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To check for a symbol, one might use symbolp. To check for a number, one might use numberp. And so on…
Why is there no booleanp to check for a boolean value? Of course I can use
(defun booleanp (x)
(or (null x)
(equal x t)))
but is there an easier (built-in) way of doing this? If not, is there a special reason, why just this predicate is missing?
746
A predicate is a boolean function whose value may be true or false, depending on the arguments to the predicate. Predicates are a generalization of propositional variables. A propositional variable is a predicate with no arguments.
A Boolean predicate returns the truth value of a Boolean expression. boolean-expression IS NOT TRUE FALSE. boolean-expression. An expression that returns the Boolean value (or representation of a Boolean value) that is to be evaluated by the function.
The predicate is a predefined functional interface in Java defined in the java. util. Function package. It helps with manageability of code, aids in unit-testing, and provides various handy functions.
Namely, the SP has to be able to parse and evaluate a Boolean predicate. Therefore, we advise against using nested terms, and utilizing only simple predicates. An additional problem is posed by lengthy predicates. The DNS-SD specification limits domain names to 253 characters.
Valued predicates refer to a set of related unary Boolean predicates that return TRUE or FALSE based on a property of their argument. IS NULL is the only way to test to see if an expression is NULL or not, and it has been present in SQL-86 and all later versions of the standard.
Given that an assertion a labels a transition t = ( u, v), with t ∈ T i, then we say a is violated if the formula f = u i ∧ ¬ a is satisfied. Valued predicates refer to a set of related unary Boolean predicates that return TRUE or FALSE based on a property of their argument.
Predicate logic is the formal logician’s bread and butter (or Sriracha and literally any other food, if that’s more your thing). Predication identifies a property and tries to connect that property to all, some, or none of a group of things.
I don't know the exact history of numberp, symbolp, the boolean type, and other type predicates, but with the availability of the generic type predicate typep it is not necessary to have a separate predicate for every type. A short way to see if something is of type boolean is (typep object 'boolean).
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I'd dare to guess that the real reason for this is that even though only NIL and T are (TYPEP 'BOOLEAN), any value is a valid boolean expression. I.e. any value except for NIL is considered true in an IF form. Thus, the usefulness of a BOOLEANP would be limited, if not harmful as it would return false for things that are perfectly valid input to the conditional forms.
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