Module niltoken

Module to provide Nil token.

Purpose

The nil value is somewhat special in Lua, and can be difficult to use in some contexts. This module offers a unified way to circumvent these difficulties.

The problem

In Lua, nil means "nothing". As such, in a table, there is no way to tell apart a key associated with the value nil from a key which doesn't exist at all in this table. For instance, { foo=123, bar=nil } is exactly the same as { foo=123 }.

However, in some contexts, Lua tables are used to represent data structures which might contain a NULL/nil value. For instance, if the server wants to indicate that it erased the variable foo, it cannot simply send the record { foo=nil }: it would be undistinguishable from the empty record { }. Similarly, Hessian has a Null value which almost naturally maps to Lua's nil; but Hessian's Null can be used as a record value, and even as a record key, whereas Lua's nil can't.

The solution

The way to address those issues of representing a null value when nil can't be used is to have a token, an arbitrary object which by convention means null. Whenever the need for such a token appears, we use the same niltoken object as this token.

It is preferable for niltoken not to be a naturally easy to serialize type, such as a table. This mainly leaves userdata and functions as good candidates. For reasons exposed below, it has been decided to represent niltoken as a function.

Converting between niltoken and nil

There are three common operations to perform on niltoken and tables likely to contain references to niltoken:

• Test for equality with niltoken, as in if foo==niltoken then ... end;
• Convert niltoken references into actual nil;
• Convert nil values into niltoken references.

Equality tests are performed by the usual == Lua operator. Conversions are performed by niltoken itself, which is a function:

• It converts niltoken references into nil: assert(niltoken(niltoken)==nil);
• It converts nil values into niltoken references: assert(niltoken(nil)==niltoken).
• It leaves everything else alone: if a~=nil and a~=niltoken then assert(niltoken(a)==a) end`.

Packaging

the niltoken function is directly returned as the niltoken module, i.e. the idiomatic way to use it is to put a local niltoken = require 'niltoken' statement at the top of modules which might need it.

Type niltoken