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Charles Roddie committed Oct 18, 2023
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Expand Up @@ -44,6 +44,9 @@ local.properties
## Visual Studio
#################

# Visual Studio cache/options directory
.vs/

## Ignore Visual Studio temporary files, build results, and
## files generated by popular Visual Studio add-ons.

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123 changes: 62 additions & 61 deletions docs/fsharp-cheatsheet.md
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Expand Up @@ -6,18 +6,18 @@ The Microsoft [F# Documentation](https://learn.microsoft.com/en-us/dotnet/fsharp

Contents
--------
[Comments](#Comments)
[Strings](#Strings)
[Basic Types and Literals](#BasicTypesAndLiterals)
[Functions](#Functions)
[Pattern Matching](#PatternMatching)
[Collections](#Collections)
[Tuples and Records](#TuplesAndRecords)
[Discriminated Unions](#DiscriminatedUnions)
[Exceptions](#Exceptions)
[Classes and Inheritance](#ClassesAndInheritance)
[Interfaces and Object Expressions](#InterfacesAndObjectExpressions)
[Active Patterns](#ActivePatterns)
[Comments](#Comments)
[Strings](#Strings)
[Basic Types and Literals](#BasicTypesAndLiterals)
[Functions](#Functions)
[Pattern Matching](#PatternMatching)
[Collections](#Collections)
[Tuples and Records](#TuplesAndRecords)
[Discriminated Unions](#DiscriminatedUnions)
[Exceptions](#Exceptions)
[Classes and Inheritance](#ClassesAndInheritance)
[Interfaces and Object Expressions](#InterfacesAndObjectExpressions)
[Active Patterns](#ActivePatterns)
[Compiler Directives](#CompilerDirectives)

<a name="Comments"></a>Comments
Expand All @@ -27,9 +27,9 @@ Block comments are placed between `(*` and `*)`. Line comments start from `//` a
(* This is block comment *)

// And this is line comment
XML doc comments come after `///` allowing us to use XML tags to generate documentation.

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

Expand All @@ -50,7 +50,7 @@ We don't even have to escape `"` with *triple-quoted strings*.

*Backslash strings* indent string contents by stripping leading spaces.

let poem =
let poem =
"The lesser world was daubed\n\
By a colorist of modest skill\n\
A master limned you in the finest inks\n\
Expand Down Expand Up @@ -81,29 +81,29 @@ See [Literals (MS Learn)](https://learn.microsoft.com/en-us/dotnet/fsharp/langua
---------
The `let` keyword also defines named functions.

let negate x = x * -1
let square x = x * x
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))
let squareNegateThenPrint x =
print (negate (square x))

### Pipe and composition operators
Pipe operator `|>` is used to chain functions and arguments together. Double-backtick identifiers are handy to improve readability especially in unit testing:

let ``square, negate, then print`` x =
let ``square, negate, then print`` x =
x |> square |> negate |> print

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

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

Composition operator `>>` is used to compose functions:

let squareNegateThenPrint' =
let squareNegateThenPrint' =
square >> negate >> print

### Recursive functions
Expand All @@ -116,7 +116,7 @@ The `rec` keyword is used together with the `let` keyword to define a recursive
*Mutually recursive* functions (those functions which call each other) are indicated by `and` keyword:

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

and odd x =
Expand Down Expand Up @@ -165,8 +165,8 @@ A *list* is an immutable collection of elements of the same type.
let list1 = [ "a"; "b" ]
// :: is prepending
let list2 = "c" :: list1
// @ is concat
let list3 = list1 @ list2
// @ is concat
let list3 = list1 @ list2

// Recursion on list using (::) operator
let rec sum list =
Expand All @@ -180,8 +180,8 @@ A *list* is an immutable collection of elements of the same type.
// Arrays use square brackets with bar
let array1 = [| "a"; "b" |]
// Indexed access using dot
let first = array1.[0]
let first = array1.[0]

### Sequences
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.

Expand Down Expand Up @@ -215,8 +215,9 @@ Lists and arrays have comprehensive sets of higher-order functions for manipulat

- `fold` starts from the left of the list (or array) and `foldBack` goes in the opposite direction

let xs' = Array.fold (fun str n ->
sprintf "%s,%i" str n) "" [| 0..9 |]
let xs' =
Array.fold (fun str n ->
sprintf "%s,%i" str n) "" [| 0..9 |]

- `reduce` doesn't require an initial accumulator

Expand Down Expand Up @@ -300,7 +301,7 @@ Records are essentially sealed classes with extra topping: default immutability,
F# Core has built-in discriminated unions for error handling, e.g., [`option`](https://learn.microsoft.com/en-us/dotnet/fsharp/language-reference/options) and [`Result`](https://learn.microsoft.com/en-us/dotnet/fsharp/language-reference/results).

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

Expand All @@ -326,49 +327,49 @@ The `failwith` function throws an exception of type `Exception`.
Exception handling is done via `try/with` expressions.

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

<a name="ClassesAndInheritance"></a>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)
member this.X = x // (2)
member this.Y = y
member this.Mag = mag
member this.Scale(s) = // (3)
member _.X = x // (2)
member _.Y = y
member _.Mag = mag
member _.Scale(s) = // (3)
Vector(x * s, y * s)
static member (+) (a : Vector, b : Vector) = // (4)
Vector(a.X + b.X, a.Y + b.Y)

Call a base class from a derived one.

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

*Upcasting* is denoted by `:>` operator.
Expand All @@ -391,8 +392,8 @@ Declare `IVector` interface and implement it in `Vector'`.
interface IVector with
member __.Scale(s) =
Vector'(x * s, y * s) :> IVector
member __.X = x
member __.Y = y
member _.X = x
member _.Y = y

Another way of implementing interfaces is to use *object expressions*.

Expand All @@ -401,9 +402,9 @@ Another way of implementing interfaces is to use *object expressions*.
abstract Age : int

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

<a name="ActivePatterns"></a>Active Patterns
---------------
Expand All @@ -413,12 +414,12 @@ Another way of implementing interfaces is to use *object expressions*.
let (|EmailDomain|) (email: string) =
let parts = email.Split '@'
parts[1]
let (EmailDomain emailDomain) = "[email protected]" // emailDomain = 'aretuza.org'
let (EmailDomain emailDomain) = "[email protected]" // emailDomain = 'aretuza.org'

// As Parameters
let (|Real|) (x: System.Numerics.Complex) =
(x.Real, x.Imaginary)
let addReal (Real aa) (Real bb) = // conversion done in the parameters
let addReal (Real aa) (Real bb) = // conversion done in the parameters
fst aa + fst bb
let addRealOut = addReal System.Numerics.Complex.ImaginaryOne System.Numerics.Complex.ImaginaryOne

Expand All @@ -427,8 +428,8 @@ Another way of implementing interfaces is to use *object expressions*.
match value with
| None -> onNone
| Some e -> e
let (Default "random citizen" name) = None // name = "random citizen"
let (Default "random citizen" name) = Some "Steve" // name = "Steve"
let (Default "random citizen" name) = None // name = "random citizen"
let (Default "random citizen" name) = Some "Steve" // name = "Steve"

*Single-case active patterns* can be thought of as a simple way to convert data to a new form.

Expand All @@ -445,7 +446,7 @@ Another way of implementing interfaces is to use *object expressions*.
let (|Phone|Email|) (s:string) =
if s.Contains '@' then Email $"Email: {s}" else Phone $"Phone: {s}"

match "[email protected]" with // output: "Email: [email protected]"
match "[email protected]" with // output: "Email: [email protected]"
| Email email -> printfn $"{email}"
| Phone phone -> printfn $"{phone}"

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