Boolean Type Proposal

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VersionNotice: This page pertains to Lua 4 and older versions of Lua that did not have a built-in Boolean type. A built-in Boolean type is available as of Lua 5.0.

Here is a plan for fixing Lua's problem with nil being used for false (which is not explained anywhere on this site yet, refer to lua-l). It still needs more work.

Add a new type "false" that has a single value of false. (This is similar to how type "nil" has a single value nil, and follows Scheme.) Examples:

print(type(false))  --> "false"

Relational operators return false for false and a value other than false/nil for true. Examples:

5 > 6         --> false
5 ~= false    --> "true"
nil == false  --> false

Logical operators consider values other than false/nil as true. If the result of a logical operator is nil (due to conjunction/disjunction with nil) it will be converted to false. In other words the result of both relational and logical operators will never be nil. Examples:

nil and 5     --> false
nil or 5      --> 5
5 and false   --> false
5 or false    --> 5
not 5         --> false

The bad news... here are some examples of what would get broken. However none seem like sane programming patterns.

if (a>b) == nil then ... end  -- testing boolean expression against nil
var3 = var1 or var2           -- expecting possible nil var3
var3 = var1 and var2          --             "

The good news... this change allows programs to distinguish between a variable assigned from a boolean expression and a non-existing variable. For example:

-- try one of these
--IsOk = nil
--IsOk = SomePredicateFunction()
--IsOk = "I don't know"

-- new style test for non-existance
if IsOk == nil then ... end

-- new style test for existance
if IsOk ~= nil then ... end

-- new style test for true/false (assuming value is not nil)
-- also: old style test for existance (assuming value is not false,
--    which holds for legacy code)
if IsOk then ... end


Implementation for Lua 4.0

I made a patch for Lua 4.0 to implement a boolean type. It does not behave exactly as written above -- the operators and and or are different. In Lua they are not strict boolean operators; they may give non-boolean results and will not evaluate their second operand if the outcome of expression is known by the first operand. So I defined them like this:

and(a,b):  if a==nil or  a==false then return a else return b end
or(a,b):   if a~=nil and a~=false then return a else return b end

where b is only evaluated when required. With this definition regularly used Lua constructs with these operators behave as before. I.e. your example above (var3 = var1 and/or var2) may give a nil to var3.

Everything else is as described as above: all comparison operators and the not operator return either false (a reserved keyword now) or the number 1 as true. The conditional statements (if/while/repeat) and not assume everything except nil and false as true. There are 3 new API functions: lua_pushfalse, lua_isfalse, and lua_istrue (not nil nor false). The new tag is named LUA_TFALSE and the type string is "false".

Some warnings:

Ah, and the patch is here: Files:wiki_insecure/users/froese/bool-patch-1

-- EdgarToernig?

Great! One minor complaint about the patch... when adding new defines (LUA_REFFALSE, LUA_TFALSE) there is no reason to change existing defines. Likewise for new opcodes. This reduces conflict with other patches. --JohnBelmonte

Maybe this should be rounded out by adding a global true set to 1.


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Last edited July 14, 2013 7:23 pm GMT (diff)