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Logical Values

Lpp chose to use 0 as nil. This allows Lpp code to be symmetric with usual C code that uses 0 and 1 as predicates. The alternative would be to have to use the null function for every predicate occurrence. For example any Lpp function f returning let or Lpp object x can be used as a predicate

     if (x) // do something
     if (f()) // do something

Note that nil serves as three important things, a logical value, the end of a list and it is also the symbol nil. The 0 as nil concept it is to be thought of as though the symbol object nil resides at memory address 0. So that everything that can be done with a symbol can be done with 0. For example

     symbolName(0) => "nil"

When a let variable has the value 0 it prints as nil. Conversely when in any s-expression is read from a stream nil is translated into 0 internally. Lpp provides a global variable True that has as its value the Lpp Symbol t, See Symbols. And Nil is defined simply as 0, so it is just as efficient to use Nil as 0 in your code except that the compiler knows that Nil is an Lpp type.

Choosing 0 as nil creates an ambiguous situation when a type can be interpreted as either int or let. For example overloaded Lpp functions that are disambiguated between those that have the same function profile but differ in an int or let argument and so the following is legitimate

     let someFunction(int i, let obj);  // i to be an int
     let someFunction(let i, let obj);  // i to be an Lpp Integer or nil

So to disambiguate you would use Nil where nil was intended

     someFunction(0, something);     // Means 0 as an int
     someFunction(L(0), something);  // Means 0 as an Lpp Integer
     someFunction(Nil, something);   // Menas 0 as Lpp nil

Nil Constant

True Constant

Nil is intended to be used in programs where the nil symbol is needed and True is intended to be used where the t symbol is needed. These are both names of constants that can not be assigned other values. Since the Lpp Nil object prints as nil and the Lpp True object prints as t, nil and t when used in this manual will refer to those objects respectively.

The name of the constant True and Nil can be redefined per compilation unit, See Redefining Predefined Names. So for example an Lpp programmer could redefine these names to be nil and t to correspond to the Common Lisp constants nil and t. After examining many C++ programs it was decided that the default should not be the names nil and t since these frequently occurred as variable names. For example t is frequently used as a variable in C++ programs to denote "time". Also, using Nil and True in Lpp programs emphasizes the fact that they are just C++ identifier names and not in fact symbols as they are in Lisp programs.

null exp Function

null returns t if given expression exp evaluates to nil and returns nil otherwise.