Sharing

Point-Free's swift-sharing gives one value a single storage location and many observers. It is the layer SQLiteData's @FetchAll and @FetchOne are built on, and it works on its own for user defaults, files, and in-memory state.

When to Use

Use this skill when:

• A value is read in several places and must stay in sync — user settings, feature flags, the signed-in user
• You want to persist to UserDefaults or a file without hand-writing the load, save, and observe loop
• You need to know why @FetchAll is empty — still loading, or failed
• You are writing a custom persistence strategy, such as Keychain-backed or remote-config-backed storage
• A build broke after upgrading Xcode with an error about a @Shared setter

Example Prompts

• "How do I persist a setting to UserDefaults and observe it from several views?"
• "My @FetchAll is empty. Is it still loading or did the query fail?"
• "I'm getting a compile error assigning to $settings.isEnabled after upgrading Xcode. What changed?"
• "How do I store an auth token with @Shared but keep it in the Keychain?"
• "How do I stop my tests from writing to the real UserDefaults?"

What This Skill Provides

• @Shared versus @SharedReader, and how to derive narrower references from either
• The three built-in strategies — .appStorage, .fileStorage, .inMemory — and their traps, including the key-format rule that degrades cross-process observation (with a runtime warning) and the single-suite-instance rule
• Mutation through withLock and Binding($shared), and the Swift 6.3 change that turned the old setter path into a compile error below Sharing 2.8.0
• Loading and error state: isLoading, loadError, saveError
• Dynamic keys — swapping the backing key at runtime to re-drive a search
• Writing a custom SharedKey / SharedReaderKey, including continuation rules
• Per-test storage quarantine and the UI-test entry-point setup
• Package traits (CustomDump, IdentifiedCollections, CasePaths) and the Swift 6.1 manifest requirement

Documentation Scope

This page documents the swift-sharing material in the axiom-data skill — the patterns Claude draws on when a value has to persist and stay in sync across your app.

• For the database layer built on top of it, see SQLiteData
• For storing secrets rather than preferences, see Keychain

Related

• SQLiteData – @FetchAll and @FetchOne wrap a Sharing SharedReader, so the loading and error APIs documented here apply to every SQLiteData fetch
• SQLiteData Reference – advanced query composition on top of those property wrappers
• Keychain – where secrets belong; reach them from @Shared through a custom key, never .appStorage

Resources

Docs: github.com/pointfreeco/swift-sharing, github.com/pointfreeco/sqlite-data

Skills: sqlitedata, sqlitedata-ref