erlandono

%% The contents of this file are subject to the Mozilla Public License
%% Version 1.1 (the "License"); you may not use this file except in
%% compliance with the License. You may obtain a copy of the License
%% at http://www.mozilla.org/MPL/
%%
%% Software distributed under the License is distributed on an "AS IS"
%% basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
%% the License for the specific language governing rights and
%% limitations under the License.
%%
%% The Original Code is Erlando.
%%
%% The Initial Developer of the Original Code is VMware, Inc.
%% Copyright (c) 2011-2013 VMware, Inc.  All rights reserved.
%%

-module(maybe_m).

-export_type(['maybe'/1]).

-behaviour(monad).
-export(['>>='/2, return/1, fail/1]).

-behaviour(monad_plus).
-export([mzero/0, mplus/2]).

-type 'maybe'(A) :: {just, A} | nothing.

-spec '>>='('maybe'(A), fun((A) -> 'maybe'(B))) -> 'maybe'(B).
'>>='({just, X}, Fun) -> Fun(X);
'>>='(nothing, _Fun) -> nothing.

-spec return(A) -> 'maybe'(A).
return(X) -> {just, X}.

-spec fail(any()) -> 'maybe'(_A).
fail(_X) -> nothing.

-spec mzero() -> 'maybe'(_A).
mzero() -> nothing.

-spec mplus('maybe'(A), 'maybe'(A)) -> 'maybe'(A).
mplus(nothing, Y) -> Y;
mplus(X, _Y) -> X.