Relational metamethods and mixed types

[Geoff Leyland <geoff_leyland@...>]

Hi,

PiL says: "Unlike arithmetic metamethods, relational metamethods do  
not support mixed types...".

Supporting mixed types would be very handy for defining constraints in  
Linear Programs, where you could go:

a_constraint = 2 * x1 + 3 * x2 + 4 * x3 <= 6

Where the type of the result of the expression on the LHS is a list of  
terms containing LP variables, and the type of the RHS is clearly a  
number.  Note that this isn't a statement about truth, testing or  
ordering, it's a constraint on the values of xi to be respected while  
solving an optimisation problem.

Unfortunately, I can't do this because the left and right-hand sides  
aren't the same types.

The pure Lua alternative I can think of is to offer easy constructors  
for term lists:

a_constraint = 2 * x1 + 3 * x2 + 4 * x3 <= make_term_list(6)

This isn't too onerous, but a better alternative would be nice.

Can anyone shed any light on:
 - why there is a restriction on mixed-type relational metamethods
 - whether the right place to patch this is around luaV_lessthan in  
lvm.c
 - whether there's anywhere else that would need to be patched -  
particularly if there's other places in luajit
 - whether anyone's code would break if mixed-type relational  
metamethods were allowed
 - whether there is any chance of allowing mixed-type relational  
metamethods in a future Lua

Thanks in advance,
Geoff