CEP-0017: Function Literals and Callable Values¶

Summary¶

Catalyst should eventually decide how anonymous functions, bound methods, callable values, and possibly closures fit with the V1 distinction between comptime-known function items and runtime-storable function pointers.

V1 function items are compile-time-known definitions. Runtime storage uses explicit function pointer types. Capturing closures and bound method values are deferred.

Example¶

Possible future shape:

const doubled = items.map(fn (x: i32) i32 {
  return x * 2
})

const threshold = 10
const filtered = items.filter(fn (x: i32) bool {
  return x > threshold
})

The first literal can be non-capturing. The second captures threshold, so it needs the future closure/capture model rather than a plain V1 function pointer.

Motivation¶

Callable values affect ABI shape, capture ownership, resource cleanup, default parameters, and overload/function-pointer selection. They should not be bolted onto function pointer syntax without a clear storage and lifetime model.

Proposed Direction¶

The proposal should cover:

• anonymous function expression syntax;
• return inference for function literals with bodies;
• bound method values and receiver capture;
• closure capture semantics;
• move, copy, and resource behavior for captures;
• runtime-storable callable value representation;
• function defaults through callable values;
• coercion to function pointers when no capture exists;
• interaction with CEP-0004: Function Overloading.

V1 Compatibility¶

V1 rejects anonymous function expressions, bound method values, and capturing closures. A function item can coerce to a runtime function pointer only in accepted expected-type contexts.

Related Details¶

• Function Types and Pointers
• Default Parameters
• Ownership and Resources