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Capability Types

Verum has two complementary dependency-injection mechanisms:

  • Contexts (using [Database, Logger]) — which resources a function needs.
  • Capabilities (Database with [Read]) — what a function is allowed to do with a resource it already has.

Capability types narrow a type's effective API at the type level, before any call site. A Database with [Read] cannot call .write(...) — not because of a runtime check, but because the method resolution refuses to find the symbol.

Syntax​

capability_type = path_type , 'with' , capability_list ;
capability_list = '[' , capability_item , { ',' , capability_item } , ']' ;
capability_item = capability_name | capability_or_expr ;
capability_or_expr = capability_name , '|' , capability_name , { '|' , capability_name } ;

Example:

fn stats(db: Database with [Read]) -> Stats {
    // db.query(...) works (Read capability provides .query)
    // db.write(...) is rejected at type check
}

The built-in capability vocabulary​

From the grammar:

Read | Write | ReadWrite | Admin | Transaction
Network | FileSystem | Query | Execute
Logging | Metrics | Config | Cache | Auth

Any identifier can serve as a custom capability, so user code is not restricted to the built-in list — the built-ins are merely the compiler-known conventions that the standard library uses.

Subtyping​

A capability type with a superset of capabilities is a subtype of one with fewer:

T with [A, B, C]  <:  T with [A, B]  <:  T with [A]  <:  T with []

This is the basis of automatic attenuation at call sites. You never write a conversion — the compiler checks that whatever capabilities the callee demands are a subset of those the caller supplies.

fn audit(db: Database with [Read, Write, Admin])
    using [AuditLog]
{
    log_stats(db);      // calls fn log_stats(db: Database with [Read])
    migrate(db);        // calls fn migrate(db: Database with [Read, Write])
    purge_deleted(db);  // calls fn purge(db: Database with [Admin])
}

All three inner calls receive a subtype of what they demand.

Composition via union​

A single capability item can be a union with |, meaning "any of these capabilities suffices":

fn log_access(resource: Resource with [Read | Execute]) { ... }

Read | Execute is a single capability slot satisfied by either. Union in a capability list is distinct from listing two separate capabilities:

[Read | Execute]       // one slot, one of the two required
[Read, Execute]        // two slots, both required

Declaring refined types​

Use type to name a particular attenuation:

type Database.Full     is Database with [Read, Write, Admin];
type Database.ReadOnly is Database with [Read];
type Database.TxScope  is Database with [Read, Write, Transaction];

The dotted name is a nested path; the dot has no runtime meaning — it's purely a naming convention to group related refinements.

A function boundary can then speak in the narrow name:

pub fn rates(db: Database.ReadOnly) -> List<Rate> {
    db.query("SELECT ...").rows()
}

pub fn migrate(db: Database.Full)
    using [MigrationLog]
{
    db.begin();
    db.write("ALTER TABLE ...");
    db.commit();
}

Interaction with the context system​

Contexts and capabilities compose orthogonally:

fn page(req: Request) -> Response
    using [Database with [Read], Logger, Analytics if cfg.feature_flag]
{
    // Database injected via `using` (DI)
    // Capabilities narrow it (type-level)
    // Analytics is conditional on a feature flag
}

The using [...] clause always wins for injection — the compiler finds the most general implementation matching the capability demand and automatically attenuates.

Stdlib protocols that respect capabilities​

Protocols in the standard library are generic over their capability demand. The common pattern:

type Query is protocol {
    fn run<D: Database with [Read]>(&self, db: D) -> Rows;
}

type Mutation is protocol {
    fn apply<D: Database with [Write]>(&self, db: D) -> RowsAffected;
}

A Mutation cannot accept a Database.ReadOnly — the protocol bound forbids it.

Capability assertion at call sites​

For cross-boundary APIs (FFI, untyped context boundaries), the assert_capability intrinsic performs a one-time runtime check and narrows the type:

fn from_ffi(raw: RawHandle) -> Database with [Read, Write] {
    let db = Database.from_raw(raw);
    assert_capability!(db, [Read, Write]);   // panics if missing
    db
}

Inside normal Verum code this escape hatch is rarely needed — the static subtype relation carries capabilities through.

Relationship with &T, &mut T, and tiers​

Capability attenuation is independent of reference tiers. You can freely combine them:

fn read_rows(db: &(Database with [Read])) -> List<Row> { ... }
fn cb_tx(db: &checked mut (Database with [Read, Write, Transaction])) { ... }

When the capability list is on a reference target, parenthesise: &(T with [R]). The unparenthesised form &T with [R] parses as "reference to T, then extend with [R]" — two different syntactic positions, same net effect, but explicit parentheses are preferred.

Examples​

Read-only file handle​

type File.Read is File with [Read];

fn count_lines(f: File.Read) -> Int {
    f.lines().count()
    // f.write(...) is a type error here.
}

Admin-only migration​

type Db.Admin is Database with [Read, Write, Admin];

pub fn migrate_v3(db: Db.Admin) using [Logger] {
    db.write("CREATE TABLE ...");
    db.write("ALTER ROLE ...");
}

A non-admin caller cannot even name the function's argument type without having [Read, Write, Admin] in scope; capability types surface privilege failures at the function boundary.

Minimal auth-safe logger​

type Logger.NoAuth is Logger with [Logging];
// Logger is presumed to potentially carry [Logging, Metrics, Config, Auth]

fn fire_and_forget(msg: Text, log: Logger.NoAuth) {
    log.info(msg);
    // log.auth_token() is a type error
}

Grammar​

capability_type     = path_type , 'with' , capability_list ;
capability_list     = '[' , capability_item , { ',' , capability_item } , ']' ;
capability_item     = capability_name | capability_or_expr ;
capability_or_expr  = capability_name , '|' , capability_name
                    , { '|' , capability_name } ;
capability_name     = 'Read' | 'Write' | 'ReadWrite' | 'Admin' | 'Transaction'
                    | 'Network' | 'FileSystem' | 'Query' | 'Execute'
                    | 'Logging' | 'Metrics' | 'Config' | 'Cache' | 'Auth'
                    | identifier ;

See also​