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README.md

F# Cheatsheet πŸ”·

An updated cheatsheet for F#.

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

Contents


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

Basic 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.

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

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

A sequence is a 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 pineapples = inventory.["Pineapples"]   // KeyNotFoundException

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.

Tuples and Records

A tuple is a grouping of unnamed but ordered values, possibly of different types:

// Tuple construction
let x = (1, "Hello")

// Triple
let y = ("one", "two", "three") 

// Tuple deconstruction / pattern
let (a', b') = x

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

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

Recursive Functions

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)

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 Choice.

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

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

Useful functions

Mapping functions

map (Array, List, Seq)

Converts all the items in a collection from one shape to another shape. Always returns the same number of items in the output collection as were passed in.

// [2; 4; 6; 8; 10; 12; 14; 16; 18; 20]
[1 .. 10] |> List.map (fun n -> n * 2)

type Person = { Name : string; Town : string }

let persons =
    [
        { Name = "Isaak"; Town = "London" }
        { Name = "Sara"; Town = "Birmingham" }
        { Name = "Tim"; Town = "London" }
        { Name = "Michelle"; Town = "Manchester" }
    ]

// ["London"; "Birmingham"; "London"; "Manchester"]
persons |> List.map (fun person -> person.Town)

map2, map3 (Array, List, Seq)

map2 and map3 are variations of map that take multiple lists. The collections must have equal lengths, except for Seq.map2 and Seq.map3 where extra elements are ignored.

let list1 = [1; 2; 3]
let list2 = [4; 5; 6]
let list3 = [7; 8; 9]

// [5; 7; 9]
(list1, list2) ||> List.map2 (fun x y -> x + y)

// [12; 15; 18]
(list1, list2, list3) |||> List.map3 (fun x y z -> x + y + z) 

mapi, mapi2 (Array, List, Seq)

mapi and mapi2 - in addition to the element, the function needs to be passed the index of each element. The only difference mapi2 and mapi is that mapi2 works with two collections.

let list1 = [9; 12; 53; 24; 35]

// [(0, 9); (1, 12); (2, 53); (3, 24); (4, 35)]
list1 |> List.mapi (fun x i -> (x, i))

// [9; 13; 55; 27; 39]
list1 |> List.mapi (fun x i -> x + i)

let list1 = [9; 12; 3]
let list2 = [24; 5; 2]

// [0; 17; 10]
(list1, list2) ||> List.mapi2 (fun i x y -> (x + y) * i) 

indexed (Array, List, Seq)

Returns a new collection whose elements are the corresponding elements of the input paired with the index (from 0) of each element.

let list1 = [23; 5; 12]

// [(0, 23); (1, 5); (2, 12)]
let result = list1 |> Array.indexed

iter, iter2, iteri, iteri2 (Array, List, Seq)

iter is the same as a for loop, the function that you pass in must return unit.

let list1 = [1; 2; 3]
let list2 = [4; 5; 6]

// Prints: 1; 2; 3; 
list1 |> List.iter (fun x -> printf "%d; " x)

// Prints: (1 4); (2 5); (3 6);
(list1, list2) ||> List.iter2 (fun x y -> printf "(%d %d); " x y)

// Prints: ([0] = 1); ([1] = 2); ([2] = 3);
list1 |> List.iteri (fun i x -> printf "([%d] = %d); " i x)

// Prints: ([0] = 1 4); ([1] = 2 5); ([2] = 3 6);
(list1, list2) ||> List.iteri2 (fun i x y -> printf "([%d] = %d %d); " i x y) 

collect (Array, List, Seq)

collect runs a specified function on each element and then collects the elements generated by the function and combines them into a new collection.

// [0; 1; 0; 1; 2; 3; 4; 5; 0; 1; 2; 3; 4; 5; 6; 7; 8; 9; 10]
let list1 = [1; 5; 10]
let list2 = [1; 2; 3]

list1 |> List.collect (fun elem -> [0 .. elem])

// [1; 2; 3; 2; 4; 6; 3; 6; 9]
list2 |> List.collect (fun x -> [for i in 1..3 -> x * i])

// Example 3
type Customer =
    {
        Id: int
        OrderId: int list
    }

let c1 = { Id = 1; OrderId = [1; 2]}
let c2 = { Id = 2; OrderId = [43]}
let c3 = { Id = 5; OrderId = [39; 56]}
let customers = [c1; c2; c3]

// [1; 2; 43; 39; 56]
let orders = customers |> List.collect(fun c -> c.OrderId)

pairwise (Array, List, Seq)

pairwise takes a collection and returns a new list of tuple pairs of the original adjacent items.

let list1 = [1; 30; 12; 20]

//  [(1, 30); (30, 12); (12, 20)]
list1 |> List.pairwise

// [31.0; 11.0; 21.0]
[ DateTime(2010,5,1)
  DateTime(2010,6,1)
  DateTime(2010,6,12)
  DateTime(2010,7,3) ]
|> List.pairwise
|> List.map(fun (a, b) -> b - a)
|> List.map(fun time -> time.TotalDays)

windowed (Array, List, Seq)

Returns a list of sliding windows containing elements drawn from the input collection. Each window is returned as a fresh collection. Unlike pairwise the windows are collections, not tuples.

// [['a'; 'b'; 'c']; ['b'; 'c'; 'd']; ['c'; 'd'; 'e']]
['a'..'e'] |> List.windowed 3

Grouping functions

groupBy (Array, List, Seq) == GroupBy() in LINQ

groupBy works exactly as the LINQ version does. The output is a collection of simple tuples. The first element of the tuple is the key, and the second element is the collection of items in that group.

type Person =
    {
        Name: string
        Town: string
    }

let persons =
    [ { Name = "Isaak"; Town = "London" }
      { Name = "Sara"; Town = "Birnmingham" }
      { Name = "Tim"; Town = "London" }
      { Name = "Michelle"; Town = "Manchester" } ]

// [("London", [{ Name = "Isaak"; Town = "London" }; { Name = "Tim"; Town = "London" }]);
//  ("Birnmingham", [{ Name = "Sara"; Town = "Birnmingham" }]);
//  ("Manchester", [{ Name = "Michelle"; Town = "Manchester" }])]
persons |> List.groupBy (fun person -> person.Town)

countBy (Array, List, Seq)

A useful derivative of groupBy is countBy. This has a similar signature, but instead of returning the items in the group, it returns the number of items in each group.

type Person = { Name: string; Town: string }

let persons =
    [
        { Name = "Isaak"; Town = "London" }
        { Name = "Sara"; Town = "Birnmingham" }
        { Name = "Tim"; Town = "London" }
        { Name = "Michelle"; Town = "Manchester" }
    ]

// [("London", 2); ("Birnmingham", 1); ("Manchester", 1)]
persons |> List.countBy (fun person -> person.Town)

partition (Array, List)

partition use predicate and a collection; it returns two collections, partitioned based on the predicate:

// Tupled result in two lists
let londonPersons, otherPersons =
    persons |> List.partition(fun p -> p.Town = "London")

If there are no matches for either half of the split, an empty collection is returned for that half.

chunkBySize (Array, List, Seq)

chunkBySize groups elements into arrays (chunks) of a given size.

let list1 = [33; 5; 16]

// int list list = [[33; 5]; [16]]
let chunkedLst = list1 |> List.chunkBySize 2

splitAt (Array, List)

splitAt splits an Array (List) into two parts at the index you specify. The first part ends just before the element at the given index; the second part starts with the element at the given index.

let xs = [| 1; 2; 3; 4; 5 |]

let left1, right1 = xs |> Array.splitAt 0   // [||] and [|1; 2; 3; 4; 5|]
let left2, right2 = xs |> Array.splitAt 1   // [|1|] and [|2; 3; 4; 5|]
let left3, right3 = xs |> Array.splitAt 5   // [|1; 2; 3; 4; 5|] and [||]
let left4, right4 = xs |> Array.splitAt 6   // InvalidOperationException

splitInto (Array, List, Seq)

Splits the input collection into at most count chunks.

// [[1; 2; 3; 4]; [5; 6; 7]; [8; 9; 10]]
// note that the first chunk has four elements
[1..10] |> List.splitInto 3

// [[1; 2; 3]; [4; 5; 6]; [7; 8]; [9; 10]]
[1..10] |> List.splitInto 4

// [[1; 2]; [3; 4]; [5; 6]; [7; 8]; [9]; [10]]
[1..10] |> List.splitInto 6

Aggregate functions

Aggregate functions take a collection of items and merge them into a smaller collection of items (often just one).

sum, average, min, max (Array, List, Seq)

All of these functions are specialized versions of a more generalized function fold.

let numbers = [1.0 .. 10.0]

let total = numbers |> List.sum         // 55.0
let average = numbers |> List.average   // 5.5
let max = numbers |> List.max           // 10.0
let min = numbers |> List.min           // 1.0

Miscellaneous functions

find (Array, List, Seq) == Single() in LINQ

find - finds the first element that matches a given condition.

let isDivisibleBy number elem = elem % number = 0

let input = [1 .. 10]

input |> List.find (isDivisibleBy 5)    // 5
input |> List.find (isDivisibleBy 11)   // KeyNotFoundException

findBack (Array, List, Seq)

let isDivisibleBy number elem = elem % number = 0

let input = [1 .. 10]

input |> List.findBack (isDivisibleBy 4)    // 8
input |> List.findBack (isDivisibleBy 11)   // KeyNotFoundException

findIndex (Array, List, Seq)

let isDivisibleBy number elem = elem % number = 0

let input = [1 .. 10]

input |> List.findIndex (isDivisibleBy 5)   // 4
input |> List.findIndex (isDivisibleBy 11)  // KeyNotFoundException

findIndexBack (Array, List, Seq)

let isDivisibleBy number elem = elem % number = 0

let input = [1..10]

input |> List.findIndexBack (isDivisibleBy 3)   // 8
input |> List.findIndexBack (isDivisibleBy 11)  // KeyNotFoundException

head, last, tail, item (Array, List, Seq)

Returns the first, last and all-but-first items in the collection.

let input = [15..22]

input |> List.head    // 15
input |> List.last    // 22
input |> List.tail    // [16; 17; 18; 19; 20; 21; 22]

item (Array, List, Seq)

Gets the element at a given index.

let input = [1..7]

input |> List.item 5      // 6
input |> List.item 8      // ArgumentException

take (Array, List, Seq)

Returns the elements of the collection up to a specified count.

let input = [1..10]

input |> List.take 5        // [1; 2; 3; 4; 5]
input |> List.take 11       // InvalidOperationException

truncate (Array, List, Seq)

Returns a collection that when enumerated returns at most N elements.

let input = [1..10]

input |> List.truncate 5    // [1; 2; 3; 4; 5]
input |> List.truncate 11   // [1; 2; 3; 4; 5; 6; 7; 8; 9; 10]

takeWhile (Array, List, Seq)

takeWhile returns each item in a new collection until it reaches an item that does not meet the predicate.

let input = [1..10]

input |> List.takeWhile (fun x -> x / 7 = 0)   // [1; 2; 3; 4; 5; 6]
input |> List.takeWhile (fun x -> x / 17 = 0)     // [1; 2; 3; 4; 5; 6; 7; 8; 9; 10]

skip (Array, List, Seq)

Returns a collection that skips N elements of the underlying sequence and then yields the remaining elements.

let input = [ for i in 1 .. 10 -> i * i ]  // [1; 4; 9; 16; 25; 36; 49; 64; 81; 100]

input |> List.skip 5                       // [36; 49; 64; 81; 100]
input |> List.skip 11                      // ArgumentException

skipWhile (Array, List, Seq)

Returns a collection, when iterated, skips elements of the underlying array (list, seq) while the given predicate returns true, and then yields the remaining elements.

let mySeq = seq { for i in 1 .. 10 -> i * i }    // 1 4 9 16 25 36 49 64 81 100

mySeq |> Seq.skipWhile (fun elem -> elem < 10)   // 16 25 36 49 64 81 100
mySeq |> Seq.skipWhile (fun elem -> elem < 101)  // Empty seq

exists (Array, List, Seq)

Tests if any element of the collection satisfies the given predicate.

let inputs = [0..3]

inputs |> List.exists (fun elem -> elem = 3)      // true
inputs |> List.exists (fun elem -> elem = 10)     // false

exists2 (Array, List, Seq)

Tests if any pair of corresponding elements of the collections satisfies the given predicate. The collections must have equal lengths, except for Seq where extra elements are ignored.

let list1to5 = [1 .. 5]           // [1; 2; 3; 4; 5]
let list0to4 = [0 .. 4]           // [0; 1; 2; 3; 4]
let list5to1 = [5 .. -1 .. 1]     // [5; 4; 3; 2; 1]
let list6to1 = [6 .. -1 .. 1]     // [6; 5; 4; 3; 2; 1]

(list1to5, list5to1) ||> List.exists2 (fun i1 i2 -> i1 = i2)    // true
(list1to5, list0to4) ||> List.exists2 (fun i1 i2 -> i1 = i2)    // false
(list1to5, list6to1) ||> List.exists2 (fun i1 i2 -> i1 = i2)    // ArgumentException

forall (Array, List, Seq)

Tests if all elements of the collection satisfy the given predicate.

let inputs = [2; 4; 6; 8; 10]

inputs |> List.forall (fun i -> i % 2 = 0)    // true
inputs |> List.forall (fun i -> i % 2 = 0)    // false

forall2 (Array, List, Seq)

Returns true if all corresponding elements of the collection satisfy the given predicate pairwise. The collections must have equal lengths, except for Seq where extra elements are ignored.

let lst1 = [0; 1; 2]
let lst2 = [0; 1; 2]
let lst3 = [2; 1; 0]
let lst4 = [0; 1; 2; 3]

(lst1, lst2) ||> List.forall2 (fun i1 i2 -> i1 = i2)    // true
(lst1, lst3) ||> List.forall2 (fun i1 i2 -> i1 = i2)    // false
(lst1, lst4) ||> List.forall2 (fun i1 i2 -> i1 = i2)    // ArgumentException

contains (Array, List, Seq)

Returns true if a collection contains an equal value:

let rushSet = ["Dirk"; "Lerxst"; "Pratt"]
let gotSet = rushSet |> List.contains "Lerxst"      // true

filter, where (Array, List, Seq)

Returns a new collection containing only the elements of the collection for which the given predicate returns true.

let data =
    [("Cats",4)
     ("Tiger",5)
     ("Mice",3)
     ("Elephants",2) ]

// [("Cats", 4); ("Mice", 3)]
data |> List.filter (fun (nm, x) -> nm.Length <= 4)
data |> List.where (fun (nm, x) -> nm.Length <= 4)

length (Array, List, Seq)

Returns the length of the collection.

[ 1 .. 100 ] |> List.length         // 100
[ ] |> List.length                  // 0
[ 1 .. 2 .. 100 ] |> List.length    // 50

distinctBy (Array, List, Seq)

Returns a collection that contains no duplicate entries according to the generic hash and equality comparisons on the keys returned by the given key-generating function.

let inputs = [-5 .. 10]

// [-5; -4; -3; -2; -1; 0; 6; 7; 8; 9; 10]
inputs  |> List.distinctBy (fun i -> abs i)

distinct (Array, List, Seq) == Distinct() in LINQ

Returns a collection that contains no duplicate entries according to generic hash and equality comparisons on the entries.

[1; 3; 9; 4; 3; 1] |> List.distinct    // [1; 3; 9; 4]
[1; 1; 1; 1; 1; 1] |> List.distinct     // [1]
[ ] |> List.distinct                    // error FS0030: Value restriction

sortBy (Array, List, Seq) == OrderBy() in LINQ

Sorts the given collection using keys given by the given projection. Keys are compared using Operators.compare.

[1; 4; 8; -2; 5] |> List.sortBy (fun x -> abs x)    // [1; -2; 4; 5; 8]

sort (Array, List, Seq)

Sorts the given list using Operators.compare.

[1; 4; 8; -2; 5] |> List.sort    // [-2; 1; 4; 5; 8]

sortByDescending (Array, List, Seq)

[-3..3] |> List.sortByDescending (fun x -> abs x)   // [-3; 3; -2; 2; -1; 1; 0]

sortDescending (Array, List, Seq)

[0..5] |> List.sortDescending    // [5; 4; 3; 2; 1; 0]

sortWith (Array, List, Seq)

Sorts the given collection using the given comparison function.

let lst = ["<>"; "&"; "&&"; "&&&"; "<"; ">"; "|"; "||"; "|||"]

let sortFunction (str1: string) (str2: string) =
    if (str1.Length > str2.Length) then
        1
    else
        -1

// ["|"; ">"; "<"; "&"; "||"; "&&"; "<>"; "|||"; "&&&"]
lst |> List.sortWith sortFunction

Array, List and Seq functions

allPairs (Array, List, Seq)

Takes 2 arrays (list, seq), and returns all possible pairs of elements.

let arr1 = [| 0; 1 |]
let arr2 = [| 4; 5 |]

(arr1, arr2) ||> Array.allPairs arr1 arr2    // [|(0, 4); (0, 5); (1, 4); (1, 5)|]

append (Array, List, Seq)

Combines 2 arrays (list, seq).

let list1 = [33; 5; 16]
let list2 = [42; 23; 18]

List.append list1 list2     // [33; 5; 16; 42; 23; 18]

averageBy (Array, List, Seq)

averageBy take a function as a parameter, and this function's results are used to calculate the values for the average.

// val avg1 : float = 2.0
[1..3] |> List.averageBy (fun elem -> float elem)

// val avg2 : float = 4.666666667
[| 1..3 |] |> Array.averageBy (fun elem -> float (elem * elem))

Seq.cache

Seq.cache creates a stored version of a sequence. Use Seq.cache to avoid reevaluation of a sequence, or when you have multiple threads that use a sequence, but you must make sure that each element is acted upon only one time. When you have a sequence that is being used by multiple threads, you can have one thread that enumerates and computes the values for the original sequence, and remaining threads can use the cached sequence.

choose (Array, List, Seq)

choose enables you to transform and select elements at the same time.

let list1 = [33; 5; 16]

// [34; 6; 17]
list1 |> List.choose (fun elem -> Some(elem + 1)) 

compareWith (Array, List, Seq)

Compare two arrays (lists, seq) by using the compareWith function. The function compares successive elements in turn, and stops when it encounters the first unequal pair. Any additional elements do not contribute to the comparison.

let sq1 = seq { 1; 2; 4; 5; 7 }
let sq2 = seq { 1; 2; 3; 5; 8 }
let sq3 = seq { 1; 3; 3; 5; 2 }

let compareSeq seq1 seq2 =
    (seq1, seq2 ||> Seq.compareWith (fun e1 e2 ->
        if e1 > e2 then 1
        elif e1 < e2 then -1
        else 0)

let compareResult1 = compareSeq sq1 sq2     // int = 1
let compareResult2 = compareSeq sq2 sq3     // int = -1

concat (Array, List, Seq)

concat is used to join any number of arrays (lists, seq).

// int list = [1; 2; 3; 4; 5; 6; 7; 8; 9]
List.concat [[1; 2; 3]; [4; 5; 6]; [7; 8; 9]]

Array.copy

Creates a new array that contains elements that are copied from an existing array. The copy is a shallow copy, which means that if the element type is a reference type, only the reference is copied, not the underlying object.

let firstArray : StringBuilder array = Array.init 3 (fun index -> new StringBuilder(""))
let secondArray = Array.copy firstArray
// Two arrays: [|; ; |]

firstArray.[0] <- new StringBuilder("Test1")
// firstArray: [|Test1; ; |]
// secondArray: [|; ; |]

firstArray.[1].Insert(0, "Test2") |> ignore
// firstArray: [|Test1; Test2; |]
// secondArray: [|; Test2; |]

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