I have no idea what I'm doing.

st.cs

public class STRunner {
    private class STToken { }
    public A RunST<A>(ISTScope<A> scope) {
        return scope.Get<STToken>().UnsafeRun(new STToken());
    }
}

public interface ISTScope<A> {
    ST<S, A> Get<S>();
}

public class ST<S, A> {
    private readonly Func<S, Tuple<A, S>> _mutableOp;
    internal A UnsafeRun(S state) { return _mutableOp(state).Item1; }

    public ST(Func<S,Tuple<A,S>> mutableOp) {
        _mutableOp = mutableOp;
    }

    public ST<S, C> SelectMany<B,C>(Func<A, ST<S, B>> f, Func<A, B, C> selector) {
        return new ST<S, C>(s => {
            var stateAndA = _mutableOp(s);
            var stB = f(stateAndA.Item1);
            var stateAndB = stB._mutableOp(stateAndA.Item2);
            return Tuple.Create(selector(stateAndA.Item1, stateAndB.Item1), stateAndB.Item2);
        });
    }
}

public class STRef<S, A> {
    private A value;
    public STRef(A a) { value = a; }

    public ST<S, A> Read() {
        return new ST<S, A>(s => Tuple.Create(value, s));
    }

    public ST<S, Unit> Write(A a) {
        return new ST<S, Unit>(s => {
            value = a;
            return Tuple.Create(new Unit(), s);
        });
    }
}

public class STScope : ISTScope<int> {
    public ST<S, int> Get<S>() {
        var v = new STRef<S, int>(5);
        return
            from a in v.Read()              // a = 5
            from b in v.Write(a*10)         // v <- 50
            from c in v.Read()              // c = 50
            select a + c;                   // return: 55
    }
}

[Test]
public void TestST() {
    var result = new STRunner().RunST(new STScope());
    result.ShouldBe(55);
}