From 3250a38411e2f1a024be19c3a7b6361f59cb9976 Mon Sep 17 00:00:00 2001
From: Matthew Sackman <matthew@rabbitmq.com>
Date: Sun, 3 Apr 2011 23:50:31 +0100
Subject: [PATCH] Implement a couple of other standard monad helpers

---
 src/monad.erl        | 20 ++++++++++++++++++++
 test/src/test_do.erl |  5 +++++
 2 files changed, 25 insertions(+)

diff --git a/src/monad.erl b/src/monad.erl
index d3417f7..1ca40e9 100644
--- a/src/monad.erl
+++ b/src/monad.erl
@@ -17,6 +17,15 @@
 -module(monad).
 
 -export([behaviour_info/1]).
+-export([join/2, sequence/2]).
+
+-ifdef(use_specs).
+-type(monad(_A) :: any()). %% urm, don't know what to do here.
+-spec(join/2 :: (atom(), monad(monad(A))) -> monad(A)).
+-spec(sequence/2 :: (atom(), [monad(A)]) -> monad([A])).
+-endif.
+
+-compile({parse_transform, erlando}).
 
 behaviour_info(callbacks) ->
     [{'>>=',  2},
@@ -25,3 +34,14 @@ behaviour_info(callbacks) ->
      {fail,   1}];
 behaviour_info(_Other) ->
     undefined.
+
+join(Monad, X) ->
+    do([Monad || Y <- X(),
+                 Y()]).
+
+sequence(Monad, Xs) ->
+    lists:foldr(fun (X, Acc) ->
+                        do([Monad || E <- X,
+                                     Es <- Acc,
+                                     return([E|Es])])
+                end, Monad:return([]), Xs).
diff --git a/test/src/test_do.erl b/test/src/test_do.erl
index 03714c8..11a8536 100644
--- a/test/src/test_do.erl
+++ b/test/src/test_do.erl
@@ -82,3 +82,8 @@ test_omega() ->
                        return({X,Y,Z})]),
     true = A =/= B,
     true = A =:= lists:usort(B).
+
+test_sequence() ->
+    List = lists:seq(1,5),
+    ListM = [do([maybe_m || return(N)]) || N <- List],
+    {just, List} = monad:sequence(maybe_m, ListM).