# 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).

## 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.

#### Why

- A type guard is the one TypeScript construct that both narrows in an `if` and
  composes into a chain, so the library needs no DSL and no transpiler.
- Because `matches` narrows, `match(value).with(pattern, handler)` types the
  handler with no cast and no runtime tag.
- `Pattern<T>` is an alias, so either name is the same type.

## The builder is immutable

#### Decision (2026-09)

`.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 to `R | V`, which cannot be represented by a
  value that changes type in place.

## `exhaustive()` is a runtime check

#### Decision (2026-09)

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

#### Why

- Proving coverage through a fluent chain would need a compiler plugin or a
  builder that tracks an uncovered-member set — out of proportion to this
  library's size.
- The union of handler return types is still type-safe; only the chain's
  completeness is unchecked.

#### Known issue

- A missing case is a runtime `Error`, not a compile error. A caller wants
  either an `.otherwise(...)` (which never throws) or to prove coverage
  themselves. Tracking uncovered members is a possible future change.

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

#### Decision (2026-09)

`P.type<T>(name)` requires the caller to supply `T`; 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

- `T` and the runtime `name` can disagree (`P.type<number>("string")` compiles),
  because nothing ties them together. Prefer `P.when` with a real type guard. A
  name-to-type conditional mapping is a possible future change.

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

#### Decision (2026-09)

`P.shape<S, T>(shape, refine?)` returns `Matcher<T>`, defaulting to `Matcher<S>`
without a `refine`.

#### Why

- The shape's values may be matchers, so `S` describes the check, not the value;
  the `refine` guard is the explicit point where the value narrows to `T`.
- Keys are checked with `in` rather than requiring an exact match, so a shape can
  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`
  alone is easy to misuse. Prefer `P.when` when a guard is already available.

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

#### Decision (2026-09)

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

#### Why

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

#### Known issue

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