srfi.259 - 型安全な手続きタグ ¶This is an extension of the concept of tagged procedures
introduced by SRFI-229 (see srfi.229 - 手続きタグ).
SRFI-229 allows to attach arbitrary object to a procedure.
SRFI-229 opens possibilities to implement interesting features on top of a very simple primitive. However, since the type and use of tag object is totally up to the user, it can be difficult to compose libraries that relies on procedure tags. What if two libraries chose to use the same type of tags (e.g. symbols)? What if one library wants to add a tag to a procedure that is already tagged by another library?
This SRFI addresses that issue. It creates a unique kind of procedure tags that can only be attached and queried by the designated procedures, so it won’t intefere with other libraries’ use of tags.
The reference implementation of this SRFI is implemented on top of
SRFI-229, allowing users to peek under the hood if they use SRFI-229
procedure-tag. In Gauche, we implement this SRFI directly
in the Gauche’s native mechanism, so SRFI-259 procedure tags are orthogonal
to SRFI-229 procedure tags.
[SRFI-259]{srfi.259}
Construct a unique procedure tag type, orthogonal to any other
procedure tags. Arguments tagger, pred, and accessor
must be identifiers, and are bound to procedures to utilize the new
procedure tag type.
The identifier tagger is bound to a procedure that takes a value and a procedure, and returns a tagged procedure that works the same as the given procedure, associating the given value to the given procedure with this specific tag type. If the given procedure is already tagged by other tag types, the returned procedure has both tag types, keeping values of each type separately.
The identifier pred is bound to a predicate, that returns
#t if the argument is a procedure with this tag type, #f
otherwise.
The identifier accessor is bound to a procedure that takes a procedure with this tag type, and returns the associated value. It is an error to pass objects other than procedures with this tag type.
A tag type created by this macro is conceptually anonymous, but for the debugging aid, Gauche uses tagger as a part of tag type name.