F# Cheat Sheet

Core syntax for F#, covering bindings, functions, pipelines, discriminated unions, and pattern matching on .NET.

2 PagesIntermediateMar 25, 2026

Values & Functions

Immutable bindings and function definitions.

fsharp

// Values and functionslet x = 42                     // immutable bindinglet name = "SkillVeris"let add a b = a + b            // function definitionlet square x = x * xprintfn "Sum: %d" (add 2 3)    // formatted printlet mutable counter = 0        // explicitly mutablecounter <- counter + 1

Pipelines & Composition

The pipe operator and function composition.

fsharp

let isEven n = n % 2 = 0[1..10]|> List.filter isEven|> List.map (fun n -> n * n)|> List.sum|> printfn "Result: %d"// Function composition with >>let addThenDouble = (+) 1 >> (*) 2

Core Types

F#'s main type-system building blocks.

record- Immutable named-field type: type Point = { X: int; Y: int }
discriminated union- Sum type: type Shape = Circle of float | Square of float
option- Represents an optional value: Some 5 or None
tuple- Fixed-size grouping: let pair = (1, "a")
unit- Type with a single value (), analogous to void
list vs array- list: T list (immutable, linked); array: T[] (mutable, fixed size)

Pattern Matching

Matching on unions and options.

fsharp

type Shape =    | Circle of radius: float    | Rectangle of width: float * height: floatlet area shape =    match shape with    | Circle r -> System.Math.PI * r * r    | Rectangle (w, h) -> w * hmatch Some 5 with| Some x when x > 0 -> printfn "positive %d" x| Some _ -> printfn "non-positive"| None -> printfn "none"