X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=code%2Ftree_monadize.ml;h=d9c5a5900e520c3fb78c918dd0abbd08f41ea8ae;hp=310b797e4f08c0bcc631194dfcd1a5f488bbc5dc;hb=0c48836d5dbaa454898800a73183d6775cc3664c;hpb=f6950034eb1c228badf3364375595032a56e3afb

diff --git a/code/tree_monadize.ml b/code/tree_monadize.ml
index 310b797e..d9c5a590 100644
--- a/code/tree_monadize.ml
+++ b/code/tree_monadize.ml
@@ -1,30 +1,105 @@
 (*
  * tree_monadize.ml
  *
- * 'a and so on are type variables in OCaml; they stand for arbitrary types
+ * If you've got some block of code that uses `unit`s and `bind`s, and you
+ * want to interpret it alternately using this monad, that monad, or another
+ * monad, you can use OCaml's module system. You'd write your code like this:
+ *) 
+
+module Reader_monad = struct
+    (* change this to suit your needs *)
+    type env = int -> int;;
+
+    type 'a m = env -> 'a;;
+    let unit a : 'a m = fun e -> a;;
+    let bind (u : 'a m) (f : 'a -> 'b m) : 'b m =
+      fun e -> f (u e) e;;
+end
+
+module State_monad = struct
+    (* change this to suit your needs *)
+    type store = int;;
+
+    type 'a m = store -> 'a * store;;
+    let unit a : 'a m  = fun s -> (a, s);;
+    let bind (u : 'a m) (f : 'a -> 'b m) : 'b m =
+      fun s -> (let (a, s') = u s in (f a) s');;
+end
+
+module List_monad = struct
+    type 'a m = 'a list;;
+    let unit a : 'a m = [a];;
+    let bind (u: 'a m) (f : 'a -> 'b m) : 'b m =
+      List.concat(List.map f u);;
+end
+
+(*
+ * Then you can replace code that looks like this:
+ *     ... reader_bind ...
+ * with code that looks like this:
+ *     ... Reader_monad.bind ...
+ * and the latter can be reformulated like this:
+ *     let open Reader_monad in ... bind ...
+ * or equivalently, like this:
+ *     Reader_monad.(... bind ...)
+ * Then you can use literally the same `... bind ...` code when writing instead:
+ *     State_monad.(... bind ...)
+ *)
+
+(* That's great, however it still requires us to repeat the
+ * `... bind ...` code every time we want to change which monad we're working
+ * with. Shouldn't there be a way to _parameterize_ the `... bind ...` code
+ * on a monad, so that we only have to write the `... bind ...` code once,
+ * but can invoke it alternately with the Reader_monad supplied as an
+ * argument, or the State_monad, or another?
+ *
+ * There is a way to do this, but it requires putting the `... bind ...` code in
+ * its own module, and making that module parameterized on some M_monad
+ * module. Also we have to explicitly declare what commonality we're expecting
+ * from M_monad modules we're going to use as parameters. We'll explain how to
+ * do this in a moment.
+ *
+ * As preparation, a general observation:
+ * 'a and so on are type variables in OCaml; they stand for arbitrary types.
  * What if you want a variable for a type constructor? For example, you want to
  * generalize this pattern:
- *      type ('a) t1 = 'a -> 'a list
- *      type ('a) t2 = 'a -> 'a option
- *      type ('a) t3 = 'a -> 'a reader
+ *      type ('a) t1 = 'a -> ('a) list
+ *      type ('a) t2 = 'a -> ('a) option
+ *      type ('a) t3 = 'a -> ('a) reader
  * and so on? OCaml won't let you do this:
- *      type ('a, 'b) t = 'a -> 'a 'b
- * to generalize on the 'b position, we instead have to use OCaml's modules,
+ *      type ('a, 'b) t = 'a -> ('a) 'b
+ * To generalize on the 'b position, we instead have to use OCaml's modules,
  * and in particular its ability to make modules parameterized on other modules
  * (OCaml calls these parameterized modules Functors, but that name is also
  * used in other ways in this literature, so I won't give in to it.)
  *
  * Here's how you'd have to define the t type from above:
- *      module T_maker(S: sig
- *          type 'a b
- *      end) = struct
- *          type 'a t = 'a -> 'a S.b
+ *      module T_maker(
+ *      (* A sig...end block specifies the type of a module
+ *       * What we're doing here is specifying the type of the 
+ *       * module parameter that will choose
+ *       * whether b = list or b = option or b = reader...
+ *       * This module parameter may supply values as well as types *)
+ *      M : sig
+ *          type ('a) b
+ *      end
+ *      ) = 
+ *      (* A struct...end block gives a module value
+ *       * What we're doing here is building a new module that makes
+ *       * use of the module that was supplied as M *)
+ *      struct
+ *          type ('a) t = 'a -> ('a) M.b
  *      end
  * And here's how you'd use it:
  *      module T_list = T_maker(struct type 'a b = 'a list end);;
  *      type 'a t1 = 'a T_list.t;;
+ *      module T_option = T_maker(struct type 'a b = 'a option end);;
+ *      type 'a t2 = 'a T_option.t;;
+ *      (* and so on *)
  *
- * I know, it seems unnecessarily complicated.
+ * I know, it seems unnecessarily complicated. Nonetheless, that's how it
+ * works. And that is also the technique we'll use to make our
+ * `... bind ...` code parametric on some M_monad module.
  *)
 
 type 'a tree = Leaf of 'a | Node of ('a tree) * ('a tree);;
@@ -38,87 +113,68 @@ let t1 = Node
                 (Leaf 7, Leaf 11)));;
 
 
-module Tree_monadizer(S: sig
+module Tree_monadizer(M : sig
+  (* the module we're using as a parameter has to supply function values
+   * for unit and bind, as well as a monadic type constructor *)
   type 'a m
   val unit : 'a -> 'a m
   val bind : 'a m -> ('a -> 'b m) -> 'b m
 end) = struct
-  let rec monadize (f: 'a -> 'b S.m) (t: 'a tree) : 'b tree S.m =
+  let rec monadize (f: 'a -> 'b M.m) (t: 'a tree) : 'b tree M.m =
     match t with
-    | Leaf a -> S.bind (f a) (fun b -> S.unit (Leaf b))
+    | Leaf a -> M.bind (f a) (fun b -> M.unit (Leaf b))
     | Node(l, r) ->
-        S.bind (monadize f l) (fun l' ->
-          S.bind (monadize f r) (fun r' ->
-            S.unit (Node (l', r'))))
+        M.bind (monadize f l) (fun l' ->
+          M.bind (monadize f r) (fun r' ->
+            M.unit (Node (l', r'))))
 end;;
 
 
-type env = int -> int;;
+(* Now we supply Reader_monad as a parameter to Tree_monadizer.
+ * We'll get back a module TreeReader that contains a single value,
+ * the monadize function specialized to the Reader monad *)
+module TreeReader = Tree_monadizer(Reader_monad);;
 
-type 'a reader = env -> 'a;;
-let unit_reader a : 'a reader = fun e -> a;;
-let bind_reader (u : 'a reader) (f : 'a -> 'b reader) : 'b reader =
-  fun e -> f (u e) e;;
 
-module TreeReader = Tree_monadizer(struct
-  type 'a m = 'a reader
-  let unit = unit_reader
-  let bind = bind_reader
-end);;
+(* Make a TreeState module containing monadize specialized to the State monad *)
+module TreeState =  Tree_monadizer(State_monad);;
 
 
-type store = int;;
+(* Make a TreeList module containing monadize specialized to the List monad *)
+module TreeList =  Tree_monadizer(List_monad);;
 
-type 'a state = store -> 'a * store;;
-let unit_state a : 'a state  = fun s -> (a, s);;
-let bind_state (u : 'a state) (f : 'a -> 'b state) : 'b state =
-  fun s -> (let (a, s') = u s in (f a) s');;
 
-module TreeState =  Tree_monadizer(struct
-  type 'a m = 'a state
-  let unit = unit_state
-  let bind = bind_state
-end);;
+(* The Continuation monad is a bit more complicated *)
+module Continuation_monad = struct
+    type ('r,'a) m = ('a -> 'r) -> 'r;;
+    let unit a : ('r,'a) m = fun k -> k a;;
+    let bind (u: ('r,'a) m) (f: 'a -> ('r,'b) m) : ('r,'b) m =
+      fun k -> u (fun a -> f a k);;
+end
 
-
-let unit_list a = [a];;
-let bind_list (u: 'a list) (f : 'a -> 'b list) : 'b list =
-  List.concat(List.map f u);;
-
-module TreeList =  Tree_monadizer(struct
-  type 'a m = 'a list
-  let unit = unit_list
-  let bind = bind_list
-end);;
-
-
-
-(* we need to a new module when the monad is parameterized on two types *)
-module Tree_monadizer2(S: sig
-  type ('a,'x) m
-  val unit : 'a -> ('a,'x) m
-  val bind : ('a,'x) m -> ('a -> ('b,'x) m) -> ('b,'x) m
+(* Since the Continuation monad is parameterized on two types---it's
+ * ('r,'a) cont not ('a) cont---we can't match the type ('a) monad that
+ * Tree_monadizer expects in its parameter. So we have to make a different
+ * Tree_monadizer2 that takes a ('r,'a) monad type constructor in its
+ * parameter instead *)
+module Tree_monadizer2(M : sig
+  type ('r,'a) m
+  val unit : 'a -> ('r,'a) m
+  val bind : ('r,'a) m -> ('a -> ('r,'b) m) -> ('r,'b) m
 end) = struct
-  let rec monadize (f: 'a -> ('b,'x) S.m) (t: 'a tree) : ('b tree,'x) S.m =
-    (* the definition is the same, the difference is only in the types *)
+  (* the body of the monadize function is the same; the only difference is in
+   * the types *)
+  let rec monadize (f: 'a -> ('r,'b) M.m) (t: 'a tree) : ('r,'b tree) M.m =
     match t with
-    | Leaf a -> S.bind (f a) (fun b -> S.unit (Leaf b))
+    | Leaf a -> M.bind (f a) (fun b -> M.unit (Leaf b))
     | Node(l, r) ->
-        S.bind (monadize f l) (fun l' ->
-          S.bind (monadize f r) (fun r' ->
-            S.unit (Node (l', r'))))
+        M.bind (monadize f l) (fun l' ->
+          M.bind (monadize f r) (fun r' ->
+            M.unit (Node (l', r'))))
 end;;
 
-type ('a,'r) cont = ('a -> 'r) -> 'r;;
-let unit_cont a : ('a,'r) cont = fun k -> k a;;
-let bind_cont (u: ('a,'r) cont) (f: 'a -> ('b,'r) cont) : ('b,'r) cont =
-  fun k -> u (fun a -> f a k);;
-
-module TreeCont =  Tree_monadizer2(struct
-  type ('a,'r) m = ('a,'r) cont
-  let unit = unit_cont
-  let bind = bind_cont
-end);;
+(* Make a TreeCont module containing monadize specialized to the Cont monad *)
+module TreeCont =  Tree_monadizer2(Continuation_monad);;
 
 
 
@@ -127,34 +183,89 @@ end);;
  * http://lambda.jimpryor.net/manipulating_trees_with_monads/
  *)
 
-let int_readerize : int -> int reader =
-  fun (a : int) (modifier : int -> int) -> modifier a;;
+
+let asker : int -> int Reader_monad.m =
+  fun (a : int) -> fun (env : int -> int) -> env a;;
+
+(* asker takes an int and returns a Reader monad that
+ * "looks up" that int in the environment (i.e. modifies it)
+ * this is structurally parallel to the function `lookup` we used
+ * before to "look up" variables in the environment *)
 
 (* double each leaf *)
-TreeReader.monadize int_readerize t1 (fun i -> i + i);;
+let env = fun i -> i + i in
+TreeReader.monadize asker t1 env;;
+
+(* You can also avoid declaring a separate toplevel TreeReader module
+ * (or even a separate Reader_monad module) by using one of these forms:
+ *     ...
+ *     let module T = Tree_monadizer(Reader_monad) in
+ *     T.monadize asker t1 env;;
+ * or:
+ *     ...
+ *     let env = fun i -> i + i in
+ *     let module Monad = struct
+ *       type env = int -> int;;
+ *       type 'a m = env -> 'a;;
+ *       let unit a : 'a m = fun e -> a;;
+ *       let bind (u : 'a m) (f : 'a -> 'b m) : 'b m =
+ *         fun e -> f (u e) e;;
+ *     end in
+ *     let module T = Tree_monadizer(Monad) in
+ *     T.monadize asker t1 env;;
+ * or:
+ *     ...
+ *     let module T = Tree_monadizer(struct
+ *       type env = int -> int;;
+ *       type 'a m = env -> 'a;;
+ *       let unit a : 'a m = fun e -> a;;
+ *       let bind (u : 'a m) (f : 'a -> 'b m) : 'b m =
+ *         fun e -> f (u e) e;;
+ *     end) in
+ *     T.monadize asker t1 env;;
+ *)
+
 
 (* square each leaf *)
-TreeReader.monadize int_readerize t1 (fun i -> i * i);;
+let env = fun i -> i * i in
+TreeReader.monadize asker t1 env;;
+
+
+
+let incrementer : int -> int State_monad.m =
+  fun (a : int) -> fun s -> (a, s+1);;
+
+(* incrementer takes an 'a and returns it wrapped in a
+ * State monad that increments the store *)
 
 (* count leaves *)
-TreeState.monadize (fun a s -> (a, s+1)) t1 0;;
+let initial_store = 0 in
+TreeState.monadize incrementer t1 initial_store;;
+
+
 
 (* replace leaves with list *)
-TreeList.monadize (fun i -> [[i;i*i]]) t1;;
+TreeList.monadize (fun i -> [ [i;i*i] ]) t1;;
+
 
-(* convert tree to list of leaves *)
-TreeCont.monadize (fun a k -> a :: k a) t1 (fun t -> []);;
 
 (* do nothing *)
-TreeCont.monadize unit_cont t1 (fun t-> t);;
+let initial_continuation = fun t -> t in
+TreeCont.monadize Continuation_monad.unit t1 initial_continuation;;
+
+(* convert tree to list of leaves *)
+let initial_continuation = fun t -> [] in
+TreeCont.monadize (fun a k -> a :: k a) t1 initial_continuation;;
 
 (* square each leaf using continuation *)
-TreeCont.monadize (fun a k -> k (a*a)) t1 (fun t -> t);;
+let initial_continuation = fun t -> t in
+TreeCont.monadize (fun a k -> k (a*a)) t1 initial_continuation;;
 
 (* replace leaves with list, using continuation *)
-TreeCont.monadize (fun a k -> k [a; a*a]) t1 (fun t -> t);;
+let initial_continuation = fun t -> t in
+TreeCont.monadize (fun a k -> k [a; a*a]) t1 initial_continuation;;
 
 (* count leaves, using continuation *)
-TreeCont.monadize (fun a k -> 1 + k a) t1 (fun t -> 0);;
-
+let initial_continuation = fun t -> 0 in
+TreeCont.monadize (fun a k -> 1 + k a) t1 initial_continuation;;