# Library design

The type-level design of the public API, and the limitations it carries. The
user-facing reference is [README § API](../README.md#api); this file records why
the API is shaped the way it is.

## A type guard is the single primitive

#### Decision (2026-09)

Every pattern constructor returns a `Matcher<T>` whose only member is
`matches: (value: unknown) => value is T`. `Pattern<T>` is an alias for
`Matcher<T>`.

#### Why

- A type guard is the one TypeScript construct that both narrows in an `if` and
  composes into a chain, so the whole library can be built from it without a DSL
  or transpiler.
- Because `matches` narrows, `match(value).with(pattern, handler)` can give the
  handler the right type with no cast and no runtime type tag.
- Keeping `Pattern<T>` as an alias means a caller can name either; the type is
  identical.

## The builder is immutable

#### Decision (2026-09)

`match(value)` returns a builder whose `.with` returns a _new_ builder rather
than mutating the current one.

#### Why

- A partially built chain can be stored and reused without one call site
  affecting another.
- `.with` widens the result type `R` to `R | V`; a value that changes type in
  place cannot be represented soundly. A new value per `.with` is what makes the
  type-level accumulation work.

## `exhaustive()` is a runtime check

#### Decision (2026-09)

`.exhaustive()` throws at runtime when no case matched. It does not statically
prove that every member of the input union has a case.

#### Why

- TypeScript cannot require a fluent call chain to cover every union member
  without a compiler plugin or a builder whose type tracks an uncovered-member
  set. That machinery is out of proportion to the library's size.
- The union-of-return-types (`R`) is still type-safe; what is not guaranteed is
  that the chain is complete.

#### Known issue

- A missing case is a runtime `Error`, not a compile error. A caller who wants
  compile-time safety must either add a `.otherwise(...)` (which never throws) or
  prove coverage themselves. Making the builder track uncovered members is a
  possible future change; it is not in scope now.

## `P.type` takes the type as a parameter

#### Decision (2026-09)

`P.type<T>(name)` requires the caller to supply `T` explicitly; it is not
inferred from the `typeof` string.

#### Why

- A runtime string cannot carry a TypeScript type. The caller pairs the two, and
  the compiler only checks that `T` is used consistently afterwards.

#### Known issue

- The type parameter and the runtime name can disagree (`P.type<number>("string")`
  compiles), because nothing ties `T` to `name`. `P.when` with a real type guard
  avoids the pairing entirely, so prefer it. A future revision could map the name
  to a type with a conditional type; that is not in scope now.

## `P.shape` narrows only through `refine`

#### Decision (2026-09)

`P.shape<S, T>(shape, refine?)` returns `Matcher<T>`, defaulting to
`Matcher<S>` when no `refine` is given.

#### Why

- The shape object's values may be matchers, so the shape's literal type is not
  the matched type: `S` describes the check, not the value. A `refine` type guard
  is the explicit point where the value is narrowed to `T`.
- Checking keys with `in` (rather than requiring exact keys) lets a shape match a
  wider object, which is how discriminated unions are handled.

#### Known issue

- Without `refine`, a discriminated-union case does not narrow, so `P.shape`
  on its own is easy to misuse. The README shows the `refine` form; prefer
  `P.when` when a guard is already available.

## `P.any` exists for non-guard predicates

#### Decision (2026-09)

`P.any<T>(predicate)` accepts a boolean predicate and a declared `T`.

#### Why

- Some checks are cheap to write as a boolean but awkward as a type guard (for
  example an `every` over an array). `P.any` is the escape hatch for those.

#### Known issue

- Like `P.type`, the declared `T` is not proven by the predicate. Prefer
  `P.when` whenever the check can be written as a guard.