A filestore is a structured collection of data files housed in a

conventional hierarchical file system. Many applications use filestores

as a poor-mans database, and the correct execution of these

applications requires that the collection of files, directories, and

symbolic links stored on disk satisfy a variety of precise

invariants. Moreover, all of these structures must have acceptable

ownership, permission, and timestamp attributes. Unfortunately,

current programming languages do not provide support for documenting

assumptions about filestores, detecting errors, or safely loading from

and storing to them.

This paper describes the design, implementation, and semantics of

Forest, a novel domain-specific language for describing

filestores. The language uses a type-based metaphor to specify the

expected structure, attributes, and invariants of filestores.

Forest generates loading and storing functions that make it easy to

connect data on disk to an isomorphic representation in memory that

can be manipulated as if it were any other data structure. Forest

also generates metadata that describes the degree to which the

structures on the disk conform to the specification, making error

detection easy. Hence, in a nutshell, Forest extends the

rigorous discipline of typed programming languages and many of

their benefits to the untyped world of file systems.

We have implemented Forest as an embedded domain-specific language

in Haskell. In addition to generating infrastructure for reading,

writing and checking file systems, our implementation generates a

type class instances that make it easy to build generic tools that

operate over arbitrary filestores. We illustrate the utility of

this infrastructure by building a file system visualizer, a file access

checker, a generic query interface, description-directed variants of

several standard UNIX shell tools and (circularly) a simple Forest

description inference engine. Finally, we formalize a core fragment

of Forest in a semantics inspired by classical tree logics and prove round-tripping laws showing that the loading and storing functions behave sensibly.