exceptions.erl

-module(exceptions).
-export([eval/2, wrap/2]).

% simple model of arithmetical expressions, like
% {add, {num, 0}, {mul, {num,1}, {var,a2}}}

% environment gives value of a variable:
% [ {a,2}, {b,1} ]

eval(_Env, {num, N}) -> N;

eval(Env, {var, V}) -> 
    lookup(V, Env);

eval(Env, {mul, E1, E2}) -> 
    eval(Env, E1) * eval(Env, E2);

eval(Env, {add, E1, E2}) -> 
    eval(Env, E1) + eval(Env, E2);

eval(Env, {'div', Num, Denom}) ->
    N = eval(Env, Num),
    D = eval(Env, Denom),
    case D of
        0 -> throw(div_by_zero);
        _NZ -> N div D
    end.

wrap(Env, Exp) ->
    try eval (Env, Exp) of
        Res -> {ok, Res}
    catch
        throw:div_by_zero -> {error, div_by_zero}
        %error:{unbound, Val} -> {error, {unbound, Val}}
    end.

% lookup
% this version assumes it doesn't fail …
% … but could use exceptions to deal with failure (exercise!)

lookup(X, []) -> erlang:error({unbound, X});
lookup(X, [{X,N}|_]) -> N;
lookup(X, [_|Rest]) -> lookup(X, Rest).