Improve the interface for constraints

There are two important changes in this commit.

Constraints are now producing an error tuple. This error tuple
in turn can be provided to a function for formatting a human
readable error message. Both the error tuple and the formatting
code are controlled by and part of the constraint function.

Constraints now also implement the reverse operation.
When constraint functions only validate, the reverse operation
will be the same as the forward operation. When they also do
some conversion then the reverse operation will reverse it.

Since constraints are now performing 3 different operations
(forward, reverse and format_error), they now take the form
of a function accepting two separate arguments. The operation
is the first argument.

In addition, the return value was changed to take the form
of {ok, Value} | {error, Reason}. The value must be returned
as-is if it was not modified.

3 files changed, 155 insertions, 44 deletions

diff --git a/src/cowboy_constraints.erl b/src/cowboy_constraints.erl
index 27c0e72..6b468fe 100644
--- a/src/cowboy_constraints.erl
+++ b/src/cowboy_constraints.erl
@@ -15,47 +15,160 @@
 -module(cowboy_constraints).
 
 -export([validate/2]).
+-export([reverse/2]).
+-export([format_error/1]).
 
 -type constraint() :: int | nonempty | fun().
 -export_type([constraint/0]).
 
--spec validate(binary(), [constraint()]) -> true | {true, any()} | false.
-validate(Value, [Constraint]) ->
-	apply_constraint(Value, Constraint);
+-type reason() :: {constraint(), any(), any()}.
+-export_type([reason/0]).
+
+-spec validate(binary(), constraint() | [constraint()])
+	-> {ok, any()} | {error, reason()}.
 validate(Value, Constraints) when is_list(Constraints) ->
-	validate_list(Value, Constraints, original);
+	apply_list(forward, Value, Constraints);
 validate(Value, Constraint) ->
-	apply_constraint(Value, Constraint).
-
-validate_list(_, [], original) ->
-	true;
-validate_list(Value, [], modified) ->
-	{true, Value};
-validate_list(Value, [Constraint|Tail], State) ->
-	case apply_constraint(Value, Constraint) of
-		true ->
-			validate_list(Value, Tail, State);
-		{true, Value2} ->
-			validate_list(Value2, Tail, modified);
-		false ->
-			false
+	apply_list(forward, Value, [Constraint]).
+
+-spec reverse(any(), constraint() | [constraint()])
+	-> {ok, binary()} | {error, reason()}.
+reverse(Value, Constraints) when is_list(Constraints) ->
+	apply_list(reverse, Value, Constraints);
+reverse(Value, Constraint) ->
+	apply_list(reverse, Value, [Constraint]).
+
+-spec format_error(reason()) -> iodata().
+format_error({Constraint, Reason, Value}) ->
+	apply_constraint(format_error, {Reason, Value}, Constraint).
+
+apply_list(_, Value, []) ->
+	{ok, Value};
+apply_list(Type, Value0, [Constraint|Tail]) ->
+	case apply_constraint(Type, Value0, Constraint) of
+		{ok, Value} ->
+			apply_list(Type, Value, Tail);
+		{error, Reason} ->
+			{error, {Constraint, Reason, Value0}}
 	end.
 
 %% @todo {int, From, To}, etc.
-apply_constraint(Value, int) ->
-	int(Value);
-apply_constraint(Value, nonempty) ->
-	nonempty(Value);
-apply_constraint(Value, F) when is_function(F) ->
-	F(Value).
+apply_constraint(Type, Value, int) ->
+	int(Type, Value);
+apply_constraint(Type, Value, nonempty) ->
+	nonempty(Type, Value);
+apply_constraint(Type, Value, F) when is_function(F) ->
+	F(Type, Value).
 
 %% Constraint functions.
 
-int(Value) when is_binary(Value) ->
-	try {true, binary_to_integer(Value)}
-	catch _:_ -> false
-	end.
+int(forward, Value) ->
+	try
+		{ok, binary_to_integer(Value)}
+	catch _:_ ->
+		{error, not_an_integer}
+	end;
+int(reverse, Value) ->
+	try
+		{ok, integer_to_binary(Value)}
+	catch _:_ ->
+		{error, not_an_integer}
+	end;
+int(format_error, {not_an_integer, Value}) ->
+	io_lib:format("The value ~p is not an integer.", [Value]).
+
+nonempty(Type, <<>>) when Type =/= format_error ->
+	{error, not_empty};
+nonempty(Type, Value) when Type =/= format_error, is_binary(Value) ->
+	{ok, Value};
+nonempty(format_error, {not_empty, Value}) ->
+	io_lib:format("The value ~p is not empty.", [Value]).
+
+-ifdef(TEST).
+
+validate_test() ->
+	F = fun(_, Value) ->
+		try
+			{ok, binary_to_atom(Value, latin1)}
+		catch _:_ ->
+			{error, not_a_binary}
+		end
+	end,
+	%% Value, Constraints, Result.
+	Tests = [
+		{<<>>, [], <<>>},
+		{<<"123">>, int, 123},
+		{<<"123">>, [int], 123},
+		{<<"123">>, [nonempty, int], 123},
+		{<<"123">>, [int, nonempty], 123},
+		{<<>>, nonempty, error},
+		{<<>>, [nonempty], error},
+		{<<"hello">>, F, hello},
+		{<<"hello">>, [F], hello},
+		{<<"123">>, [F, int], error},
+		{<<"123">>, [int, F], error},
+		{<<"hello">>, [nonempty, F], hello},
+		{<<"hello">>, [F, nonempty], hello}
+	],
+	[{lists:flatten(io_lib:format("~p, ~p", [V, C])), fun() ->
+		case R of
+			error -> {error, _} = validate(V, C);
+			_ -> {ok, R} = validate(V, C)
+		end
+	end} || {V, C, R} <- Tests].
+
+reverse_test() ->
+	F = fun(_, Value) ->
+		try
+			{ok, atom_to_binary(Value, latin1)}
+		catch _:_ ->
+			{error, not_an_atom}
+		end
+	end,
+	%% Value, Constraints, Result.
+	Tests = [
+		{<<>>, [], <<>>},
+		{123, int, <<"123">>},
+		{123, [int], <<"123">>},
+		{123, [nonempty, int], <<"123">>},
+		{123, [int, nonempty], <<"123">>},
+		{<<>>, nonempty, error},
+		{<<>>, [nonempty], error},
+		{hello, F, <<"hello">>},
+		{hello, [F], <<"hello">>},
+		{123, [F, int], error},
+		{123, [int, F], error},
+		{hello, [nonempty, F], <<"hello">>},
+		{hello, [F, nonempty], <<"hello">>}
+	],
+	[{lists:flatten(io_lib:format("~p, ~p", [V, C])), fun() ->
+		case R of
+			error -> {error, _} = reverse(V, C);
+			_ -> {ok, R} = reverse(V, C)
+		end
+	end} || {V, C, R} <- Tests].
+
+int_format_error_test() ->
+	{error, Reason} = validate(<<"string">>, int),
+	Bin = iolist_to_binary(format_error(Reason)),
+	true = is_binary(Bin),
+	ok.
+
+nonempty_format_error_test() ->
+	{error, Reason} = validate(<<>>, nonempty),
+	Bin = iolist_to_binary(format_error(Reason)),
+	true = is_binary(Bin),
+	ok.
+
+fun_format_error_test() ->
+	F = fun
+		(format_error, {test, <<"value">>}) ->
+			formatted;
+		(_, _) ->
+			{error, test}
+	end,
+	{error, Reason} = validate(<<"value">>, F),
+	formatted = format_error(Reason),
+	ok.
 
-nonempty(<<>>) -> false;
-nonempty(Value) when is_binary(Value) -> true.
-%% @todo Perhaps return true for any other type except empty list?
+-endif.
diff --git a/src/cowboy_req.erl b/src/cowboy_req.erl
index ab77a68..3eca379 100644
--- a/src/cowboy_req.erl
+++ b/src/cowboy_req.erl
@@ -780,10 +780,10 @@ filter([Key|Tail], Map) ->
 	true = maps:is_key(Key, Map),
 	filter(Tail, Map).
 
-filter_constraints(Tail, Map, Key, Value, Constraints) ->
-	case cowboy_constraints:validate(Value, Constraints) of
-		true ->
-			filter(Tail, Map);
-		{true, Value2} ->
-			filter(Tail, Map#{Key => Value2})
+filter_constraints(Tail, Map, Key, Value0, Constraints) ->
+	case cowboy_constraints:validate(Value0, Constraints) of
+		{ok, Value} ->
+			filter(Tail, Map#{Key => Value});
+		{error, Reason} ->
+			exit(Reason)
 	end.
diff --git a/src/cowboy_router.erl b/src/cowboy_router.erl
index 7e70025..53ead4a 100644
--- a/src/cowboy_router.erl
+++ b/src/cowboy_router.erl
@@ -277,14 +277,12 @@ check_constraints([Field|Tail], Bindings) when is_atom(Field) ->
 check_constraints([Field|Tail], Bindings) ->
 	Name = element(1, Field),
 	case Bindings of
-		#{Name := Value} ->
+		#{Name := Value0} ->
 			Constraints = element(2, Field),
-			case cowboy_constraints:validate(Value, Constraints) of
-				true ->
-					check_constraints(Tail, Bindings);
-				{true, Value2} ->
-					check_constraints(Tail, Bindings#{Name => Value2});
-				false ->
+			case cowboy_constraints:validate(Value0, Constraints) of
+				{ok, Value} ->
+					check_constraints(Tail, Bindings#{Name => Value});
+				{error, _} ->
 					nomatch
 			end;
 		_ ->