"Sometimes an additional storage area is needed to hold objects. The " { $emphasis "retain stack" } " is an auxilliary stack for this purpose. Objects can be moved between the data and retain stacks using the following two words:"
{ $subsection >r }
{ $subsection r> }
"The top of the data stack is ``hidden'' between " { $link >r } " and " { $link r> } ":"
"Words must not leave objects on the retain stack, nor expect values to be there on entry. The retain stack is for local storage within a word only, and occurrences of " { $link >r } " and " { $link r> } " must be balanced inside a single quotation. One exception is the following trick involving " { $link if } "; values may be pushed on the retain stack before the condition value is computed, as long as both branches of the " { $link if } " pop the values off the retain stack before returning:"
"The following pair of words invoke words and quotations reflectively:"
{ $subsection call }
{ $subsection execute }
"These words are used to implement " { $emphasis "combinators" } ", which are words that take code from the stack. Note that combinator definitions must be followed by the " { $link POSTPONE:inline } " declaration in order to compile in the optimizing compiler; for example:"
"Quotations also implement the sequence protocol, and can be manipulated with sequence words; see " { $link "quotations" } "."
{ $see-also "combinators" } ;
ARTICLE: "booleans""Booleans"
"In Factor, any object that is not " { $link f } " has a true value, and " { $link f } " has a false value. The " { $link t } " object is the canonical true value."
{ $subsection f }
{ $subsection t }
"The " { $link f } " object is the unique instance of the " { $link f } " class; the two are distinct objects. The latter is also a parsing word which adds the " { $link f } " object to the parse tree at parse time. To refer to the class itself you must use " { $link POSTPONE:POSTPONE: } " or " { $link POSTPONE:\ } " to prevent the parsing word from executing."
$nl
"Here is the " { $link f } " object:"
{ $example "f .""f" }
"Here is the " { $link f } " class:"
{ $example "\\ f .""POSTPONE: f" }
"They are not equal:"
{ $example "f \\ f = .""f" }
"Here is an array containing the " { $link f } " object:"
{ $example "{ f } .""{ f }" }
"Here is an array containing the " { $link f } " class:"
{ $example "{ POSTPONE: f } .""{ POSTPONE: f }" }
"The " { $link f } " object is an instance of the " { $link f } " class:"
{ $example "f class .""POSTPONE: f" }
"The " { $link f } " class is an instance of " { $link word } ":"
{ $example "\\ f class .""word" }
"On the other hand, " { $link t } " is just a word, and there is no class which it is a unique instance of."
{ $example "t \\ t eq? .""t" }
"Many words which search collections confuse the case of no element being present with an element being found equal to " { $link f } ". If this distinction is imporant, there is usually an alternative word which can be used; for example, compare " { $link at } " with " { $link at* } "."
$nl
"A tuple cannot delegate to " { $link f } " at all, since a delegate of " { $link f } " actually denotes that no delegate is set. See " { $link set-delegate } ".";
ARTICLE: "conditionals""Conditionals and logic"
"The basic conditionals:"
{ $subsection if }
{ $subsection when }
{ $subsection unless }
"Forms abstracting a common stack shuffle pattern:"
{ $subsection if* }
{ $subsection when* }
{ $subsection unless* }
"Another form abstracting a common stack shuffle pattern:"
{ $subsection ?if }
"Sometimes instead of branching, you just need to pick one of two values:"
{ $subsection ? }
"Forms which abstract away common patterns involving multiple nested branches:"
"There are two distinct notions of ``sameness'' when it comes to objects. You can test if two references point to the same object (" { $emphasis "identity comparison" } "), or you can test if two objects are equal in a domain-specific sense, usually by being instances of the same class, and having equal slot values (" { $emphasis "value comparison" } "). Both notions of equality are equality relations in the mathematical sense."
{ $values { "x"object } } { $description "Moves the top of the data stack to the retain stack." } ;
HELP:r>( -- x)
{ $values { "x"object } } { $description "Moves the top of the retain stack to the data stack." } ;
HELP:datastack( -- ds)
{ $values { "ds"array } }
{ $description "Outputs an array containing a copy of the data stack contents right before the call to this word, with the top of the stack at the end of the array." } ;
HELP:set-datastack( ds-- )
{ $values { "ds"array } }
{ $description "Replaces the data stack contents with a copy of an array. The end of the array becomes the top of the stack." } ;
HELP:retainstack( -- rs)
{ $values { "rs"array } }
{ $description "Outputs an array containing a copy of the retain stack contents right before the call to this word, with the top of the stack at the end of the array." } ;
HELP:set-retainstack( rs-- )
{ $values { "rs"array } }
{ $description "Replaces the retain stack contents with a copy of an array. The end of the array becomes the top of the stack." } ;
HELP:callstack( -- cs)
{ $values { "cs"callstack } }
{ $description "Outputs a copy of the call stack contents, with the top of the stack at the end of the vector. The stack frame of the caller word is " { $emphasis "not" } " included." } ;
HELP:set-callstack( cs-- )
{ $values { "cs"callstack } }
{ $description "Replaces the call stack contents. The end of the vector becomes the top of the stack. Control flow is transferred immediately to the new call stack." } ;
{ $contract "Outputs the hashcode of an object. The hashcode operation must satisfy the following properties:"
{ $list
{ "if two objects are equal under " { $link = } ", they must have equal hashcodes" }
{ "if the hashcode of an object depends on the values of its slots, the hashcode of the slots must be computed recursively by calling " { $link hashcode* } " with a " { $snippet "level" } " parameter decremented by one. This avoids excessive work while still computing well-distributed hashcodes. The " { $link recursive-hashcode } " combinator can help with implementing this logic" }
{ "the hashcode should be a " { $link fixnum } ", however returning a " { $link bignum } " will not cause any problems other than potential performance degradation."
"the hashcode is only permitted to change between two invocations if the object was mutated in some way" }
}
"If mutable objects are used as hashtable keys, they must not be mutated in such a way that their hashcode changes. Doing so will violate bucket sorting invariants and result in undefined behavior." } ;
HELP:hashcode
{ $values { "obj"object } { "code"fixnum } }
{ $description "Computes the hashcode of an object with a default hashing depth. See " { $link hashcode* } " for the hashcode contract." } ;
"Tests if two objects are equal. If " { $snippet "obj1" } " and " { $snippet "obj2" } " point to the same object, outputs " { $link t } ". Otherwise, calls the " { $link equal? } " generic word."
"User code should call " { $link = } " instead; that word first tests the case where the objects are " { $link eq? } ", and so by extension, methods defined on " { $link equal? } " assume they are never called on " { $link eq? } " objects."
$nl
"Method definitions should ensure that this is an equality relation, modulo the assumption that the two objects are not " { $link eq? } ". That is, for any three non-" { $link eq? } " objects " { $snippet "a" } ", " { $snippet "b" } " and " { $snippet "c" } ", we must have:"
"To define a tuple class such that two instances are only equal if they are both the same instance, we can add a method to " { $link equal? } " which always returns " { $link f } ". Since " { $link = } " handles the case where the two objects are " { $link eq? } ", this method will never be called with two " { $link eq? } " objects, so such a definition is valid:"
"The default implementation treats the two objects as sequences, and recursively compares their elements. So no extra work is required to compare sequences lexicographically."
{ $contract "Outputs a new object equal to the given object. This is not guaranteed to actually copy the object; it does nothing with immutable objects, and does not copy words either. However, sequences and tuples can be cloned to obtain a shallow copy of the original." } ;
{ $description "Outputs an object's type number, between zero and one less than " { $link num-types } ". This is implementation detail and user code should call " { $link class } " instead." } ;
{ $description "Convert a generalized boolean into a boolean. That is, " { $link f } " retains its value, whereas anything else becomes " { $link t } "." } ;
{ $description "For " { $link f } " outputs " { $link t } " and for anything else outputs " { $link f } "." }
{ $notes "This word implements boolean not, so applying it to integers will not yield useful results (all integers have a true value). Bitwise not is the " { $link bitnot } " word." } ;
{ $description "If both inputs are true, outputs " { $snippet "obj2" } ". otherwise outputs " { $link f } "." }
{ $notes "This word implements boolean and, so applying it to integers will not yield useful results (all integers have a true value). Bitwise and is the " { $link bitand } " word." }
{ $examples
"Usually only the boolean value of the result is used, however you can also explicitly rely on the behavior that if both inputs are true, the second is output:"
{ $description "If both inputs are false, outputs " { $link f } ". otherwise outputs the first of " { $snippet "obj1" } " and " { $snippet "obj2" } " which is true." }
{ $notes "This word implements boolean inclusive or, so applying it to integers will not yield useful results (all integers have a true value). Bitwise inclusive or is the " { $link bitor } " word." }
{ $examples
"Usually only the boolean value of the result is used, however you can also explicitly rely on the behavior that the result will be the first true input:"
{ $description "Tests if at exactly one object is not " { $link f } "." }
{ $notes "This word implements boolean exclusive or, so applying it to integers will not yield useful results (all integers have a true value). Bitwise exclusive or is the " { $link bitxor } " word." } ;
{ $description "Alternative conditional form that preserves the " { $snippet "cond" } " value if it is true."
$nl
"If the condition is true, it is retained on the stack before the " { $snippet "true" } " quotation is called. Otherwise, the condition is removed from the stack and the " { $snippet "false" } " quotation is called."
$nl
"The following two lines are equivalent:"
{ $code "X [ Y ] [ Z ] if*""X dup [ Y ] [ drop Z ] if" } } ;
{ $description "If the condition is " { $link f } ", the " { $snippet "false" } " quotation is called with the " { $snippet "default" } " value on the stack. Otherwise, the " { $snippet "true" } " quotation is called with the condition on the stack."
$nl
"The following two lines are equivalent:"
{ $code "[ X ] [ Y ] ?if""dup [ nip X ] [ drop Y ] if" } } ;
{ $description "Starts the front-end processor (FEP), which is a low-level debugger which can inspect memory addresses and the like. The FEP is also entered when a critical error occurs." }
{ $notes
"The term FEP originates from the Lisp machines of old. According to the Jargon File,"
$nl
{ $strong "fepped out" } " /fept owt/ " { $emphasis "adj." } " The Symbolics 3600 LISP Machine has a Front-End Processor called a `FEP' (compare sense 2 of box). When the main processor gets wedged, the FEP takes control of the keyboard and screen. Such a machine is said to have `fepped out' or `dropped into the fep'."
{ $description "Outputs a byte-by-byte copy of the given object. User code should call " { $link clone } " instead." } ;
HELP:declare
{ $values { "spec""an array of class words" } }
{ $description "Declares that the elements at the top of the stack are instances of the classes in " { $snippet "spec" } "." }
{ $warning "The compiler blindly trusts declarations, and false declarations can lead to crashes, memory corruption and other undesirable behavior." }
{ $examples
"The optimizer cannot do anything with the below code:"
{ $code "2 + 10 *" }
"However, if we declare that the top of the stack is a " { $link float } ", then type checks and generic dispatch are eliminated, and the compiler can use unsafe intrinsics:"
{ $description "Outputs an object's tag number, between zero and one less than " { $link num-tags } ". This is implementation detail and user code should call " { $link class } " instead." } ;
{ $description "Reads an object from the Factor VM's environment table. User code never has to read the environment table directly; instead, use one of the callers of this word." } ;
{ $description "Writes an object to the Factor VM's environment table. User code never has to write to the environment table directly; instead, use one of the callers of this word." } ;
HELP:object
{ $class-description
"The class of all objects. If a generic word defines a method specializing on this class, the method is used as a fallback, if no other applicable method is found. For instance:"
{ $description "Partial application. Outputs a " { $link callable } " which first pushes " { $snippet "obj" } " and then calls " { $snippet "quot" } "." }
{ $class-description "The class of objects created by " { $link curry } ". These objects print identically to quotations and implement the sequence protocol, however they only use two cells of storage; a reference to the object and a reference to the underlying quotation." }
{ $notes "Even if " { $snippet "obj" } " is a word, it will be pushed as a literal."
$nl
"This operation is efficient and does not copy the quotation." }
{ $notes "In most cases, tail recursion should be used, because it is simpler both in terms of implementation and conceptually. However in some cases this combinator expresses intent better and should be used."
$nl
"Strictly speaking, the " { $snippet "tail" } " is not necessary, since the following are equivalent:"
{ $code
"[ P ] [ Q ] [ T ] while"
"[ P ] [ Q ] [ ] while T"
}
"However, depending on the stack effects of " { $snippet "pred" } " and " { $snippet "quot" } ", the " { $snippet "tail" } " quotation might need to be non-empty in order to balance out the stack effect of branches for stack effect inference." } ;
