FileSystem

IFileSystem makes file access an explicit, mockable dependency. Every operation returns Flow<'env, FileSystemError, _> and fails with FileSystemError rather than throwing. Read the FileSystem guide for supplying the service and representative usage; this page is the operation catalog.

Whole-file reads and writes

Whole-file reads and writes come in text, line, and byte forms, each with an encoding-explicit and an asynchronous variant: readAllTextWithEncoding, readAllTextAsync, readAllBytes, writeAllBytes, and the rest of the readAllText, readAllLines, writeAllLines, writeAllText, appendAllText, and appendAllLines family.

The Async members on IFileSystem (including ReadAllTextAsync, ReadAllBytesAsync, and WriteAllTextAsync) have signatures such as ReadAllTextAsync : path: string -> Flow<unit, FileSystemError, string> — no token argument, and an environment type of unit. The FileSystem module functions you call from a workflow (FileSystem.readAllTextAsync, FileSystem.readAllBytesAsync, FileSystem.writeAllTextAsync, and the rest) wrap those members, so they return Flow<'env, FileSystemError, _> with when 'env :> IHasFileSystem. The runtime passes its cancellation token to the underlying I/O, so interrupting a workflow stops a large read in progress.

Files and metadata

fileExists, exists, deleteFile, copyFile, and moveFile cover the other common operations. getFileLength returns a file's size in bytes. File metadata has getters and setters for attributes and the creation, last-access, and last-write times in both local and UTC forms, such as getFileLastWriteTimeUtc, setFileAttributes, and so on.

Symbolic links are first class: createFileSymbolicLink, createDirectorySymbolicLink, getSymbolicLinkTarget (which returns None when the path is not a link), and resolveSymbolicLinkTarget, whose boolean argument decides whether to follow the whole chain or stop at the immediate target.

Streams and scopes

openRead, openText, openWrite, createFile, createText, appendText, and the openFile family return open handles. Acquire open handles inside a scope to ensure disposal; see scopes and resources.

Directories and paths

createDirectory creates missing parents. deleteDirectory path recursive takes the recursion flag explicitly, so a recursive delete is visible at the call site. Listing comes in eager (getFiles, getDirectories, getFileSystemEntries) and lazy (enumerateFiles, enumerateDirectories, enumerateFileSystemEntries) forms, each taking a search pattern and a SearchOption. enumerateFileSystemInfos directory pattern searchOption yields FileSystemEntry values with path, kind, size, and timestamps in one listing, avoiding separate metadata calls per entry.

Path manipulation is also on the service: combine, getFullPath, getFileName, getExtension, getRelativePath, getTempPath, getRandomFileName, and the rest. These are pure string operations on .NET, but routing them through the service keeps platform-specific separator and rooting behaviour substitutable in tests.