Lazy.fs

namespace FSharpPlus

/// Additional operations on Lazy - delayed computations
[<RequireQualifiedAccess>]
module Lazy =

    /// <summary>Creates a Lazy value from another Lazy value, mapping through a function.</summary>
    /// <param name="mapping">The mapping function.</param>
    /// <param name="x">The Lazy value.</param>
    ///
    /// <returns>The mappeed value.</returns>
    let map (mapping: 'T -> 'U) (x: Lazy<'T>) = Lazy<_>.Create (fun () -> mapping x.Value) : Lazy<'U>

    /// <summary>Creates a Lazy value from a pair of Lazy values, using a mapping function to combine them.</summary>
    /// <param name="mapping">The mapping function.</param>
    /// <param name="x">The first Lazy value.</param>
    /// <param name="y">The second Lazy value.</param>
    ///
    /// <returns>The combined value.</returns>
    let map2 (mapping: 'T->'U->'V) (x: Lazy<'T>) (y: Lazy<'U>) = Lazy<_>.Create (fun () -> mapping x.Value y.Value)
    
    /// <summary>Creates a Lazy value from three Lazy values, using a function to combine them.</summary>
    /// <param name="mapping">The mapping function.</param>
    /// <param name="x">The first Lazy value.</param>
    /// <param name="y">The second Lazy value.</param>
    /// <param name="z">The third Lazy value.</param>
    ///
    /// <returns>The combined value.</returns>
    let map3 (mapping: 'T->'U->'V->'W) (x: Lazy<'T>) (y: Lazy<'U>) (z: Lazy<'V>) = Lazy<_>.Create (fun () -> mapping x.Value y.Value z.Value)

    /// <summary>Applies a Lazy value to a Lazy function.</summary>
    /// <param name="f">The Lazy function.</param>
    /// <param name="x">The Lazy value.</param>
    /// <returns>A Lazy value of the function applied to the value.</returns>
    let apply (f: Lazy<'T->'U>) (x: Lazy<'T>) : Lazy<'U> = Lazy<_>.Create (fun () -> f.Value x.Value)