F# Cheatsheet 🔷

An updated cheatsheet for F#.

This cheatsheet glances over some of the common syntax of F#.

Comments
Strings
Types and Literals
Printing Things
Loops
Values
Functions
Pattern Matching
Collections
Records
Discriminated Unions
Exceptions
Classes and Inheritance
Interfaces and Object Expressions
Casting and Conversions
Active Patterns
Compiler Directives
Acknowledgments

Comments

Line comments start from // and continue until the end of the line. Block comments are placed between (* and *).

// And this is line comment
(* This is block comment *)

XML doc comments come after /// allowing us to use XML tags to generate documentation.

/// The `let` keyword defines an (immutable) value
let result = 1 + 1 = 2

Strings

The F# string type is an alias for System.String type. See Strings.

/// Create a string using string concatenation
let hello = "Hello" + " World"

Use verbatim strings preceded by @ symbol to avoid escaping control characters (except escaping " by "").

let verbatimXml = @"<book title=""Paradise Lost"">"

We don't even have to escape " with triple-quoted strings.

let tripleXml = """<book title="Paradise Lost">"""

Backslash strings indent string contents by stripping leading spaces.

let poem = 
    "The lesser world was daubed\n\
     By a colorist of modest skill\n\
     A master limned you in the finest inks\n\
     And with a fresh-cut quill."

Interpolated strings let you write code in "holes" inside of a string literal:

let name = "Phillip"
let age = 30
printfn $"Name: {name}, Age: {age}"

let str = $"A pair of braces: {{}}"
printfn $"Name: %s{name}, Age: %d{age}" // typed

Types and Literals

Most numeric types have associated suffixes, e.g., uy for unsigned 8-bit integers and L for signed 64-bit integer.

let b, i, l, ul = 86uy, 86, 86L, 86UL

// val ul: uint64 = 86UL
// val l: int64 = 86L
// val i: int = 86
// val b: byte = 86uy

Other common examples are F or f for 32-bit floating-point numbers, M or m for decimals, and I for big integers.

let s, f, d, bi = 4.14F, 4.14, 0.7833M, 9999I

// val bi: System.Numerics.BigInteger = 9999
// val d: decimal = 0.7833M
// val f: float = 4.14
// val s: float32 = 4.14f

See Literals for complete reference.

and keyword is used for definining mutually recursive types and functions:

type A = 
  | Aaa of int 
  | Aaaa of C
and C = 
  { Bbb : B }
and B() = 
  member x.Bbb = Aaa 10

Floating point and signed integer values in F# can have associated units of measure, which are typically used to indicate length, volume, mass, and so on:

[<Measure>] type kg
let m1 = 10.0<kg>
let m2 = m1 * 2.0 // type inference for result 
let add30kg m =   // type inference for input and output
    m + 30.0<kg>
add30 2.0<kg>     // val it: float<kg> = 32.0

Printing Things

Print things to console with printfn:

printfn "Hello, World"

printfn $"The time is {System.DateTime.Now}"

You can also use Console.WriteLine:

open System

Console.WriteLine $"The time is {System.DateTime.Now}"

Constrain types with %d, %s, and print structured values with %A:

let data = [1..10]

printfn $"The numbers %d{1} to %d{10} are %A{data}"

Omit holes and apply arguments:

printfn "The numbers %d to %d are %A" 1 10 data

See Plaintext Formatting

Loops

for...in

For loops:

let list1 = [1; 5; 100; 450; 788]

for i in list1 do
    printf "%d" i           // 1 5 100 450 788

let seq1 = seq { for i in 1 .. 10 -> (i, i * i) }

for (a, asqr) in seq1 do
    // 1 squared is 1
    // ...
    // 10 squared is 100
    printfn "%d squared is %d" a asqr

for i in 1 .. 10 do
    printf "%d " i          // 1 2 3 4 5 6 7 8 9 10

// for i in 10 .. -1 .. 1 do
for i = 10 downto 1 do
    printf "%i " i          // 10 9 8 7 6 5 4 3 2 1

for i in 1 .. 2 .. 10 do
    printf "%d " i             // 1 3 5 7 9

for c in 'a' .. 'z' do
    printf "%c " c          // a b c ... z

// Using of a wildcard character (_)
// when the element is not needed in the loop.
let mutable count = 0

for _ in list1 do
    count <- count + 1

while...do

While loops:

let mutable mutVal = 0
while mutVal < 10 do        // while (not) test-expression do
    mutVal <- mutVal + 1

Values

Values have different names based on length, called unit, single value and tuples.

// unit (no value)
let nothing = ()

// single value
let single = 1 // same as `let single = (1)`

Functions that return void in C# will return the unit type in F#.

A tuple is a grouping of unnamed but ordered values, with lenght equal or bigger than 2 and possibly of different types:

// 2-tuples
let x = (1, "Hello")

// 3-tuples
let y = ("one", "two", "three") 

// Tuple deconstruction
let (a', b') = x
let (c', d', e') = y

// The first and second elements of a tuple can be obtained using `fst`, `snd`, or pattern matching:
let c' = fst (1, 2)
let d' = snd (1, 2)
  
let print' tuple =
    match tuple with
    | (a, b) -> printfn "Pair %A %A" a b

Functions

The let keyword also defines named functions.

let pi () = 3.14159 // function with no arguments. () is called unit type
pi ()               // it's necessary to use () to call the function

let negate x = x * -1 
let square x = x * x 
let print x = printfn $"The number is: %d{x}"

let squareNegateThenPrint x = 
    print (negate (square x))

Double-backtick identifiers are handy to improve readability especially in unit testing:

let ``square, negate, then print`` x = 
    print (negate (square x))

Pipe operator

The pipe operator |> is used to chain functions and arguments together:

let squareNegateThenPrint x = 
    x |> square |> negate |> print

This operator is essential in assisting the F# type checker by providing type information before use:

let sumOfLengths (xs : string []) = 
    xs 
    |> Array.map (fun s -> s.Length)
    |> Array.sum

Composition operator

The composition operator >> is used to compose functions:

let squareNegateThenPrint = 
    square >> negate >> print

Pattern Matching

Pattern matching is primarily through match keyword;

let rec fib n =
    match n with
    | 0 -> 0
    | 1 -> 1
    | _ -> fib (n - 1) + fib (n - 2)

Use when to create filters or guards on patterns:

let sign x = 
    match x with
    | 0 -> 0
    | x when x < 0 -> -1
    | x -> 1

Pattern matching can be done directly on arguments:

let fst (x, _) = x

or implicitly via function keyword:

/// Similar to `fib`; using `function` for pattern matching
let rec fib2 = function
    | 0 -> 0
    | 1 -> 1
    | n -> fib2 (n - 1) + fib2 (n - 2)

See Pattern Matching.

Collections

Lists

Lists are immutable collection of elements of the same type.

// Lists use square brackets and `;` delimiter
let list1 = ["a"; "b"]

// :: is prepending
let list2 = "c" :: list1

// @ is concat    
let list3 = list1 @ list2   

// Recursion on list using (::) operator
let rec sum list = 
    match list with
    | [] -> 0
    | x :: xs -> x + sum xs

Arrays

Arrays are fixed-size, zero-based, mutable collections of consecutive data elements.

// Arrays use square brackets with bar
let array1 = [| "a"; "b" |]

// Indexed access using dot
let first1 = array1.[0]   
let first2 = array1[0]    // F# 6

Sequences == IEnumerable

Sequences are logical series of elements of the same type. Individual sequence elements are computed only as required, so a sequence can provide better performance than a list in situations in which not all the elements are used.

// Sequences can use yield and contain subsequences
seq {
    // "yield" adds one element
    yield 1
    yield 2

    // "yield!" adds a whole subsequence
    yield! [5..10]
}

The yield can normally be omitted:

// Sequences can use yield and contain subsequences
seq {
    1
    2
    yield! [5..10]
}

Mutable Dictionaries (from BCL)

Create a dictionary, add two entries, remove an entry, lookup an entry

open System.Collections.Generic

let inventory = Dictionary<string, float>()

inventory.Add("Apples", 0.33)
inventory.Add("Oranges", 0.5)

inventory.Remove "Oranges"

// Read the value. If not exists - throw exception.
let bananas1 = inventory.["Apples"]
let bananas2 = inventory["Apples"]   // F# 6

Additional F# syntax:

// Generic type inference with Dictionary
let inventory = Dictionary<_,_>()   // or let inventory = Dictionary()

inventory.Add("Apples", 0.33)

dict == IDictionary in BCL

dict creates immutable dictionaries. You can’t add and remove items to it.

open System.Collections.Generic

let inventory : IDictionary<string, float> =
    ["Apples", 0.33; "Oranges", 0.23; "Bananas", 0.45]
    |> dict

let bananas = inventory.["Bananas"]     // 0.45
let bananas2 = inventory["Bananas"]     // 0.45, F# 6

inventory.Add("Pineapples", 0.85)       // System.NotSupportedException
inventory.Remove("Bananas")             // System.NotSupportedException

Quickly creating full dictionaries:

[ "Apples", 10; "Bananas", 20; "Grapes", 15 ] |> dict |> Dictionary

Map

Map is an immutable key/value lookup. Allows safely add or remove items.

let inventory =
    Map ["Apples", 0.33; "Oranges", 0.23; "Bananas", 0.45]

let apples = inventory.["Apples"]
let apples = inventory["Apples"] // F# 6
let pineapples = inventory.["Pineapples"]   // KeyNotFoundException
let pineapples = inventory["Pineapples"]    // KeyNotFoundException on F# 6 too

let newInventory =              // Creates new Map
    inventory
    |> Map.add "Pineapples" 0.87
    |> Map.remove "Apples"

Safely access a key in a Map by using TryFind. It returns a wrapped option:

let inventory =
    Map ["Apples", 0.33; "Oranges", 0.23; "Bananas", 0.45]

inventory.TryFind "Apples"      // option = Some 0.33
inventory.TryFind "Unknown"     // option = None

Useful Map functions include map, filter, partition:

let cheapFruit, expensiveFruit =
    inventory
    |> Map.partition(fun fruit cost -> cost < 0.3)

Dictionaries, dict, or Map?

Use Map as your default lookup type:
- It’s immutable
- Has good support for F# tuples and pipelining.
Use the dict function
- Quickly generate an IDictionary to interop with BCL code.
- To create a full Dictionary.
Use Dictionary:
- If need a mutable dictionary.
- Need specific performance requirements. (Example: tight loop performing thousands of additions or removals).

Generating lists

The same list [ 1; 3; 5; 7; 9 ] can be generated in various ways.

[ 1; 3; 5; 7; 9 ]
[ 1..2..9 ]
[ for i in 0..4 -> 2 * i + 1 ]
List.init 5 (fun i -> 2 * i + 1)

The array [| 1; 3; 5; 7; 9 |] can be generated similarly:

[| 1; 3; 5; 7; 9 |]
[| 1..2..9 |]
[| for i in 0..4 -> 2 * i + 1 |]
Array.init 5 (fun i -> 2 * i + 1)

Functions on collections

Lists and arrays have comprehensive functions for manipulation.

List.map transforms every element of the list (or array)
List.iter iterates through a list and produces side effects

These and other functions are covered below. All these operations are also available for sequences.

Records

Records represent simple aggregates of named values, optionally with members:

// Declare a record type
type Person = { Name : string; Age : int }

// Create a value via record expression
let paul = { Name = "Paul"; Age = 28 }

// 'Copy and update' record expression
let paulsTwin = { paul with Name = "Jim" }

Records can be augmented with properties and methods:

type Person with
     member x.Info = (x.Name, x.Age)

Records are essentially sealed classes with extra topping: default immutability, structural equality, and pattern matching support.

let isPaul person =
    match person with
    | { Name = "Paul" } -> true
    | _ -> false

Recursion

The rec keyword is used together with the let keyword to define a recursive function:

let rec fact x =
    if x < 1 then 1
    else x * fact (x - 1)

Mutually recursive functions (those functions which call each other) are indicated by and keyword:

let rec even x =
   if x = 0 then true 
   else odd (x - 1)

and odd x =
   if x = 0 then false
   else even (x - 1)

rec also can be used to define strings like this:

let rec name = nameof name

Discriminated Unions

Discriminated unions (DU) provide support for values that can be one of a number of named cases, each possibly with different values and types.

type Tree<'T> =
    | Node of Tree<'T> * 'T * Tree<'T>
    | Leaf


let rec depth input =
    match input with
    | Node(l, _, r) -> 1 + max (depth l) (depth r)
    | Leaf -> 0

F# Core has a few built-in discriminated unions for error handling, e.g., Option and Result.

Using Option:

let optionPatternMatch input =
    match input with
    | Some i -> printfn "input is an int=%d" i
    | None -> printfn "input is missing"

optionPatternMatch (Some 1)
optionPatternMatch None

Using Result:

let resultPatternMatch input =
    match input with
    | Ok i -> printfn "Success with code %d" i
    | Error e -> printfn "Error with code %d" e

resultPatternMatch (Ok 0)
resultPatternMatch (Error 1)

Single-case discriminated unions are often used to create type-safe abstractions with pattern matching support:

type OrderId = Order of string

// Create a DU value
let orderId = Order "12"

// Use pattern matching to deconstruct single-case DU
let (Order id) = orderId

Exceptions

The failwith function throws an exception of type Exception.

let divideFailwith x y =
    if y = 0 then 
        failwith "Divisor cannot be zero." 
        else x / y

Exception handling is done via try/with expressions.

let divide x y =
    try
        Some (x / y)
    with :? System.DivideByZeroException -> 
        printfn "Division by zero!"
        None

The try/finally expression enables you to execute clean-up code even if a block of code throws an exception. Here's an example which also defines custom exceptions.

exception InnerError of string
exception OuterError of string
  
let handleErrors x y =
   try 
       try 
           if x = y then raise (InnerError("inner"))
           else raise (OuterError("outer"))
       with InnerError(str) -> 
          printfn "Error1 %s" str
   finally
       printfn "Always print this."

Classes and Inheritance

This example is a basic class with (1) local let bindings, (2) properties, (3) methods, and (4) static members.

type Vector(x: float, y: float) =
    let mag = sqrt(x * x + y * y)               // (1) - local let binding

    member this.X = x                           // (2) property
    member this.Y = y                           // (2) property
    member this.Mag = mag                       // (2) property

    member this.Scale(s) =                       // (3) method
        Vector(x * s, y * s)

    static member (+) (a : Vector, b : Vector) = // (4) static method
        Vector(a.X + b.X, a.Y + b.Y)

Call a base class from a derived one:

type Animal() =
    member _.Rest() = ()
           
type Dog() =
    inherit Animal()
    member _.Run() =
        base.Rest()

Interfaces and Object Expressions

Declare IVector interface and implement it in Vector'.

type IVector =
    abstract Scale : float -> IVector

type Vector(x, y) =
    interface IVector with
        member __.Scale(s) =
            Vector(x * s, y * s) :> IVector
            
    member __.X = x
    
    member __.Y = y

Another way of implementing interfaces is to use object expressions.

type ICustomer =
    abstract Name : string
    abstract Age : int

let createCustomer name age =
    { new ICustomer with
        member __.Name = name
        member __.Age = age }

Casting and Conversions

int 3.1415     // float to int = 3
int "3"        // string to int = 3
float 3        // int to float = 3.0
float "3.1415" // string to float = 3.1415
string 3       // int to string = "3"
string 3.1415  // float to string = "3.1415"

Upcasting is denoted by :> operator.

let dog = Dog() 
let animal = dog :> Animal

In many places type inference applies upcasting automatically:

let exerciseAnimal (animal: Animal) = () 

let dog = Dog()

exerciseAnimal dog   // no need to upcast dog to Animal

Dynamic downcasting (:?>) might throw an InvalidCastException if the cast doesn't succeed at runtime.

let shouldBeADog = animal :?> Dog

Active Patterns

Complete active patterns:

let (|Even|Odd|) i = 
  if i % 2 = 0 then Even else Odd

let testNumber i =
    match i with
    | Even -> printfn "%d is even" i
    | Odd -> printfn "%d is odd" i

Parameterized, partial active patterns:

let (|DivisibleBy|_|) divisor n = 
  if n % divisor = 0 then Some DivisibleBy else None

let fizzBuzz input =
    match input with
    | DivisibleBy 3 & DivisibleBy 5 -> "FizzBuzz" 
    | DivisibleBy 3 -> "Fizz" 
    | DivisibleBy 5 -> "Buzz" 
    | i -> string i

Partial active patterns share the syntax of parameterized patterns but their active recognizers accept only one argument.

Compiler Directives

Load another F# source file into F# Interactive (dotnet fsi).

#load "../lib/StringParsing.fs"

Reference a .NET package:

#r "nuget: FSharp.Data"                // latest non-beta version
#r "nuget: FSharp.Data,Version=4.2.2"  // specific version

Specifying a package source:

#i "nuget: https://my-remote-package-source/index.json"

#i """nuget: C:\path\to\my\local\source"""

Reference a specific .NET assembly file:

#r "../lib/FSharp.Markdown.dll"

Include a directory in assembly search paths:

#I "../lib"
#r "FSharp.Markdown.dll"

Other important directives are conditional execution in FSI (INTERACTIVE), conditional for compiled code (COMPILED) and querying current directory (__SOURCE_DIRECTORY__).

#if INTERACTIVE
let path = __SOURCE_DIRECTORY__ + "../lib"
#else
let path = "../../../lib"
#endif

Acknowledgments

Thanks goes to these people/projects:

adelarsq/fsharp-cheatsheet

adelarsq

Reviews

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