calc.erl

%Khayyam Saleem, Ryan Edelstein
%We pledge our honors that we have abided by the Stevens Honor System.
%Finished before Prof. reviewed in lecture, feeling very good.
-module(calc).
-compile([calc/2]).

calc([], {var, _S})->
    erlang:error('Variable not in env');
calc([[K,V]|_T], {var, S}) when S == K ->
    {const, V};
calc([[_K,_V]|T], {var, S}) ->
    calc(T, {var, S});
calc(_Env, {const, I})->
    {const, I};
calc(_Env, {add, {const, A}, {const, B}}) ->
    {const, A+B};
calc(Env, {add, E1, E2})->
    calc(Env, {add, calc(Env, E1), calc(Env, E2)});
calc(_Env, {sub, {const, A}, {const, B}}) ->
    {const, A-B};
calc(Env, {sub, E1, E2})->
    calc(Env, {sub, calc(Env, E1), calc(Env, E2)});
calc(_Env, {mul, {const, A}, {const, B}}) ->
    {const, A*B};
calc(Env, {mul, E1, E2})->
    calc(Env, {mul, calc(Env, E1), calc(Env, E2)});
calc(_Env, {divi, {const, A}, {const, B}}) ->
    {const, A div B};
calc(Env, {divi, E1, E2})->
    calc(Env, {divi, calc(Env, E1), calc(Env, E2)});
calc(_Env, {eq, {const, A}, {const, B}})->
    {bool, A == B};
calc(_Env, {eq, {bool, A}, {bool, B}})->
    {bool, A == B};
calc(Env, {eq, E1, E2})->
    calc(Env, {eq, calc(Env, E1), calc(Env, E2)});
calc(_Env, {fact, {const, A}}) when A < 0 ->
    erlang:error('cannot compute fact of neg const');
calc(_Env, {fact, {bool, _}})->
    erlang:error('Type error: cannot compute fact of bool');
calc(_Env, {fact, {const, 0}})->
    {const, 1};
calc(Env, {fact, E1})->
    calc(Env, {mul, calc(Env, E1), calc(Env, {fact, calc(Env, {sub, calc(Env, E1), {const, 1}})})}).

testEnv() ->
    [["x", 5], ["y", 3]].

main(_) ->
    io:fwrite("~p.~n", [
                         calc(testEnv(), {fact, {var, "x"}})
                       ]).
% calc(testEnv(), {eq, {add, {mul, {var, "x"},
%                                  {const, 2}},
%                            {const, -30}},
%                      {sub, {const, 10},
%                            {mul, {var, "y"},
%                                  {add, {const, 5},
%                                        {divi, {const, 25},
%                                               {var, "x"}}}}}})