Re: Soliciting criticism of memoize function

["Peter Cawley" <lua@...>]

Gé Weijers: It doesn't check that f takes a single argument

2008/5/16 Gé Weijers <ge@weijers.org>:
> Is there anything wrong with the following:
>
> local function memoize(f)
>    local function index(t, x)
>        local v = f(x)
>        t[x] = v
>        return v
>    end
>
>    local cache = setmetatable({}, {__mode = 'k', __index = index})
>
>    return function(x)
>        return cache[x]
>    end
> end
>
>
> On May 14, 2008, at 6:40 PM, Norman Ramsey wrote:
>
>> I would be pleased if readers of this list could think of any improvements
>> to the following memoize function:
>>
>>  function memoize (f)
>>    -- f must take at most one argument and return at most one result
>>      local cache = setmetatable({ }, { __mode = 'k' })
>>      local function update(x, v, ...) -- f(x) returns v, ...
>>        assert(select('#', ...) == 0)
>>        cache[x] = v
>>        return v
>>      end
>>
>>      return function (x, ...)
>>               assert(select('#', ...) == 0) -- cache works with at most
>> one arg
>>               local v = cache[x]
>>               if v ~= nil then
>>                 return v
>>               else
>>                 return update(x, f(x))
>>               end
>>             end
>>  end
>>
>> I'm especially concerned whether I have the cache's __mode metamethod
>> set correctly, as I always get confused about the semantics of weak
>> keys and weak values.
>>
>>
>> Norman
>
> --
> Gé Weijers
> ge@weijers.org
>
>
>
>