undefined_p, undefined_symbol_p

Arguments

Returns

t if the value of s_exp is not defined; otherwise nil.

Description

These two functions perform a similar task, checking to see if the s_exp is defined. However, they differ in two important ways:

• undefined_p examines the value of s_exp directly, whereas undefined_symbol_p expects the value of s_exp to be a symbol, and examines the value of that resulting symbol.

• undefined_p evaluates its argument in a protected scope where any "Symbol is undefined" errors will be trapped and disregarded. undefined_symbol_p evaluates its argument without protection, so it is possible that a "Symbol is undefined" error could be thrown if the evaluation of s_exp generates such an error.

Example

    Gamma> a = #xyz
    xyz
    Gamma> undefined_p (a);
    nil
    Gamma> undefined_symbol_p (a);
    t
    Gamma> xyz = t;
    t
    Gamma> undefined_symbol_p (a);
    nil
    
    
    Gamma> undefined_p (y);
    t
    Gamma> undefined_symbol_p (y);
    Symbol is undefined: y
    debug 1>
    		

See the GTK: Sending Queries section or the Photon: Sending Queries section of the History Functions chapter in the Cogent Tools Demo and Tutorials book for examples of the undefined_p function used in context.

See Also

Data Types and Predicates