Type-Position Macros

A type-position macro rewrites TypeScript types rather than values. It’s invoked with angle brackets:

const $Nullable = macroRules({
  kind: "type",
  expand: macroRules`
    ($t:Type) => $t | null | undefined
  `,
});

type MaybeUser = $Nullable<User>;
// → type MaybeUser = User | null | undefined;

Requirements

The object form is mandatory. A tag-form macroRules`…` always declares a value macro; there is no way to mark it as a type macro. You must use macroRules({ kind: "type", expand: … }).

Only Type and Tt fragment kinds are accepted in type-position patterns. Any other kind produces a dedicated error.

Zero-argument references

A bare $Foo with no type arguments matches only an arm whose pattern is ():

const $Id = macroRules({
  kind: "type",
  expand: macroRules`
    () => string
    ($t:Type) => $t
  `,
});

type A = $Id;        // → string
type B = $Id<number>; // → number

Qualified names never resolve — a.$Foo is not a macro reference.

Sharing modes are rejected

Types have no runtime representation, so a type macro cannot share a runtime helper. Setting mode to share-only or share-anyway with kind: "type" is an error — and so is supplying runtime or call at all, independently of the mode.

Cross-position misuse

The two directions behave differently:

• Using a value macro in type position is a hard error: “macro $X is value-only; cannot use it in type position”.
• Using a type macro in value position falls through to the ordinary no-arm-matched path, since the value-position matcher never sees its type arms.

No backtracking

Type-position repetition matching is greedy with no retry. A pattern like ($($t:Type),* $last:Type) that works in value position will never match in type position. Keep the variable-length group last, or write one arm per arity. See Repetitions & Backtracking.

Composition

Type macros compose with each other exactly like value macros, and nested expansions run in type context:

const $Wrap = macroRules({
  kind: "type",
  expand: macroRules`($t:Type) => Array<$Nullable<$t>>`,
});