"When a source file is reloaded, the parser compares the previous list of definitions with the current list; any definitions which are no longer present in the file are removed by a call to " { $link forget } ". A warning message is printed if any other definitions still depend on the removed definitions."
$nl
"The parser also catches forward references when reloading source files. This is best illustrated with an example. Suppose we load a source file " { $snippet "a.factor" } ":"
{ $code
"USING: io sequences ;"
"IN: a"
": hello \"Hello\" ;"
": world \"world\" ;"
": hello-world hello "" world 3append print ;"
}
"The definitions for " { $snippet "hello" } ", " { $snippet "world" } ", and " { $snippet "hello-world" } " are in the dictionary."
$nl
"Now, after some heavily editing and refactoring, the file looks like this:"
{ $code
"USING: namespaces ;"
"IN: a"
": hello \"Hello\" % ;"
": hello-world [ hello "" % world ] \"\" make ;"
": world \"world\" % ;"
}
"Note that the developer has made a mistake, placing the definition of " { $snippet "world" } " " { $emphasis "after" } " its usage in " { $snippet "hello-world" } "."
$nl
"If the parser did not have special checks for this case, then the modified source file would still load, because when the definition of " { $snippet "hello-world" } " on line 4 is being parsed, the " { $snippet "world" } " word is already present in the dictionary from an earlier run. The developer would then not discover this mistake until attempting to load the source file into a fresh image."
$nl
"Since this is undesirable, the parser explicitly raises an error if a source file refers to a word which is in the dictionary, but defined after it is used."
{ $subsection forward-error }
"If a source file raises a " { $link forward-error } " when loaded into a development image, then it would have raised a " { $link no-word } " error when loaded into a fresh image."
$nl
"The parser also catches duplicate definitions. If an artifact is defined twice in the same source file, the earlier definition will never be accessible, and this is almost always a mistake, perhaps due to a bad choice of word names, or a copy and paste error. The parser raises an error in this case."
{ $subsection redefine-error } ;
ARTICLE: "definitions""Definitions"
"A " { $emphasis "definition" } " is an artifact read from a source file. This includes words, methods, and help articles. Words for working with definitions are found in the " { $vocab-link "definitions" } " vocabulary. Implementations of the definition protocol include pathnames, words, methods, and help articles."
