Function assertGuardEqual

In the TS-SDK we duplicate the type and guard definition for each type as the inferred type from io-ts does not produce a good type export when used with typedoc. Sadly this means we have to maintain two definitions for each type which can lead to errors, so we need a mechanism to assert that the type and guard are equal. Normally we could simply do:

interface Foo {
    prop: string;
}
const FooGuard: t.Type<Foo> = t.type({
    prop: t.string
})
Copy

And the explicit type assertion in the FooGuard ensure us that the type are the same. But when we include Branded or Newtype types in the definition, the full type is not the same

interface Foo {
    prop: ContractId;
}
// This throws a type error
const FooGuard: t.Type<Foo> = t.type({
    prop: ContractIdGuard
})
Copy

To understand the error we need to know that t.Type<A> = t.Type<A, O = A, I = unknown> where A is the underlying type, O is the Output type that we get when we call FooGuard.encode and I is the Input type we expect when we call FooGuard.decode. When we use a Branded or Newtype guard, the Output type is different from the underlying type, so we get an error.

The assertGuardEqual function can be used to only assert on the representation type A and leave the Input and Output type unchanged.

interface Foo {
    prop: ContractId;
}
const FooGuard = assertGuardEqual(proxy<Foo>(), t.type({
    prop: ContractIdGuard
}))
Copy