Semipredicate problem

In computer programming, a semipredicate problem occurs when a subroutine intended to return a useful value can fail, but the signalling of failure uses an otherwise valid return value. The problem is that the caller of the subroutine cannot tell what the result means in this case.

In computer programming, a semipredicate problem occurs when a subroutine intended to return a useful value can fail, but the signalling of failure uses an otherwise valid return value. The problem is that the caller of the subroutine cannot tell what the result means in this case. Returning a tuple of values. This is conventional in languages (such as Python) that have a built-in tuple data type and special syntax for handling these: in Python, x, y = f() calls the function f returning a pair of values and assigns the elements of the pair to two variables. Secondary return values as in Common Lisp. All expressions have a primary value, but secondary values might be returned to interested callers. For example, the GETHASH function returns the value of the given key in an associative map, or a default value otherwise. However, it also returns a secondary boolean indicating whether the value was found, making it possible to distinguish between the "no value was found" and "the value found was equal to default value" cases. This is different from returning a tuple, in that secondary return values are optional if a caller does not care about them, it may ignore them completely, whereas tuple-valued returns are merely syntactic sugar for returning and unpacking a list, and every caller must always know about and consume all items returned. Languages with call by reference or equivalents, such as call by address using pointers can allow for multivalued return by designating some parameters as output parameters. In this case, the function could just return the error value, with a variable intended to store the actual result being passed to the function. This is analogous to the use of an exit status to store an error code and to streams for returning content. A variant of output parameters is used in object-oriented languages that use call by sharing, where a mutable object is passed to a function, and the object is mutated to return values. Logic programming languages such as Prolog have no return values. Instead, unbound logical variables are used as output parameters, to be unified with values constructed in a predicate call. For example, a numeric function normally returns a number (int or float), and while zero might be a valid response, false is not. Similarly, a function that normally returns a string might sometimes return the empty string as a valid response, but return false on failure. This process of type-juggling necessitates care in testing the return value: e.g., in PHP, use === (i.e., equal and of same type) rather than just == (i.e., equal, after automatic type conversion). It works only when the original function is not meant to return a boolean value, and still requires that information about the error be conveyed via other means. fn find(key: String) -> Option { if key == "hello" { Some(key) } else { None } }

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