From f4457664b4e5de53cff4bd079b8ea561ae66f1f0 Mon Sep 17 00:00:00 2001 From: Jim Pryor Date: Sun, 12 Dec 2010 23:20:39 -0500 Subject: [PATCH] manip trees tweak Signed-off-by: Jim Pryor --- manipulating_trees_with_monads.mdwn | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/manipulating_trees_with_monads.mdwn b/manipulating_trees_with_monads.mdwn index e309faab..0d9e33df 100644 --- a/manipulating_trees_with_monads.mdwn +++ b/manipulating_trees_with_monads.mdwn @@ -293,7 +293,7 @@ it through: Later, we will talk more about controlling the order in which nodes are visited. One more revealing example before getting down to business: replacing -`state` everywhere in `tree_monadize` with `list` gives us +`state` everywhere in `tree_monadize` with `list` lets us do: # let decider i = if i = 2 then [20; 21] else [i];; # tree_monadize decider t1;; @@ -311,11 +311,11 @@ one for each choice of `int`s for its leaves. Now for the main point. What if we wanted to convert a tree to a list of leaves? - type ('a, 'r) continuation = ('a -> 'r) -> 'r;; + type ('r,'a) continuation = ('a -> 'r) -> 'r;; let continuation_unit a = fun k -> k a;; let continuation_bind u f = fun k -> u (fun a -> f a k);; - let rec tree_monadize (f : 'a -> ('b, 'r) continuation) (t : 'a tree) : ('b tree, 'r) continuation = + let rec tree_monadize (f : 'a -> ('r,'b) continuation) (t : 'a tree) : ('r,'b tree) continuation = match t with | Leaf a -> continuation_bind (f a) (fun b -> continuation_unit (Leaf b)) | Node (l, r) -> continuation_bind (tree_monadize f l) (fun l' -> @@ -368,7 +368,7 @@ interesting functions for the first argument of `tree_monadize`: It's not immediately obvious to us how to simulate the List monadization of the tree using this technique. We could simulate the tree annotating example by setting the relevant -type to `('a, 'state -> 'result) continuation`. +type to `(store -> 'result, 'a) continuation`. Andre Filinsky has proposed that the continuation monad is able to simulate any other monad (Google for "mother of all monads"). -- 2.11.0