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On 22-Dec-05, at 6:31 PM, Nick Trout wrote:

With the new ... syntax can you do (something like)?

function memoize(func)
    return setmetatable({}, {__index =
	 function(self, ...)
       local v = func(...)
       self[...] = v
       return v

No, sadly. '...' is not a first-class object. You *could* do that with first-class immutable tuples, but Lua does not seem likely to acquire such a datatype. You could do this by interning {...} (or, for 5.0, arg) but the general case of interning {a, b, c} is pretty expensive in Lua, so it's unlikely that this would be a win without direct language support.

Python is adding (@) decorators to make this easy. If you could alter
the Lua syntax (token stream) you could do it in Lua:

function myexpensivefn(a,b,c) --code-- end

I can actually live with

  myexpensivefn = memoize(function(a) ... end)

but I can see the appeal of the Python sugar. However, it should be noted that memoizing turns a function into a table (of sorts); you can arrange for the table to have a __call metamethod as well, but that increases the cost of usage quite a bit.

The fact that the resulting object is technically a table makes it slightly less general than a real function, since there are various places in the Lua core where a check is specifically made for the type TTFUNCTION. For example, a table (even with __call metamethod) cannot be used as a metamethod itself (except for __index/__newindex, where the table would be used as a table, with the same functionality.)

Question: should it be possible to distinguish between a table and a function of one parameter? That is, does Lua really need both foo() and foo[] syntaxes?