How to Declare and Define an Boolean data type variable in F# ?

// AbundantCode F# Tutorials and Code snippets
open System
[<EntryPoint>]
let main argv = 
    //How to Declare and Define an Boolean data type variable in F# ?
    let booleanType = true
    Console.WriteLine(booleanType)
    let retval = Console.ReadLine()
    0 // return an integer exit code

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