Re: absence of a __hash metamethod

[Mark Hamburg <mhamburg@...>]

Canonicalized, write-once tuples. Not that those are available...

But essentially, if you have a way to hash a table, then what you probably
want to do is:

    hashToTable = setmetatable( {}, { __mode = "v" } )

    tableWithComplexKeys = setmetatable( { },
        __index = function( t, k )
            local hash_k = hash( k )
            local kk = hashToTable[ hash_k ]
            return t[ kk ]
                -- Uses the fact that t[ nil ] will be nil
        end,
        __index = function( t, k, v )
            local hash_k = hash( k )
            local kk = hashToTable[ hash_k ]
            if kk == nil then
                hashToTable[ hash_k ] = k
                kk = k
            end
            t[ kk ] = v
        end
    }

You could also choose to index the resultant table with uniquely generated
values tied to the hashes in order to avoid problems with the values of the
keys being mutated:

    hashToTable = setmetatable( {}, { __mode = "v" } )

    tableWithComplexKeys = setmetatable( { },
        __index = function( t, k )
            local hash_k = hash( k )
            local kk = hashToTable[ hash_k ]
            return t[ kk ]
                -- Uses the fact that t[ nil ] will be nil
        end,
        __index = function( t, k, v )
            local hash_k = hash( k )
            local kk = hashToTable[ hash_k ]
            if kk == nil then
                kk = { } -- private value
                hashToTable[ hash_k ] = kk
            end
            t[ kk ] = v
        end
    }


Mark