fix(and): fixing typing for and

src/and.test.ts

@@ -1,9 +1,39 @@
 import * as $ from './index.js'
 
 test('and', () => {
-  const r: { name: string } = $.and(
-    $.object({ firstName: $.nullishOr($.string), lastName: $.nullishOr($.string), name: $.string }),
+  const r = $.and(
+    $.object({ firstName: $.nullishOr($.string), lastName: $.nullishOr($.string), name: $.unknown }),
     $.object({ name: $.string })
   )(JSON.parse('{"name":"Foo","role":"admin"}'))
-  expect(r).toEqual({ name: 'Foo', role: 'admin' })
+  const name: string = r.name
+  expect(name).toEqual('Foo')
+})
+
+test('parametric', () => {
+  const known = {
+    string: $.string,
+    number: $.number
+  } as const
+  const assertType = $.oneOf(
+    'string',
+    'number'
+  )
+  const assertUnknown = $.exact({
+    type: assertType,
+    value: $.unknown
+  })
+  const assert =
+    <T extends undefined | 'number' | 'string'>(type: T) =>
+      $.and(
+        assertUnknown,
+        value =>
+          $.object({
+            type: $.eq(type ?? value.type),
+            value: known[type ?? value.type]
+          })(value)
+      )
+  assert('number')(JSON.parse('{"type":"number","value":42}'))
+  expect(() => assert('number')(JSON.parse('{"type":"number","value":"42"}'))).toThrow()
+  assert('string')(JSON.parse('{"type":"string","value":"42"}'))
+  expect(() => assert('string')(JSON.parse('{"type":"string","value":42}'))).toThrow()
 })

src/and.ts

@@ -1,12 +1,14 @@
 import type { Assert } from './prelude.js'
 
-type $<T> = Assert<T>
+type Z<A, B> =
+  (value: A) =>
+    B
 
 type And = {
-  <A>(a: $<A>): $<A>
-  <A, B>(a: $<A>, b: $<B>): $<A & B>
-  <A, B, C>(a: $<A>, b: $<B>, c: $<C>): $<A & B & C>
-  <A, B, C, D>(a: $<A>, b: $<B>, c: $<C>, d: $<D>): $<A & B & C & D>
+  <A>(a: Assert<A>): Assert<A>
+  <A, B>(a: Assert<A>, b: Z<A, B>): Assert<A & B>
+  <A, B, C>(a: Assert<A>, b: Z<A, B>, c: Z<B, C>): Assert<A & B & C>
+  <A, B, C, D>(a: Assert<A>, b: Z<A, B>, c: Z<B, C>, d: Z<C, D>): Assert<A & B & C & D>
 }
 
 const and: And =