Implement a couple of other standard monad helpers

rabbitmq · Apr 3, 2011 · 3250a38 · 3250a38
1 parent f115d3e
commit 3250a38
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Showing 2 changed files with 25 additions and 0 deletions.
diff --git 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
@@ -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).