(core) Use GristObjCode in CellValue

Summary: Makes type checking a bit stronger

Test Plan: it just has to compile

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3065

app/plugin/GristData.ts

@@ -1,4 +1,20 @@
-export type CellValue = number|string|boolean|null|[string, ...unknown[]];
+// Letter codes for CellValue types encoded as [code, args...] tuples.
+export const enum GristObjCode {
+  List            = 'L',
+  Dict            = 'O',
+  DateTime        = 'D',
+  Date            = 'd',
+  Skip            = 'S',
+  Censored        = 'C',
+  Reference       = 'R',
+  ReferenceList   = 'r',
+  Exception       = 'E',
+  Pending         = 'P',
+  Unmarshallable  = 'U',
+  Versions        = 'V',
+}
+
+export type CellValue = number|string|boolean|null|[GristObjCode, ...unknown[]];
 
 export interface RowRecord {
   id: number;

app/plugin/objtypes.ts

@@ -4,7 +4,7 @@
  */
 // tslint:disable:max-classes-per-file
 
-import {CellValue} from 'app/plugin/GristData';
+import { CellValue, GristObjCode } from 'app/plugin/GristData';
 import isPlainObject = require('lodash/isPlainObject');
 
 // The text to show on cells whose values are pending.
@@ -165,32 +165,32 @@ export function encodeObject(value: unknown): CellValue {
     if (value == null) {
       return null;
     } else if (value instanceof Reference) {
-      return ['R', value.tableId, value.rowId];
+      return [GristObjCode.Reference, value.tableId, value.rowId];
     } else if (value instanceof ReferenceList) {
-      return ['r', value.tableId, value.rowIds];
+      return [GristObjCode.ReferenceList, value.tableId, value.rowIds];
     } else if (value instanceof Date) {
       const timestamp = value.valueOf() / 1000;
       if ('timezone' in value) {
-        return ['D', timestamp, (value as GristDateTime).timezone];
+        return [GristObjCode.DateTime, timestamp, (value as GristDateTime).timezone];
       } else {
         // TODO Depending on how it's used, may want to return ['d', timestamp] for UTC midnight.
-        return ['D', timestamp, 'UTC'];
+        return [GristObjCode.DateTime, timestamp, 'UTC'];
       }
     } else if (value instanceof CensoredValue) {
-      return ['C'];
+      return [GristObjCode.Censored];
     } else if (value instanceof RaisedException) {
-      return ['E', value.name, value.message, value.details];
+      return [GristObjCode.Exception, value.name, value.message, value.details];
     } else if (Array.isArray(value)) {
-      return ['L', ...value.map(encodeObject)];
+      return [GristObjCode.List, ...value.map(encodeObject)];
     } else if (isPlainObject(value)) {
-      return ['O', mapValues(value as any, encodeObject, {sort: true})];
+      return [GristObjCode.Dict, mapValues(value as any, encodeObject, {sort: true})];
     }
   } catch (e) {
     // Fall through to return a best-effort representation.
   }
   // We either don't know how to convert the value, or failed during the conversion. Instead we
   // return an "UnmarshallableValue" object, with repr() of the value to show to the user.
-  return ['U', UnknownValue.safeRepr(value)];
+  return [GristObjCode.Unmarshallable, UnknownValue.safeRepr(value)];
 }