X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=manipulating_trees_with_monads.mdwn;h=315cb68a9e58726a8d8c6cb65aeca027794d82be;hp=c92065c41bec611a3625d6b58ac2671408646b37;hb=bf8d964cc93f6b0b44a432bca8c94b1374c05e1f;hpb=f6950034eb1c228badf3364375595032a56e3afb

diff --git a/manipulating_trees_with_monads.mdwn b/manipulating_trees_with_monads.mdwn
index c92065c4..315cb68a 100644
--- a/manipulating_trees_with_monads.mdwn
+++ b/manipulating_trees_with_monads.mdwn
@@ -13,7 +13,7 @@ From an engineering standpoint, we'll build a tree transformer that
 deals in monads.  We can modify the behavior of the system by swapping
 one monad for another.  We've already seen how adding a monad can add
 a layer of funtionality without disturbing the underlying system, for
-instance, in the way that the reader monad allowed us to add a layer
+instance, in the way that the Reader monad allowed us to add a layer
 of intensionality to an extensional grammar, but we have not yet seen
 the utility of replacing one monad with other.
 
@@ -81,14 +81,14 @@ supplying the appropriate `int -> int` operation in place of `double`:
 	- : int tree =ppp
 	Node (Node (Leaf 4, Leaf 9), Node (Leaf 25, Node (Leaf 49, Leaf 121)))
 
-Note that what `tree_map` does is take some global, contextual
+Note that what `tree_map` does is take some unchanging contextual
 information---what to do to each leaf---and supplies that information
 to each subpart of the computation.  In other words, `tree_map` has the
-behavior of a reader monad.  Let's make that explicit.
+behavior of a Reader monad.  Let's make that explicit.
 
 In general, we're on a journey of making our `tree_map` function more and
 more flexible.  So the next step---combining the tree transformer with
-a reader monad---is to have the `tree_map` function return a (monadized)
+a Reader monad---is to have the `tree_map` function return a (monadized)
 tree that is ready to accept any `int -> int` function and produce the
 updated tree.
 
@@ -113,9 +113,7 @@ tree` in which each leaf `i` has been replaced with `f i`.
 With previous readers, we always knew which kind of environment to
 expect: either an assignment function (the original calculator
 simulation), a world (the intensionality monad), an integer (the
-Jacobson-inspired link monad), etc.  In the present case, it will be
-enough to expect that our "environment" will be some function of type
-`int -> int`.
+Jacobson-inspired link monad), etc.  In the present case, we expect that our "environment" will be some function of type `int -> int`. "Looking up" some `int` in the environment will return us the `int` that comes out the other side of that function.
 
 	type 'a reader = (int -> int) -> 'a;;  (* mnemonic: e for environment *)
 	let reader_unit (a : 'a) : 'a reader = fun _ -> a;;
@@ -135,16 +133,44 @@ But we can do this:
 
 	let rec tree_monadize (f : 'a -> 'b reader) (t : 'a tree) : 'b tree reader =
 	    match t with
-	    | Leaf i -> reader_bind (f i) (fun i' -> reader_unit (Leaf i'))
-	    | Node (l, r) -> reader_bind (tree_monadize f l) (fun x ->
-	                       reader_bind (tree_monadize f r) (fun y ->
-	                         reader_unit (Node (x, y))));;
+	    | Leaf a -> reader_bind (f a) (fun b -> reader_unit (Leaf b))
+	    | Node (l, r) -> reader_bind (tree_monadize f l) (fun l' ->
+	                       reader_bind (tree_monadize f r) (fun r' ->
+	                         reader_unit (Node (l', r'))));;
 
 This function says: give me a function `f` that knows how to turn
-something of type `'a` into an `'b reader`, and I'll show you how to
-turn an `'a tree` into an `'b tree reader`.  In more fanciful terms,
-the `tree_monadize` function builds plumbing that connects all of the
-leaves of a tree into one connected monadic network; it threads the
+something of type `'a` into an `'b reader`---this is a function of the same type that you could bind an `'a reader` to---and I'll show you how to
+turn an `'a tree` into an `'b tree reader`.  That is, if you show me how to do this:
+
+	              ------------
+	  1     --->  |    1     |
+	              ------------
+
+then I'll give you back the ability to do this:
+
+	              ____________
+	  .           |    .     |
+	__|___  --->  |  __|___  |
+	|    |        |  |    |  |
+	1    2        |  1    2  |
+	              ------------
+
+And how will that boxed tree behave? Whatever actions you perform on it will be transmitted down to corresponding operations on its leaves. For instance, our `int reader` expects an `int -> int` environment. If supplying environment `e` to our `int reader` doubles the contained `int`:
+
+	              ------------
+	  1     --->  |    1     |  applied to e  ~~>  2
+	              ------------
+
+Then we can expect that supplying it to our `int tree reader` will double all the leaves:
+
+	              ____________
+	  .           |    .     |                      .
+	__|___  --->  |  __|___  | applied to e  ~~>  __|___
+	|    |        |  |    |  |                    |    |
+	1    2        |  1    2  |                    2    4
+	              ------------
+
+In more fanciful terms, the `tree_monadize` function builds plumbing that connects all of the leaves of a tree into one connected monadic network; it threads the
 `'b reader` monad through the original tree's leaves.
 
 	# tree_monadize int_readerize t1 double;;
@@ -161,10 +187,10 @@ result:
 	- : int tree =
 	Node (Node (Leaf 4, Leaf 9), Node (Leaf 25, Node (Leaf 49, Leaf 121)))
 
-Now that we have a tree transformer that accepts a reader monad as a
+Now that we have a tree transformer that accepts a *reader* monad as a
 parameter, we can see what it would take to swap in a different monad.
 
-For instance, we can use a state monad to count the number of leaves in
+For instance, we can use a State monad to count the number of leaves in
 the tree.
 
 	type 'a state = int -> 'a * int;;
@@ -177,14 +203,14 @@ modification whatsoever, except for replacing the (parametric) type
 
 	let rec tree_monadize (f : 'a -> 'b state) (t : 'a tree) : 'b tree state =
 	    match t with
-	    | Leaf i -> state_bind (f i) (fun i' -> state_unit (Leaf i'))
-	    | Node (l, r) -> state_bind (tree_monadize f l) (fun x ->
-	                       state_bind (tree_monadize f r) (fun y ->
-	                         state_unit (Node (x, y))));;
+	    | Leaf a -> state_bind (f a) (fun b -> state_unit (Leaf b))
+	    | Node (l, r) -> state_bind (tree_monadize f l) (fun l' ->
+	                       state_bind (tree_monadize f r) (fun r' ->
+	                         state_unit (Node (l', r'))));;
 
 Then we can count the number of leaves in the tree:
 
-	# tree_monadize (fun a s -> (a, s+1)) t1 0;;
+	# tree_monadize (fun a -> fun s -> (a, s+1)) t1 0;;
 	- : int tree * int =
 	(Node (Node (Leaf 2, Leaf 3), Node (Leaf 5, Node (Leaf 7, Leaf 11))), 5)
 	
@@ -199,6 +225,7 @@ Then we can count the number of leaves in the tree:
 	        |  |
 	        7  11
 
+Why does this work? Because the operation `fun a -> fun s -> (a, s+1)` takes an `int` and wraps it in an `int state` monadic box that increments the state. When we give that same operations to our `tree_monadize` function, it then wraps an `int tree` in a box, one that does the same state-incrementing for each of its leaves.
 
 One more revealing example before getting down to business: replacing
 `state` everywhere in `tree_monadize` with `list` gives us
@@ -211,7 +238,8 @@ One more revealing example before getting down to business: replacing
 
 Unlike the previous cases, instead of turning a tree into a function
 from some input to a result, this transformer replaces each `int` with
-a list of `int`'s.
+a list of `int`'s. We might also have done this with a Reader monad, though then our environments would need to be of type `int -> int list`. Experiment with what happens if you supply the `tree_monadize` based on the List monad an operation like `fun -> [ i; [2*i; 3*i] ]`. Use small trees for your experiment.
+
 
 <!--
 FIXME: We don't make it clear why the fun has to be int -> int list list, instead of int -> int list
@@ -227,27 +255,28 @@ of leaves?
 	
 	let rec tree_monadize (f : 'a -> ('b, 'r) continuation) (t : 'a tree) : ('b tree, 'r) continuation =
 	    match t with
-	    | Leaf i -> continuation_bind (f i) (fun i' -> continuation_unit (Leaf i'))
-	    | Node (l, r) -> continuation_bind (tree_monadize f l) (fun x ->
-	                       continuation_bind (tree_monadize f r) (fun y ->
-	                         continuation_unit (Node (x, y))));;
+	    | Leaf a -> continuation_bind (f a) (fun b -> continuation_unit (Leaf b))
+	    | Node (l, r) -> continuation_bind (tree_monadize f l) (fun l' ->
+	                       continuation_bind (tree_monadize f r) (fun r' ->
+	                         continuation_unit (Node (l', r'))));;
+
+We use the Continuation monad described above, and insert the
+`continuation` type in the appropriate place in the `tree_monadize` code. Then if we give the `tree_monadize` function an operation that converts `int`s into `'b`-wrapping Continuation monads, it will give us back a way to turn `int tree`s into corresponding `'b tree`-wrapping Continuation monads.
 
-We use the continuation monad described above, and insert the
-`continuation` type in the appropriate place in the `tree_monadize` code.
-We then compute:
+So for example, we compute:
 
-	# tree_monadize (fun a k -> a :: (k a)) t1 (fun t -> []);;
+	# tree_monadize (fun a -> fun k -> a :: k a) t1 (fun t -> []);;
 	- : int list = [2; 3; 5; 7; 11]
 
-We have found a way of collapsing a tree into a list of its leaves.
+We have found a way of collapsing a tree into a list of its leaves. Can you trace how this is working? Think first about what the operation `fun a -> fun k -> a :: k a` does when you apply it to a plain `int`, and the continuation `fun _ -> []`. Then given what we've said about `tree_monadize`, what should we expect `tree_monadize (fun a -> fun k -> a :: k a` to do?
 
-The continuation monad is amazingly flexible; we can use it to
+The Continuation monad is amazingly flexible; we can use it to
 simulate some of the computations performed above.  To see how, first
 note that an interestingly uninteresting thing happens if we use
 `continuation_unit` as our first argument to `tree_monadize`, and then
 apply the result to the identity function:
 
-	# tree_monadize continuation_unit t1 (fun i -> i);;
+	# tree_monadize continuation_unit t1 (fun t -> t);;
 	- : int tree =
 	Node (Node (Leaf 2, Leaf 3), Node (Leaf 5, Node (Leaf 7, Leaf 11)))
 
@@ -255,41 +284,41 @@ That is, nothing happens.  But we can begin to substitute more
 interesting functions for the first argument of `tree_monadize`:
 
 	(* Simulating the tree reader: distributing a operation over the leaves *)
-	# tree_monadize (fun a k -> k (square a)) t1 (fun i -> i);;
+	# tree_monadize (fun a -> fun k -> k (square a)) t1 (fun t -> t);;
 	- : int tree =
 	Node (Node (Leaf 4, Leaf 9), Node (Leaf 25, Node (Leaf 49, Leaf 121)))
 
 	(* Simulating the int list tree list *)
-	# tree_monadize (fun a k -> k [a; square a]) t1 (fun i -> i);;
+	# tree_monadize (fun a -> fun k -> k [a; square a]) t1 (fun t -> t);;
 	- : int list tree =
 	Node
 	 (Node (Leaf [2; 4], Leaf [3; 9]),
 	  Node (Leaf [5; 25], Node (Leaf [7; 49], Leaf [11; 121])))
 
 	(* Counting leaves *)
-	# tree_monadize (fun a k -> 1 + k a) t1 (fun i -> 0);;
+	# tree_monadize (fun a -> fun k -> 1 + k a) t1 (fun t -> 0);;
 	- : int = 5
 
 We could simulate the tree state example too, but it would require
-generalizing the type of the continuation monad to
+generalizing the type of the Continuation monad to
 
 	type ('a, 'b, 'c) continuation = ('a -> 'b) -> 'c;;
 
 If you want to see how to parameterize the definition of the `tree_monadize` function, so that you don't have to keep rewriting it for each new monad, see [this code](/code/tree_monadize.ml).
 
 
-The binary tree monad
+The Binary Tree monad
 ---------------------
 
 Of course, by now you may have realized that we have discovered a new
-monad, the binary tree monad:
+monad, the Binary Tree monad:
 
 	type 'a tree = Leaf of 'a | Node of ('a tree) * ('a tree);;
-	let tree_unit (a: 'a) = Leaf a;;
+	let tree_unit (a: 'a) : 'a tree = Leaf a;;
 	let rec tree_bind (u : 'a tree) (f : 'a -> 'b tree) : 'b tree =
 	    match u with
 	    | Leaf a -> f a
-	    | Node (l, r) -> Node ((tree_bind l f), (tree_bind r f));;
+	    | Node (l, r) -> Node (tree_bind l f, tree_bind r f);;
 
 For once, let's check the Monad laws.  The left identity law is easy:
 
@@ -364,3 +393,136 @@ called a
 [SearchTree](http://hackage.haskell.org/packages/archive/tree-monad/0.2.1/doc/html/src/Control-Monad-SearchTree.html#SearchTree)
 that is intended to represent non-deterministic computations as a tree.
 
+
+What's this have to do with tree\_mondadize?
+--------------------------------------------
+
+So we've defined a Tree monad:
+
+	type 'a tree = Leaf of 'a | Node of ('a tree) * ('a tree);;
+	let tree_unit (a: 'a) : 'a tree = Leaf a;;
+	let rec tree_bind (u : 'a tree) (f : 'a -> 'b tree) : 'b tree =
+	    match u with
+	    | Leaf a -> f a
+	    | Node (l, r) -> Node (tree_bind l f, tree_bind r f);;
+
+What's this have to do with the `tree_monadize` functions we defined earlier?
+
+	let rec tree_monadize (f : 'a -> 'b reader) (t : 'a tree) : 'b tree reader =
+	    match t with
+	    | Leaf a -> reader_bind (f a) (fun b -> reader_unit (Leaf b))
+	    | Node (l, r) -> reader_bind (tree_monadize f l) (fun l' ->
+	                       reader_bind (tree_monadize f r) (fun r' ->
+	                         reader_unit (Node (l', r'))));;
+
+... and so on for different monads?
+
+The answer is that each of those `tree_monadize` functions is adding a Tree monad *layer* to a pre-existing Reader (and so on) monad. So far, we've defined monads as single-layered things. Though in the Groenendijk, Stokhoff, and Veltmann homework, we had to figure out how to combine Reader, State, and Set monads in an ad-hoc way. In practice, one often wants to combine the abilities of several monads. Corresponding to each monad like Reader, there's a corresponding ReaderT **monad transformer**. That takes an existing monad M and adds a Reader monad layer to it. The way these are defined parallels the way the single-layer versions are defined. For example, here's the Reader monad:
+
+	(* monadic operations for the Reader monad *)
+
+	type 'a reader =
+		env -> 'a;;
+	let unit (a : 'a) : 'a reader =
+		fun e -> a;;
+	let bind (u: 'a reader) (f : 'a -> 'b reader) : 'b reader =
+		fun e -> (fun v -> f v e) (u e);;
+
+We've just beta-expanded the familiar `f (u e) e` into `(fun v -> f v e) (u e)`, in order to factor out the parts where any Reader monad is being supplied as an argument to another function. Then if we want instead to add a Reader layer to some arbitrary other monad M, with its own M.unit and M.bind, here's how we do it:
+
+	(* monadic operations for the ReaderT monadic transformer *)
+
+	(* We're not giving valid OCaml code, but rather something
+	 * that's conceptually easier to digest.
+	 * How you really need to write this in OCaml is more circuitous...
+	 * see http://lambda.jimpryor.net/code/tree_monadize.ml for some details. *)
+
+	type ('a, M) readerT =
+		env -> 'a M;;
+	(* this is just an 'a M reader; but don't rely on that pattern to generalize *)
+
+	let unit (a : 'a) : ('a, M) readerT =
+		fun e -> M.unit a;;
+
+	let bind (u : ('a, M) readerT) (f : 'a -> ('b, M) readerT) : ('b, M) readerT =
+		fun e -> M.bind (u e) (fun v -> f v e);;
+
+Notice the key differences: where before we just returned `a`, now we instead return `M.unit a`. Where before we just supplied value `u e` of type `'a reader` as an argument to a function, now we instead `M.bind` the `'a reader` to that function. Notice also the differences in the types.
+
+What is the relation between Reader and ReaderT? Well, suppose you started with the Identity monad:
+
+	type 'a identity = 'a;;
+	let unit (a : 'a) : 'a = a;;
+	let bind (u : 'a) (f : 'a -> 'b) : 'b = f u;;
+
+and you used the ReaderT transformer to add a Reader monad layer to the Identity monad. What do you suppose you would get?
+
+The relations between the State monad and the StateT monadic transformer are parallel:
+
+	(* monadic operations for the State monad *)
+
+	type 'a state =
+		store -> ('a * store);;
+
+	let unit (a : 'a) : 'a state =
+		fun s -> (a, s);;
+
+	let bind (u : 'a state) (f : 'a -> 'b state) : 'b state =
+		fun s -> (fun (a, s') -> f a s') (u s);;
+
+We've used `(fun (a, s') -> f a s') (u s)` instead of the more familiar `let (a, s') = u s in f a s'` in order to factor out the part where a value of type `'a state` is supplied as an argument to a function. Now StateT will be:
+
+	(* monadic operations for the StateT monadic transformer *)
+
+	type ('a, M) stateT =
+		store -> ('a * store) M;;
+	(* notice this is not an 'a M state *)
+
+	let unit (a : 'a) : ('a, M) stateT =
+		fun s -> M.unit (a, s);;
+
+	let bind (u : ('a, M) stateT) (f : 'a -> ('b, M) stateT) : ('b, M) stateT =
+		fun s -> M.bind (u s) (fun (a, s') -> f a s');;
+
+Do you see the pattern? Where ordinarily we'd return an `'a` value, now we instead return an `'a M` value. Where ordinarily we'd supply a `'a state` value as an argument to a function, now we instead `M.bind` it to that function.
+
+Okay, now let's do the same thing for our Tree monad.
+
+	(* monadic operations for the Tree monad *)
+
+	type 'a tree =
+		Leaf of 'a | Node of ('a tree) * ('a tree);;
+
+	let unit (a: 'a) : 'a tree =
+		Leaf a;;
+
+	let rec bind (u : 'a tree) (f : 'a -> 'b tree) : 'b tree =
+	    match u with
+	    | Leaf a -> (fun b -> b) (f a) (* see below *)
+	    | Node (l, r) -> (fun l' r' -> Node (l', r')) (bind l f) (bind r f);;
+
+	(* monadic operations for the TreeT monadic transformer *)
+
+	type ('a, M) treeT =
+		'a tree M;;
+
+	let unit (a: 'a) : ('a, M) tree =
+		M.unit (Leaf a);;
+
+	let rec bind (u : ('a, M) tree) (f : 'a -> ('b, M) tree) : ('b, M) tree =
+	    match u with
+	    | Leaf a -> M.bind (f a) (fun b -> M.unit (Leaf b))
+	    | Node (l, r) -> M.bind (bind l f) (fun l' ->
+							M.bind (bind r f) (fun r' ->
+								M.unit (Node (l', r'));;
+
+Compare this definition of `bind` for the TreeT monadic transformer to our earlier definition of `tree_monadize`, specialized for the Reader monad:
+
+	let rec tree_monadize (f : 'a -> 'b reader) (t : 'a tree) : 'b tree reader =
+	    match t with
+	    | Leaf a -> reader_bind (f a) (fun b -> reader_unit (Leaf b))
+	    | Node (l, r) -> reader_bind (tree_monadize f l) (fun l' ->
+	                       reader_bind (tree_monadize f r) (fun r' ->
+	                         reader_unit (Node (l', r'))));;
+
+