F# lambda

last modified July 17, 2023

In this article, we show how to work with lambda expressions in F#.

A lambda is an anonymous function which is not bound to an identifier. Lambda expressions are defined with the fun keyword. Lambdas are typically short and concise functions that are not meant to be reused.

F# simple lambda

In the following example, we define a simple lambda.

let inc x = x + 1 

let v = inc 90
printfn "%d" v

let r = (fun x -> x + 1) 100
printfn "%d" r

The program contains a simple function and a lambda.

let inc x = x + 1 

This is a definition of a function called inc, which increments its parameter.

let v = inc 90
printfn "%d" v

The inc function is called and the result is printed.

let r = (fun x -> x + 1) 100

A lambda function is defined and immediately called. The function is anonymous and cannot be reused.

λ dotnet fsi main.fsx

F# lambda with high-order functions

Lambdas are often passed to high-order functions to define predicates.

let vals = [ 2; 1; -5; 8; 9; -2; 0; 5; 4 ]

let res = List.map(fun e -> e * 2) vals
printfn "%A" res

let res2 = List.filter(fun e -> e < 0) vals
printfn "%A" res2

In the example, we use two lambdas to create predicates for List.map and List.filter functions.

λ dotnet fsi main.fsx
[4; 2; -10; 16; 18; -4; 0; 10; 8]
[-5; -2]

F# lambda argument types

The argument types are provided within round brackets, separately for each argument.

let msg = (fun (name: string)(age: int) -> $"{name} is {age} years old") "John Doe" 34
printfn "%s" msg

We have a lambda expression with two parameters: one string and one integer. The arguments and their types are defined in () brackets.

λ dotnet fsi main.fsx
John Doe is 34 years old

F# lambda with let binding

We can use a let binding in a lambda.

let names =
    [ "John Doe"
      "Lucy Smith"
      "Benjamin Young"
      "Robert Brown"
      "Thomas Moore"
      "Linda Black"
      "Adam Smith"
      "Jane Smith" ]

|> List.sortBy (fun e -> let a = e.Split(" ") in Array.get a 1)
|> printfn "%A"

In the program, we sort a list of names by surnames.

|> List.sortBy (fun e -> let a = e.Split(" ") in Array.get a 1)

We split each name by a space and then retrieve the second element of the split.

λ dotnet fsi main.fsx
["Linda Black"; "Robert Brown"; "John Doe"; "Thomas Moore"; "Lucy Smith";
 "Adam Smith"; "Jane Smith"; "Benjamin Young"]

In this article we have worked with lambdas in F#.


My name is Jan Bodnar and I am a passionate programmer with many years of programming experience. I have been writing programming articles since 2007. So far, I have written over 1400 articles and 8 e-books. I have over eight years of experience in teaching programming.