new (title, author, price) =           // Constructor
      {title = title; author = author; price = price;}
end
let book = new Book("FSharp Programming", "Chris Smith", 100.0)   // Calling Constructor
printfn "Title  : %s " book.title
printfn "Author : %s"  book.author
printfn "Price  : $%0.2f"  book.price   

27) What is Discriminated Union in F#?

It is a useful data structure. It helps to store heterogeneous data. The Union is used to represent tree data structures. It provides cases, and each case consists of heterogeneous data.

Example

type Calcualte =  
    | Add of val1 : int * val2 : int  
    | Multiply of val1 : int * val2 : int  
  
let compute vall =  
   match vall with  
     | Add (val1, val2) -> val1+val2  
     | Multiply (val1, val2)->val1*val2  
  
let addition = compute (Add(10,10))  
let multiplication = compute (Multiply(2,5))  
  
printf "Addition = %d\nMultiplication = %d" addition multiplication  

28) What is Object in F#?

The Object is a real-world entity. It can be anything like - cell phone, car, football, etc.

The Object is an instance of the class we can access all the members of the class by using object of this class.

Let's see an example of how to create an object in F#.

29) What is a class in F#?

The Class is a template or blueprint of an object. It is used to encapsulate data members and member methods. It can contain fields, methods, constructor, static method, etc.

Example

type Student (id,name)= 
 class
  member x.show = 
  printf "%d \n%s" id name 

 end
let a = new Student (12,"FSharp")
a.show

30) What is a constructor in F#?

In F#, Constructor is somewhat different than other .Net languages. There are always primary constructors that may or may not have parameters. The Scope of these parameters is throughout the class.

Example

type Employee(name) = 
 class
  do printf "%s" name
 end
let e = new Employee("FSharp")

31) What is a self in F#?

In F#, a self is used to refer the current object of class type. Self is the same as this keyword in C# and Java. You can name the self-identifier however you want. You are not restricted to names such as this or self as in .Net languages.

Example

type Employee(id,name) as this =
    let id = id
    let name = name
    do this.Display()   		// This is how we can use self(this) object
    member this.Display() =
        printf "%d %s" id name
let e =new Employee(100, "Rajkumar")

32) What is static in F#?

In F#, static is a keyword. It is used to make the static field or static method. Static is not the part of the object. It has its memory space to store static data. It is used to share common properties among objects.

Example

type Account(accno,name) = class
 static let rateOfInterest = 8.8
 member this.display()=
  printfn "%d %s %0.2f" accno name rateOfInterest 
end
let a1 = new Account(101,"Rajkumar")
let a2 = new Account(102, "john")
a1.display()
a2.display()

33) What is an inheritance in F#?

In F#, inheritance is a process in which child class acquires all the properties and behaviors of its parent class automatically. It is used to reuse the code.

Example

type Employee(name:string) =
 class
  member this.ShowName() = printf"Name = %s\n" name
 end
type Manager(name,salary:int) =
 class
  inherit Employee(name)
  member this.ShowSalary() = printf"Salary = $%d" salary
 end
let manager = new Manager("Rajkumar",10000)
manager.ShowName()
manager.ShowSalary()

34) What is method overriding in F#?

Method overriding is a feature of Object-oriented programming approach. It helps to achieve polymorphism. We can achieve method overriding using inheritance.

Example

type Employee() =
 class
  abstract ShowName : unit -> unit
  default this.ShowName() = printfn"This is base class method"
 end 
type Manager() =
 class
  inherit Employee()
  override this.ShowName() = printf "This is derived class method"
 end

let employee = new Employee()
let manager = new Manager()
employee.ShowName()
manager.ShowName()

35) What is an abstract class?

Abstract classes are used to provide the full implementation of class members. It may contain non-abstract methods. A class that inherits abstract class must provide an implementation of all abstract methods of the abstract class.

Example

[<AbstractClass>]
type AbstractClass() = 
 class
  abstract member ShowClassName : unit -> unit
 end
type DerivedClass()  = 
 class
  inherit AbstractClass()
  override this.ShowClassName() = printf "This is derived class."
 end

let a = new DerivedClass()
a.ShowClassName()

36) What is an interface in F#?

F# provides Interface type. It provides pure abstraction. It is a collection of abstract methods.

Example

type IEmployee =
   abstract member ShowName : unit -> unit
type Manager(id:int, name:string) =
   interface IEmployee with
      member this.ShowName() = printfn "Id = %d \nName = %s" id name

let manager = new Manager(100,"RajKumar")
//manager.ShowName()    // error: you can't directly access interface abstract method by using class object.
// so, we need to upcast class type object to interface type by using :> operator.
(manager :> IEmployee).ShowName()

37) What is type extension in F#?

Type extension allows you to add new members to your previously defined object type.

Example

type ClassExtension() =
 member this.ShowBefore() = printfn"Class before extension"

// Define type extension.
type ClassExtension with
 member this.ShowAfter() = printfn"Class after extension"

let classExtension = new ClassExtension()
classExtension.ShowBefore()
classExtension.ShowAfter()

38) What is a delegate in F#?

In F#, delegates are reference types. It allows us to call the function as an object. It is a feature of this language. It gives an advantage over the other functional programming languages.

Example

type Deligate() =
  static member mul(a : int, b : int) = a * b
  member x.Mul(a : int, b : int) = a * b
type Multiply = delegate of (int * int) -> int

let getIt (d : Multiply) (a : int) (b: int) =
   d.Invoke(a, b)
let d : Multiply = new Multiply( Deligate.mul )
for (a, b) in [(5, 8) ] do
  printfn "%d * %d = %d" a b (getIt d a b)

39) What is the object expression in F#?

F# object expression is a special expression. It creates a new instance of anonymous object type which is based on an existing base type, interface, or set of interfaces.

Example

let ex =
 {
      new System.Exception() with member x.ToString() = "Hello FSharp" 
 }
printfn "%A" ex

40) What is Exception handling?

Exception handling is a standard mechanism to handle abnormal termination of the program. The Exception is a situation that occurs during program execution. It may lead to terminate program abnormally like divide by zero or a null pointer.

Example

let ExExample a b =
 let mutable c = 0
 c <- (a/b)
 printfn "Rest of the code"

ExExample 10 0

41) What is a try-with block in F#?

In F#, you can create a user-defined exception. It provides flexibility to define custom exceptions according to requirement.

Example

let ExExample a b =
  let mutable c = 0
  try
    c <- (a/b)
  with
    | :? System.DivideByZeroException as e -> printfn "%s" e.Message
  printfn "Rest of the code"

ExExample 10 0

42) What is FailWith and InvalidArg in F#?

In F#, you can throw exceptions explicitly. You are allowed to throw a custom exception. You can also throw exceptions by using predefined methods of Exception like Failwith and InvalidArgs.

Example of FailWith keyword

let TestAgeValidation age  =
  try
     if (age<18) then failwith "Sorry, Age must be greater than 18"
  with
     | Failure(e) -> printfn "%s" e; 
  printf "Rest of the code"
TestAgeValidation 10

Example of InvalidArg keyword

let TestInvalidArgument age =
 if(age<18) then
  invalidArg "TestInvalidArgument" ("Sorry, Age must be greater than 18")

TestInvalidArgument 10

43) What is an assertion in F#?

The assert expression is a debugging feature of F#. You can use it to test an expression. It generates a system error dialog box upon failure in Debug mode.

Example

let divide (x:int, y:int):int =
 assert (x>0)
 let z = (x/y)
 z

let result = divide(10,2)
printf "%d" result

44) What is a module in F#?

The Module is a collection of classes, functions, and types. It helps to organize related code so we can maintain code easily.

Example

open Arithmetic
printf "%d" (sub 20 10)

45) What is the access control in F#?

Access control specifies the accessibility of code. By using these, you can specify the scope of data, method, class, etc.

There are 3 types of access control in F#.

Public: Public access control also known as default. It is accessible for all. If you don't specify any access control explicitly in your code, by default it follows public access control.
Private: F# provides private keyword to declare private members in the class or type. The scope of private is limited to the local block in which it is declared.
Internal: Internal access control is only accessible from the same assembly. An assembly is a file which is automatically generated by your compiler after compilation of F# code.

46) What is Resource management in F#?

F# manages resources with the help of use and using keywords. Resources may be data, a file or network, etc. It acquires a resource from the operating system or other service providers so that it can be provided to other application.

Example

open System.IO
let ReleaseFile fileName content =
    using (System.IO.File.CreateText(fileName)) ( fun textFile ->
        textFile.WriteLine("{0}", content.ToString() )
    )

ReleaseFile "file/textFile.txt" "This is a text file. \nIt contains data."

47) What are the attributes in F#?

In F#, the attribute is used to enable metadata for a program code construct. The attribute can be applied to any construct like function, module, method, and type.

Example

open System
[<Obsolete("Do not use. Use newFunction instead.")>]
let updateSystem() = 
 printf "updating..."

updateSystem

48) What is a signature in F#?

In F#, the signature file contains information about the public signatures. Signatures can be of a set of program elements, such as types, namespaces, and modules.

Signature file named as signature.fsi

namespace FSharpPrograms
  module Arithmetic =
    val add : int * int -> int
    val sub : int * int -> int

49) What is an open keyword in F#?

An import declaration specifies a module or namespace. You can reference its elements without using a fully qualified name.

Example

open System
Console.WriteLine("Hello, this is F# here.")

50) What is the purpose of 'base' keyword?

The 'base' keyword is used as the name of the base class object.

51) What is the purpose of 'begin' keyword?

It is used to signify the starting of a code block.

52) What is the purpose of 'elif' keyword?

It is used same as else if branching.

53) What is the purpose of 'yield' keyword?

Yield keyword finds its use in the sequence expressions to produce a sequence value.

54) What is the purpose of 'rec' keyword?

It is used to indicate a recursive function.

55) What is the purpose of 'extern' keyword?

It is used to indicate that the program element declared is defined in some other assembly or binary.

56) Write the syntax for declaration of discriminated unions.

type name = 
| case-identifier1 [of [ fieldname1 : ] type1 [ * [ fieldname2 : ] 
type2 ...]
| case-identifier2 [of [fieldname3 : ]type3 [ * [ fieldname4 : ]type4 ...]

57) 'Variables in F# are immutable' Explain.

It means, once the value is assigned to a variable it cannot be altered.

58) What is the use of 'raise' function?

It is used for the indication of error occurrence.

59) What is lazy computation in F#?

Lazy computation is a feature of F#. Lazy computation does not evaluate immediately. It is executed when the result is needed.

Example

let add x y  = x+y
let result = lazy (add 10 10)
printfn "%d" (result.Force())

60) What is XML documentation in F#?

In F#, you can produce documentation from triple-slash (///) code comments. XML comments can precede declarations in code files (.fs) or signature (.fsi) files.

Example

let add x y  = x+y
let result = lazy (add 10 10)
printfn "%d" (result.Force())

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