April 2002


Most of my work till this point had not required 'interesting' operations with file systems, being restricted to creating and opening files in externally determined locations, then reading and writing the files' contents. When it became necessary to ennumerate the contents of directories, and to deal with permissions and locking I took the opportunity to define an API that would be consistent across Windows, Mac and UNIX whilst cleanly accomodating their differences.

The ZooLib file API doesn't address more esoteric file systems like VMS or IBM's MVS. It models a hierarchical system, with accomodations for special roots (like Windows' UNC paths and Mac Classic's multiple volumes with the same name). Files are byte-oriented, and as with other stream operations ZooLib's file Open and Create return a ref counted streamer object, and optionally an error enum providing more detail in the case of failure. The representation of a pointer to a node in a file system also follows ZooLib's practice of a using a smart object with value semantics.

The ZFile suite was the first place I formalized the notion of a 'trail'. This was another case where it was important to come up with a new name that was suggestive of an existing notion, but distinct enough from it that its different semantics could be defined. Picture a tree; a trail is simply the list of steps to be taken to navigate from some node to some other node. A step is either to traverse the link from the node to its parent, or to traverse the link from the node to one of its children. We represent a step as a string. An empty string indicates a 'bounce', a traversal to the parent of the node. Any other string indicates a traversal to the node with that name. Whether the name is attached to the child node, or attached to the link to the child node doesn't matter in this model.

The notion of a trail through a tree shows up in assets and in the handling of HTTP requests. Theoretically it could also be applied to tuples but I'm not quite ready to harmonize everything to that extent.

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