Re: Changing syntax

[David Jones <drj@...>]

On Feb 12, 2004, at 12:45, andrew cooke wrote:

> Hi,
>
> As far as I can tell, Lua evaluates arguments to functions eagerly.  Is
> there any way of treating blocks of code lazily?
>
> For example, if I want to add a "case" statement to Lua that looks
> something like:
>
> case x
> of   1 do ... end
> of   2 do ... end
> else   do ... end
>
> would I be able to?  My initial idea was to define case as a vararg
> function, but if arguments are evaluated eagerly then this won't work 
> (I
> believe).
>
> The syntax in the example above isn't important - what I'm looking for 
> is
> a way of handling sections of code as first class objects, and for them
> not to be evaluated when passed as arguments, I think.
Well, functions are sections of code and they are first class objects.

This function calls one of its arguments according to the value of the 
first argument:

function select(x, ...) return arg[x]() end

And here it is being used.  Notice that arguments 2 and 3 are anonymous 
functions.

> select(2, function () print "apple" end, function () print "banana" 
end)
banana

It's almost not worth writing the select function.  You can use a table 
of functions instead:

a = {
  function () print "apple" end,
  function () print "banana" end,
}

> a[1]()
apple

Hope that helps.

Tuomo Valkonen's suggestion, which I picked up whilst I was writing 
this reply, is similar.

David Jones