The support for generalised monad tail-recursion release that I did in the early hours of Christmas Day needed some support functions, as also requested here, so I have added the following functions to the Monad module:

| Function | Behaviour | | ------- | ------- | | Monad.forever | Repeatedly runs a monad's effect | | Monad.replicate | Runs a monad's effect n times, collecting the results in an Iterable<A> | | Monad.accumWhile | Repeatedly runs a monad's effect while the predicate returns true, collecting the results in an Iterable<A> | | Monad.accumWhileM | Repeatedly runs a monad's effect while the monadic predicate returns true, collecting the results in | an Iterable<A> | | Monad.accumUntil | Repeatedly runs a monad's effect until the predicate returns true, collecting the results in an Iterable<A> | | Monad.accumUntilM | Repeatedly runs a monad's effect until the monadic predicate returns true, collecting the results in an Iterable<A> |

The functions themselves are quite good demonstrations of Monad.recur, so I have pasted the source-code below just FYI:

/// <summary>
/// This is equivalent to the infinite loop below without the stack-overflow issues:
///
///     K〈M, A〉go =>
///         ma.Bind(_ => go);
/// 
/// </summary>
/// <param name="ma">Computation to recursively run</param>
/// <typeparam name="M">Monad</typeparam>
/// <typeparam name="A">Bound value type</typeparam>
/// <typeparam name="B">'Result' type, there will never be a result of `B`, but the monad rules may exit the loop
/// with an alternative value; and so `B` is still valid</typeparam>
/// <returns>A looped computation</returns>
[Pure]
public static K<M, A> forever<M, A>(K<M, A> ma)
    where M : Monad<M> =>
    forever<M, A, A>(ma);

/// <summary>
/// This is equivalent to the infinite loop below without the stack-overflow issues:
///
///     K〈M, A〉go =>
///         ma.Bind(_ => go);
/// 
/// </summary>
/// <param name="ma">Monadic computation to recursively run</param>
/// <typeparam name="M">Monad</typeparam>
/// <typeparam name="A">Bound value type</typeparam>
/// <typeparam name="B">'Result' type, there will never be a result of `B`, but the monad rules may exit the loop
/// with an alternative value; and so `B` is still valid</typeparam>
/// <returns>A looped computation</returns>
[Pure]
public static K<M, B> forever<M, A, B>(K<M, A> ma) 
    where M : Monad<M> => 
    recur<M, A, B>(default!, _ => ma.Map(_ => Next<A, B>.UnsafeDefault));

/// <summary>
/// Running the monadic computation `ma` a fixed number of times (`count`) collecting the results
/// </summary>
/// <param name="ma">Monadic computation to repeatedly run</param>
/// <param name="count">Number of times to replicate monadic computation</param>
/// <typeparam name="M">Monad</typeparam>
/// <typeparam name="A">Bound value type</typeparam>
/// <returns>A lifted iterable of values collected</returns>
[Pure]
public static K<M, Iterable<A>> replicate<M, A>(K<M, A> ma, int count)
    where M : Monad<M> =>
    recur<M, (Iterable<A> Items, int Remain), Iterable<A>>(
        ([], count),
        acc => ma * (a => acc.Remain > 0 
                      ? Next.Loop<(Iterable<A> Items, int Remain), Iterable<A>>((acc.Items.Add(a), acc.Remain - 1))
                      : Next.Done<(Iterable<A> Items, int Remain), Iterable<A>>(acc.Items.Add(a))));

/// <summary>
/// Keep running the monadic computation `ma` collecting the result values until a result value
/// yielded triggers a `true` value when passed to the `f` predicate
/// </summary>
/// <param name="ma">Monadic computation to repeatedly run</param>
/// <param name="f">Predicate</param>
/// <typeparam name="M">Monad</typeparam>
/// <typeparam name="A">Bound value type</typeparam>
/// <returns>A lifted iterable of values collected</returns>
[Pure]
public static K<M, Iterable<A>> accumUntil<M, A>(K<M, A> ma, Func<A, bool> f)
    where M : Monad<M> =>
    recur<M, Iterable<A>, Iterable<A>>(
        [],
        acc => ma * (a => f(a) ? Next.Done<Iterable<A>, Iterable<A>>(acc)
                               : Next.Loop<Iterable<A>, Iterable<A>>(acc.Add(a))));

/// <summary>
/// Keep running the monadic computation `ma` collecting the result values until a result value
/// yielded triggers a `true` value when passed to the `f` predicate
/// </summary>
/// <param name="ma">Monadic computation to repeatedly run</param>
/// <param name="f">Predicate</param>
/// <typeparam name="M">Monad</typeparam>
/// <typeparam name="A">Bound value type</typeparam>
/// <returns>A lifted iterable of values collected</returns>
[Pure]
public static K<M, Iterable<A>> accumUntilM<M, A>(K<M, A> ma, Func<A, K<M, bool>> f)
    where M : Monad<M> =>
    recur<M, Iterable<A>, Iterable<A>>(
        [],
        acc => ma >> (a => f(a) * (r => r ? Next.Done<Iterable<A>, Iterable<A>>(acc)
                                          : Next.Loop<Iterable<A>, Iterable<A>>(acc.Add(a)))));

/// <summary>
/// Keep running the monadic computation `ma` collecting the result values until a result value
/// yielded triggers a `true` value when passed to the `f` predicate
/// </summary>
/// <param name="ma">Monadic computation to repeatedly run</param>
/// <param name="f">Predicate</param>
/// <typeparam name="M">Monad</typeparam>
/// <typeparam name="A">Bound value type</typeparam>
/// <returns>A lifted iterable of values collected</returns>
[Pure]
public static K<M, Iterable<A>> accumWhile<M, A>(K<M, A> ma, Func<A, bool> f)
    where M : Monad<M> =>
    recur<M, Iterable<A>, Iterable<A>>(
        [],
        acc => ma * (a => f(a) ? Next.Loop<Iterable<A>, Iterable<A>>(acc.Add(a))
                               : Next.Done<Iterable<A>, Iterable<A>>(acc)));

/// <summary>
/// Keep running the monadic computation `ma` collecting the result values until a result value
/// yielded triggers a `true` value when passed to the `f` predicate
/// </summary>
/// <param name="ma">Monadic computation to repeatedly run</param>
/// <param name="f">Predicate</param>
/// <typeparam name="M">Monad</typeparam>
/// <typeparam name="A">Bound value type</typeparam>
/// <returns>A lifted iterable of values collected</returns>
[Pure]
public static K<M, Iterable<A>> accumWhileM<M, A>(K<M, A> ma, Func<A, K<M, bool>> f)
    where M : Monad<M> =>
    recur<M, Iterable<A>, Iterable<A>>(
        [],
        acc => ma >> (a => f(a) * (r => r ? Next.Loop<Iterable<A>, Iterable<A>>(acc.Add(a))
                                          : Next.Done<Iterable<A>, Iterable<A>>(acc))));