Lua Expressions in Mercury

    % An expression performs an evaluation based upon the values availible
    % to a Lua state, returning the stack index containing the value of the
    % evaluated expression.
    %
    % Note: an expression that removes values from the stack that it did
    % not add should be expected to produce undefined behavior.
    %
:- type expression == impure func(lua) = index.

:- type unop == impure func(expression, lua) = index.

:- type binop == impure func(expression, expression, lua) = index.


:- func eval(expression, lua) = T is semidet.

:- some [T] det_eval(expression, lua) = T.