15.2 An illustrative example of user defined streams

15.2.3 Stream directionality

Streams can be defined for input only, output only, or both. In our example, theunicode-ls-stream class needs to be able to read from a file and write to a file, and we therefore defined it to inherit from an input and an output stream class. We could have defined disjoint classes instead, one inheriting fromfundamental-character-input-stream and the other fromfundamental-character-output-stream. This would have allowed us to rely on the default methods for the direction predicates. However, given that we have defined one bi-directional stream class, we must define our own methods for the direction predicates.

(defmethod input-stream-p ((stream unicode-ls-stream))
  (input-stream-p (ls-stream-file-stream stream)))

(defmethod output-stream-p ((stream unicode-ls-stream))
  (output-stream-p (ls-stream-file-stream stream)))

The above code allows us to "trampoline" the correct direction predicate functionality fromfile-stream, using thels-stream-file-stream accessor we defined previously.

input-stream-p

Generic Function

output-stream-p

Generic Function

input-stream-p stream

output-stream-p stream

: Package:common-lisp
: The predicatesinput-stream-p andoutput-stream-p are implemented as generic functions. Their default methods returnt if stream is respectively an input or output stream. If the user wants to implement a stream with no inherent directionality (and thus does not includefundamental-input-stream orfundamental-output-stream) but for which the directionality depends on the instance, then suitable methods should be provided.