// Here's a helper that composes UpdateState with Leaf and
// Branch in the original unlabeled tree. This looks very
// hairy in C#, but in Haskell it's quite short. Here's what
// we do with leaves:
//
// > mkm :: Tr anytype -> Labeled (Lt anytype)
// > mkm (Lf x)
// > = do n <- updateState -- call updateState; "n" is of type "S"
// > return (Lf (n,x)) -- "return" does the heavy lifting
// > -- of creating the Monad from a value.
//
// The "do" notation is just Haskell syntactic sugar for
// precisely the following call of @bind:
//
// updateState >>= \n -> return (Lf (n, x))
//
// which says "call update state, which returns an instance of
// the state monad, then @bind it to the variable n in a
// function that returns a leaf node labeled by the given
// state value." We translate this directly into C# below.
//
// The Branch case is a trivial recursion. Notice that
// updateState only gets called on leaves.
//
// > mkm (Br l r)
// > = do l' <- mkm l
// > r' <- mkm r
// > return (Br l' r')
//
// Notice this is private:
private static SM <Tr <Scp <a> > > MkM <a>(Tr <a> t)
{
if (t is Lf <a> )
{
// Call UpdateState to get an instance of
// SM<int>. Shove it (@bind it) through a lambda
// expression that converts ints to SM<Tr<Scp<a>>
// using the "closed-over" contents from the input
// Leaf node:
var lf = (t as Lf <a>);
return(SM <int> .@bind
(UpdateState(),
(n => SM <Tr <Scp <a> > > .@return
(new Lf <Scp <a> >
{
contents = new Scp <a>
{
label = n,
lcpContents = lf.contents
}
}
)
)
));
}
else if (t is Br <a> )
{
var br = (t as Br <a>);
var oldleft = br.left;
var oldright = br.right;
return(SM <Tr <Scp <a> > > .@bind
(MkM <a>(oldleft),
(newleft => SM <Tr <Scp <a> > > .@bind
(MkM <a>(oldright),
(newright => SM <Tr <Scp <a> > > .@return
(new Br <Scp <a> >
{
left = newleft,
right = newright
}
)
)
)
)
));
}
else
{
throw new Exception("MakeMonad/MLabel: impossible tree subtype");
}
}