Addendum

Memoization

💡 Idea: reduce the calculation time of a function

❓ How to: cache results → next call with same arguments, return cached result

👉 In practice : function memoizeN from library FSharpPlus

⚠️ Caution : As with any optimization, use it when you need it and check (measure) that it works without any additional inconvenience.

☝ Not to be confused with lazy expression...

Lazy expression

Syntax sugar to create a .NET Lazy<'T> object from an expression

• Expression not evaluated immediately but on 1st request (Thunk)

• Interesting for improving performance without overcomplexifying the code

let printAndForward x = printfn $"{x}"; x

let a = lazy (printAndForward "a")

let b = printAndForward "b"
// > b

printfn $"{a.Value} and {b}"
// > a
// > a and b

printfn $"{a.Value} and c"
// > a and c

Lazy active pattern

Extracts the value from a Lazy object

let triple (Lazy i) = 3 * i // Lazy<int> -> int

let v =
    lazy
        (printf "eval!"
         5 + 5) // Lazy<int>

triple v
// eval!val it: int = 30

triple v
// val it: int = 30

Organizing functions

3 ways to organize functions = 3 places to declare them:

• Module: function declared within a module 📍

• Nested : function declared inside a value or inside another function

  • 💡 Encapsulate helpers used just locally

  • ☝ Parent function parameters accessible to function nested

• Method : function defined as a method in a type...

Methods

• Defined with the member keyword rather than let

• Choice of the self-identifier: this, me, self, _...

• Choice of the parameters style:

  • Tuplified: OOP style

  • Curried: FP style

Method Examples

type Product =
    { SKU: string; Price: float }

    // Tuple style, `this`
    member this.TupleTotal(qty, discount) =
        (this.Price * float qty) - discount

    // Curried style, `me`
    member me.CurriedTotal qty discount =
        (me.Price * float qty) - discount

Function vs Method

Feature	Function	Method
Naming convention	camelCase	PascalCase
Currying	✅ yes	✅ if not tuplified nor overridden
Named parameters	❌ no	✅ if tuplified
Optional parameters	❌ no	✅ if tuplified
Overload	❌ no	✅ if tuplified
Parameter inference (declaration)	➖ Possible	➖ yes for this, possible for the other parameters
Argument inference (usage)	✅ yes	❌ no, object type annotation needed
High-order function argument	✅ yes	➖ yes with shorthand member, no with lambda otherwise
inline supported	✅ yes	✅ yes
Recursive	✅ yes with rec	✅ yes

Function vs Delegate

We can define a delegate in F# with the syntax: type Fn = delegate of args: Args -> Return

👉 A delegate adds a thin layer on top of a function signature, like an object with a single method Invoke:

• Pros: we can use named arguments when invoking the delegate

• Cons:

  • extra work to wrap the function (or the method) in the delegate and to execute the function by calling the Invoke(...) method

  • cumbersome syntax when there are several parameters, to differentiate between tuple and curried parameters:

    • Tuple: type MyFn = delegate of (int * int) -> int

    • Curried: type MyFn = delegate of int * int -> int

It's not a very common use case in F#.

Interop with the BCL

void method

A .NET void method is seen in F♯ as returning unit.

let list = System.Collections.Generic.List<int>()
list.Add
(* IntelliSense Ionide:
abstract member Add:
   item: int
      -> unit
*)

Conversely, an F♯ function returning unit is compiled into a void method.

let ignore _ = ()

Equivalent C# based on SharpLab:

public static void ignore<T>(T _arg) {}

Calling a BCL method with N arguments

A .NET method with several arguments is "pseudo-tuplified":

• All arguments must be specified (1)

• Partial application of parameters is not supported (2)

• Calls don't work with a real F♯ tuple ⚠️ (3)

System.String.Compare("a", "b") // ✅ (1)
System.String.Compare "a","b"   // ❌
System.String.Compare "a"       // ❌ (2)

let tuple = ("a","b")
System.String.Compare tuple     // ❌ (3)

out Parameter - In C♯

out used to have multiple output values from a method → Ex : Int32.TryParse, Dictionary<,>.TryGetValue :

if (int.TryParse(maybeInt, out var value))
    Console.WriteLine($"It's the number {value}.");
else
    Console.WriteLine($"{maybeInt} is not a number.");

out Parameter - In F♯

Output can be consumed as a tuple 👍

  match System.Int32.TryParse maybeInt with
  | true, i  -> printf $"It's the number {value}."
  | false, _ -> printf $"{maybeInt} is not a number."

Instantiate a class with new?

// (1) new allowed but not recommended
type MyClass(i) = class end

let c1 = MyClass(12)      // 👍
let c2 = new MyClass(234) // 👌 mais pas idiomatique

// (2) IDisposable => `new` required, `use` replaces `let` (otherwise it's a compiler warning)
open System.IO
let fn () =
    use f = new FileStream("hello.txt", FileMode.Open)
    f.Close()

Calling an overloaded method

• Compiler may not understand which overload is being called

• Tips: call with named argument

let createReader fileName =
    new System.IO.StreamReader(path = fileName)
    // ☝️ Param `path` → `filename` inferred as `string`

let createReaderByStream stream =
    new System.IO.StreamReader(stream = stream)
    // ☝️ Param `stream` of type `System.IO.Stream`