Re: Forward declaration of a table and (closure) function

[Ross Berteig <Ross@...>]

At 06:21 PM 7/13/2011, Steve Litt wrote:
>Hi all,
>
>In a program I'm doing "OOP" using a table called Columns and a
>function called Columns.new() containing several other functions
>that become, for want of a better word, "methods".
>
>The purpose of this construct is as use-modifiable setup, so I'd
>like to have the calls to it at the very top of the program, where
>users expect user-modifiable stuff to be. So what I'd like at the
>top would be something like:
>
>columns = Columns.new({})
>-- THESE *MUST* BE IN SPREADSHEET COLUMN ORDER!!!
>-- IF THE SPREADSHEET CHANGES, CHANGE THIS LIST
>columns.newCol("recvdate"  , {quantfield=false, fcn=this})
>columns.newCol("fname"     , {quantfield=false,  fcn=this})
>columns.newCol("lname"     , {quantfield=false,  fcn=this})

You want something like this:

require "spreadsheet"
local columns = spreadsheet.new{}
-- THESE *MUST* BE IN SPREADSHEET COLUMN ORDER!!!
-- IF THE SPREADSHEET CHANGES, CHANGE THIS LIST
columns:newCol("recvdate"  , {quantfield=false, fcn=this})
columns:newCol("fname"     , {quantfield=false,  fcn=this})
columns:newCol("lname"     , {quantfield=false,  fcn=this})


Note the use of colon notation for the calls to newCol() but not 
to the call to new(). Requiring the file begin with a call to 
require() is not unreasonable. However, if this file is really 
intended to be edited by a user as if it were a configuration 
file your system could insert that call ahead of parsing the 
user's file.

>Trouble is, these bomb if Columns and Columns.new() are defined
>below them. I can do this above them:
>
>local Columns
>
>and then define Columns below, but if I do this:
>
>local Columns, Columns.new

That can't work because you are actually calling the value 
stored in Columns.new before you've stored a function there. The 
forward declaration of a Columns variable doesn't permit you to 
index a field of the table it doesn't hold yet, after all.

>then it bombs with:
>slitt@mydesk:/d/at/lua/massmail$ ./colnames.lua
>/usr/bin/lua: ./colnames.lua:3: unexpected symbol near '.'
>slitt@mydesk:/d/at/lua/massmail$
>
>Is there a syntax to forward declare both Columns and 
Columns.new(),
>or will I have to byte the bullet and make those definitions in a
>separate file that I must require?

Using require() has the advantage that you separate out the 
mechanism and isolate it from the parts the user is expected to 
modify.


Ross Berteig                               Ross@CheshireEng.com
Cheshire Engineering Corp.           http://www.CheshireEng.com/