Mutables in Gjallarhorn

One core type in Gjallarhorn is a Mutable.

A Mutable extends the concept of a signal by allowing the current value to be changed.

The simplest Mutable can be created via Mutable.create:

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#r "Gjallarhorn.dll"
open Gjallarhorn

// Create a mutable variable
let m = Mutable.create 42

// Prints "Value = 42"
printfn "Value = %d" m.Value

m.Value <- 24
// Prints "Value = 24"
printfn "Value = %d" m.Value

As it's name suggests, Mutable.create takes an initial value and wraps it into a Mutable, which is represented via the IMutatable<'a> interface.

IMutatable<'a> inherits ISignal<'a>, but extends the .Value property to be both gettable and settable, which we can use to fetch or mutate the current value at any point.

As all Mutables are also signals, the Signal module functionality still works:

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// Create a mutable variable
let m = Mutable.create 0

Signal.Subscription.create (fun currentValue -> printfn "Value is now %d" currentValue) m 

// After this is set, a print will occur
// Prints: "Value is now 1"
m.Value <- 1

// After this is set, a second print will occur with the new value
// Prints: "Value is now 2"
m.Value <- 2

Now, let's move on to Validation in Gjallarhorn.

val m : obj

Full name: Mutables.m

val printfn : format:Printf.TextWriterFormat<'T> -> 'T

Full name: Microsoft.FSharp.Core.ExtraTopLevelOperators.printfn