Allow to decode t as opt t

This is a strawman PR to implement coercion of
`t` to `opt t` when decoding values.

packages/candid/src/idl.test.ts

@@ -180,7 +180,9 @@ test('IDL encoding (arraybuffer)', () => {
   IDL.encode([IDL.Vec(IDL.Nat8)], [new Uint8Array()]);
   IDL.encode([IDL.Vec(IDL.Nat8)], [new Uint8Array(100).fill(42)]);
   IDL.encode([IDL.Vec(IDL.Nat16)], [new Uint16Array(200).fill(42)]);
-  expect(() => IDL.encode([IDL.Vec(IDL.Int8)], [new Uint16Array(10).fill(420)])).toThrow(/Invalid vec int8 argument/);
+  expect(() => IDL.encode([IDL.Vec(IDL.Int8)], [new Uint16Array(10).fill(420)])).toThrow(
+    /Invalid vec int8 argument/,
+  );
 });
 
 test('IDL encoding (array)', () => {
@@ -677,3 +679,16 @@ test('should decode matching optional fields if wire type contains additional fi
     b: ['123'],
   });
 });
+
+test('should coerce value to opt value', () => {
+  const encoded = IDL.encode([IDL.Text], ['hello']);
+  const value = IDL.decode([IDL.Opt(IDL.Text)], encoded)[0] as [string];
+  expect(value).toEqual(['hello']);
+});
+
+test('should not coerce nested opt opt', () => {
+  const encoded = IDL.encode([IDL.Text], ['hello']);
+  expect(() => IDL.decode([IDL.Opt(IDL.Opt(IDL.Text))], encoded)).toThrow(
+    'type mismatch: type on the wire text, expect type opt opt text',
+  );
+});

packages/candid/src/idl.ts

@@ -914,10 +914,25 @@ export class OptClass<T> extends ConstructType<[T] | []> {
     typeTable.add(this, concat(opCode, buffer));
   }
 
+  public checkType(t: Type): Type {
+    try {
+      return super.checkType(t);
+    } catch (e) {
+      // try to coerce t to opt t
+      if (!(t instanceof OptClass) && !(this._type instanceof OptClass)) {
+        if (this._type.checkType(t)) {
+          return t;
+        }
+      }
+      // rethrow if opt coercion is not applicable
+      throw e;
+    }
+  }
+
   public decodeValue(b: Pipe, t: Type): [T] | [] {
     const opt = this.checkType(t);
     if (!(opt instanceof OptClass)) {
-      throw new Error('Not an option type');
+      return [this._type.decodeValue(b, opt)];
     }
     switch (safeReadUint8(b)) {
       case 0: