Firm up on derive typings & results (#1489)

* Firm up on derive typings & results

* Move assertion (eslint vs ts, round 1, fight!)

packages/api-derive/src/balances/fees.ts

@@ -3,37 +3,37 @@
 // of the Apache-2.0 license. See the LICENSE file for details.
 
 import { ApiInterfaceRx } from '@polkadot/api/types';
+import { Balance } from '@polkadot/types/interfaces';
 import { DerivedFees } from '../types';
 
-import BN from 'bn.js';
 import { Observable, of } from 'rxjs';
 import { map } from 'rxjs/operators';
 
 import { drr } from '../util/drr';
 
-type Result = [BN, BN, BN, BN, BN];
+type Result = [Balance, Balance, Balance, Balance, Balance];
 
-function querySubtrateV2 (api: ApiInterfaceRx): Observable<Result> {
+function queryV2 (api: ApiInterfaceRx): Observable<Result> {
   return of([
     // get values from api.const for substrate versions post spec_version: 101
     // https://github.com/paritytech/substrate/pull/2883/files#diff-5e5e1c3aec9ddfde0a9054d062ab3db9R131
-    api.consts.balances.creationFee,
-    api.consts.balances.existentialDeposit,
-    api.consts.balances.transactionBaseFee,
-    api.consts.balances.transactionByteFee,
-    api.consts.balances.transferFee
-  ]) as any as Observable<Result>;
+    api.consts.balances.creationFee as Balance,
+    api.consts.balances.existentialDeposit as Balance,
+    api.consts.balances.transactionBaseFee as Balance,
+    api.consts.balances.transactionByteFee as Balance,
+    api.consts.balances.transferFee as Balance
+  ]);
 }
 
-function querySubtrateV1 (api: ApiInterfaceRx): Observable<Result> {
-  return api.queryMulti([
+function queryV1 (api: ApiInterfaceRx): Observable<Result> {
+  return api.queryMulti<Result>([
     // Support older versions and get values from storage
     api.query.balances.creationFee,
     api.query.balances.existentialDeposit,
     api.query.balances.transactionBaseFee,
     api.query.balances.transactionByteFee,
     api.query.balances.transferFee
-  ]) as any as Observable<Result>;
+  ]);
 }
 
 /**
@@ -44,7 +44,7 @@ function querySubtrateV1 (api: ApiInterfaceRx): Observable<Result> {
  * <BR>
  *
  * ```javascript
- * api.derive.balances.fees(([creationFee, transferFee]) => {
+ * api.derive.balances.fees(({ creationFee, transferFee }) => {
  *   console.log(`The fee for creating a new account on this chain is ${creationFee} units. The fee required for making a transfer is ${transferFee} units.`);
  * });
  * ```
@@ -53,8 +53,8 @@ export function fees (api: ApiInterfaceRx): () => Observable<DerivedFees> {
   return (): Observable<DerivedFees> => {
     return (
       api.consts.balances
-        ? querySubtrateV2(api)
-        : querySubtrateV1(api)
+        ? queryV2(api)
+        : queryV1(api)
     ).pipe(
       map(([creationFee, existentialDeposit, transactionBaseFee, transactionByteFee, transferFee]): DerivedFees => ({
         creationFee,