X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=manipulating_trees_with_monads.mdwn;h=ba990d10b63229290fa195be8cf24c967a084593;hp=3b43c9a51938a55e014d86462e449ca6d0d76f3a;hb=b95d7b2ff99ed407766dd37253d5d728b41b64e0;hpb=22b8f082095efe49f44171ffa67246a58e298402

diff --git a/manipulating_trees_with_monads.mdwn b/manipulating_trees_with_monads.mdwn
index 3b43c9a5..ba990d10 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.
 
@@ -84,11 +84,11 @@ supplying the appropriate `int -> int` operation in place of `double`:
 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.
 
@@ -190,7 +190,7 @@ result:
 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;;
@@ -238,7 +238,7 @@ 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. 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.
+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.
 
 
 <!--
@@ -260,17 +260,17 @@ of leaves?
 	                       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. 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.
 
 So for example, we compute:
 
-	# tree_monadize (fun a -> fun 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. 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 _ -> []`.
+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
@@ -300,25 +300,25 @@ interesting functions for the first argument of `tree_monadize`:
 	- : 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:
 
@@ -393,3 +393,137 @@ 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 -> f a;;
+	    | Node (l, r) -> (fun l' r' -> Node (l', r')) (bind l f) (bind r f);;
+
+	(* monadic operations for the TreeT monadic transformer *)
+	(* NOTE THIS IS NOT YET WORKING --- STILL REFINING *)
+
+	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'))));;
+
+