Trouble Understanding Function with tryPromise and Decode in TypeScript

// ...
  merchantByApiToken: (customerApiToken) =>
    Effect.tryPromise({
      try: async () => await heyMantleClient.getCustomer(customerApiToken),
      catch: (unknown) => new MantleError({ unknown }),
    }).pipe(
      Effect.flatMap((res) =>
        S.decode(Customer)({
          id: res.id,
          subscription: res.subscription
            ? {
                id: res.subscription.id,
                plan: {
                  id: res.subscription.plan.id,
                  features: res.subscription.plan.features,
                },
              }
            : null,
        })
      )
    ),
// ...

(property) MantleClient.merchantByApiToken: (apiToken: string) => Effect.Effect<Customer, MantleError | ParseError, never>