VIP-3: A!E!R is an alias for Effect<A, E, R>

Status: accepted by Will, 2026-10-28.

Decision

effect function load(id): User!NotFound!Users { … }
interface Users { find(id: string): User!NotFound }

• A!E!R is a type alias, nothing more. It means exactly what Effect<A, E, R> means wherever a type can be written, and it behaves exactly the same.

• An omitted slot is never, as Effect's own defaults make it. The meaning never depends on where the type is written. In particular, an omitted slot is never inferred, so User!NotFound means Effect<User, NotFound, never> even on a function with a body.

• To have the type inferred, leave out the annotation (VIP-1).

• Amended 2026-10-29: ! beside other type operators. ! does not mix with |, &, function types or conditional types without parentheses: type T = A | B!E is a syntax error with a quick fix that parenthesizes, to (A | B)!E (Effect.Effect<A | B, E>) or A | (B!E) (A | Effect.Effect<B, E>). Decided by the agent on 2026-10-29 under Will's rule "when I state a principle, the agent derives the concrete rule and acts; I can override later" (decided detail 47).

• Amended 2026-10-29: function-type returns. (id: string) => User!NotFound is (id: string) => (User!NotFound), as () => A | B is () => (A | B); likewise for constructor types. ! still does not mix with | or &, and a conditional type's parts still need parentheses. Decided by the agent on 2026-10-29 under Will's rule "when I state a principle, the agent derives the concrete rule and acts; I can override later" (decided detail 80).

• Amended 2026-10-30: a line break ends an Effect type. Each !, and the slot after it, start on the line where the previous slot ends; a slot in parentheses may span lines. TypeScript stops its postfix type operators at a preceding line break, and a slot has no closing token, so now(): number!! then later(): number!! stay two members and a ! with no slot on its line is TS17019. Decided by the agent on 2026-10-30 under Will's rule "when I state a principle, the agent derives the concrete rule and acts; I can override later".

• Amended 2026-10-30: migration warning. The transitional compiler reports EK4853 when --fix can preserve requirements that the retired omitted-slot rule inferred by writing them explicitly. An omitted slot still means never; omitting the whole return annotation instead infers the complete contract. The warning retains its emitted compatibility code until a coordinated compiler/documentation renumber.

Amendment: ordinary source operand boundaries

An Effect slot accepts ordinary TypeScript source types at the type-operator level. At the optional slot after !!, prefer the present R when those same-line tokens start such an operand, retaining the existing exclusion of an unparenthesized {. Negative literal types remain admitted: number!!-1 means Effect.Effect<number, never, -1>, and x as number!! - 1 consumes that requirement. Parenthesize the whole assertion, (x as number!!) - 1, to select subtraction; its operands still receive ordinary TypeScript checking. This is syntax-based, independent of symbol lookup and spacing. Parser-only JSDoc forms (?T, T?, !T, *) are not source type operands, even inside parentheses: ? and * at an optional-slot boundary remain expression operators where admitted, while an additional ! follows the existing bang/non-null rules. Existing line-break and type-operator restrictions remain.

• Amended 2026-10-30: surrounding type and expression punctuation. Only optional R after !! excludes an unparenthesized object type; required slots admit it. An optional tuple element marks the completed Effect type, keeping omission and indexed undefined. At the optional-R boundary, < continues a comparison and [ starts a tuple requirement; a generic function requirement needs parentheses. Existing as/satisfies expression continuations and contextual type aliases keep their ordinary roles. Prefer present R between complete source parses, with no symbol lookup; a recovery tree that strands the rest of the expression does not compete. A complete parse includes TypeScript’s automatic semicolon insertion: x as A!!as at the end of a line, then B on the next, is two statements with R = as, while x as A!! as B on one line is one chained assertion. Appendix A states these boundaries and their checked examples.

Prior art

Zig's error unions write NotFound!User, error first. VibeLang writes the success type first because Effect<A, E, R> lists it first.

Will's words

"Keep it but get rid of that complexity where we can infer R. It's simply an alias for Effect<...> and should work the exact same as if we wrote it the long form way"

Footnote: old VibeLang

Old VibeLang is a reference only; nothing in it is part of the language unless a VIP adds it. For implementers, this VIP replaces:

The part of D127 and D309 under which an omitted slot was inferred on a function with a body.

• Amended 2026-11-01: the retired bare global alias. The language's shorthand A!E!R expands to Effect.Effect<A, E, R>; no normative global Effect<A, E, R> alias exists. While the legacy prelude alias is accepted, a use resolving to that alias reports migration warning EK4856. The quick fix and --fix write A!E!R where it means the same type in that position without new parentheses, and Effect.Effect<A, E, R> through the real Effect namespace binding everywhere else: Effect<number> becomes number!!, while Effect<A | B, E>, Effect<A, E>[] and an extends or implements heritage element get the qualified form. Existing parentheses and written default slots stay; where the namespace binding cannot be kept, the fix is refused. Emitted declarations already print Effect.Effect<…> and do not change (decided detail 260, 2026-11-02). Imported or local types named Effect retain their ordinary meanings. Remove the alias only in a coordinated compiler/codemod/docs release; thereafter a namespace-as-type use is TypeScript's TS2709, and an absent binding follows native name resolution.

• Amended 2026-11-03: written infer slots retire. An annotation means what the same annotation means on the equivalent Effect TypeScript: A!E!R is Effect.Effect<A, E, R>, and omitted slots are never. There is no one-slot inference form. Ordinary conditional-type infer remains as in TypeScript. Migration error EK4857 replaces a legacy written infer slot with the checker's inferred type, or removes an explicitly effect-marked callable's whole annotation when all slots are infer, preserving declaration types under VIP-34's safe-fix checks. For the same immediate written infer channel in the explicit legacy return-slot set, including an unmarked body-bearing callable whose Effect annotation selected the legacy model, EK4857 replaces TS1338 while this migration exists, following VIP-34's specific-diagnostic rule; independent native errors remain. Valid conditional-type inference and native infer outside the migration keep TypeScript's rules and diagnostics. After retirement, TS1338 remains wherever TypeScript rejects that occurrence. New source omits the annotation to infer its type. This follows Will's 2026-11-02 ruling: "Effect syntax is just syntax sugar. Anything you can do with the normal syntax you would ideally be able to do with the shorthand syntax."