Erlang parameter validation

t.erl

% Post/Query string parameter validation.
%
% Mark Allen, mrallen1@yahoo.com
%
% Specs [{{name, "param_name"}, {required, true}, {type, function}}, ... ]
% Input [{"name", "value}, ...] just you'd get from post or query string parameters

-module(t).
-export([test/0, test1/0, test2/0]).

find_spec(P, S) ->
    case S of
        {{name, P}, R, V} ->
            {{name, P}, R, V};
        {{name, P}, V} ->
            {{name, P}, {required, false}, V};
        {_, _, _} ->
            skip;
        {_, _} ->
            skip
    end.

validate_required(I, S) ->
    InputParams = [ Name || {Name, _Value} <- I ],
    R = [ case Bool of 
                    true ->
                        ParamName;
                    false ->
                        false 
                 end || {{_, ParamName}, {required, Bool}, {_, _}} <- S],
    ReqParams = lists:filter(fun(A) -> A =/= false end, R),
    V = [ lists:member(N, InputParams) || N <- ReqParams ],
    [ case Bool of 
        true ->
            {ok, Name};
        false ->
            {missingreq, Name} 
      end || {Name, Bool} <- lists:zip(ReqParams, V) ].

validate_specification(S, Acc) ->
    case S of
        {{name, Name}, {required, Bool}, {Type, F}} ->
            Member = lists:member(Type, [int, string, atom, list, tuple]),
            if 
                is_list(Name), is_boolean(Bool), Member == true, is_function(F) ->
                    {ok, Name};
                true ->
                    {badspec, Name}
            end;
        {{name, Name}, {Type, F}} ->
            validate_specification({{name, Name}, {required, false}, {Type, F}}, Acc);
        [H|T] ->
            validate_specification(T, [validate_specification(H, Acc) | Acc]);
        [] ->
            lists:filter(fun(A) -> is_tuple(A) end, Acc)
    end.
    
validate_params(Input, Spec) ->
    [ case lists:filter(fun(A) -> A =/= skip end, [ find_spec(Name, S) || S <- Spec ]) of 
            [H|_T] ->
                {{name, Name}, {required, _Bool}, {Type, F}} = H,
                try
                    {ok, Coerce} = coerce_type(Type, Value),
                    case F(Coerce) of
                        true ->
                            {ok, Name, Coerce};
                        false ->
                            {badparam, Name, Value}
                    end
                catch
                    _:_ ->
                        {badparam, Name, Value}
                end;
            [] ->
                {missingspec, Name} 
          end || {Name, Value} <- Input ].

coerce_type(Type, Value) ->
    case Type of
        int when is_list(Value) ->
            try
                V = list_to_integer(Value),
                {ok, V}
            catch
                _:_ ->
                    {badparam, Value}
            end;
        int when is_integer(Value) ->
            {ok, Value};
        string when is_list(Value) ->
                {ok, Value};
        binary when is_list(Value) ->
            try
                V = list_to_binary(Value),
                {ok, V}
            catch
                _:_ ->
                    {badparam, Value}
            end;
        binary when is_binary(Value) ->
            {ok, Value};
        tuple when is_list(Value) ->
            try
                V = list_to_tuple(Value),
                {ok, V}
            catch
                _:_ ->
                    {badparam, Value}
            end;
        tuple when is_tuple(Value) ->
            {ok, Value};
        list when is_list(Value) ->
            {ok, Value}
    end.

test() ->
    Spec = [ 
        {{name, "foo"}, {required, true}, {int, fun(A) -> A > 50 end}},
        {{name, "bar"}, {required, false}, {string, fun(A) -> A =:= "quux" end}},
        {{name, "guz"}, {required, crap}, {string, fun(A) -> A =:= "quux" end}},
        {{name, gurz}, {required, crap}, {string, fun(A) -> A =:= "quux" end}},
        {{name, "baz"}, {int, fun(A) -> A =:= 0 end}},
        {{name, "bak"}, {crap, fun(A) -> A =:= 0 end}},
        {{name, "bez"}, {string, crap}},
        {{name, "vap"}, {int, fun(A) -> A < 0 end}}
    %    {{name, "zod"}},
    %    {{name, "zux"}, {int}},
    %    {{name, "faz"}, {fun(_A) -> true end}}
    ],
 validate_specification(Spec, []).

test1() ->
    Spec = [ 
        {{name, "foo"}, {required, true}, {int, fun(A) -> A > 50 end}},
        {{name, "bar"}, {required, false}, {string, fun(A) -> A =:= "quux" end}},
        {{name, "baz"}, {int, fun(A) -> A =:= 0 end}},
        {{name, "vap"}, {int, fun(A) -> A < 0 end}}
    ],
    Input = [{"foo", "abc"}, {"bar", "quux"}, {"weasel", "bat"}],

    validate_params(Input, Spec).

test2() -> 
    Spec = [ 
        {{name, "foo"}, {required, true}, {int, fun(A) -> A > 50 end}},
        {{name, "bar"}, {required, true}, {string, fun(A) -> A =:= "quux" end}},
        {{name, "baz"}, {int, fun(A) -> A =:= 0 end}},
        {{name, "vap"}, {int, fun(A) -> A < 0 end}}
    ],
    Input = [{"foo", "abc"}, {"barf", "quux"}, {"baz", "bat"}],

    validate_required(Input, Spec).