How to pass optional parameter in Motoko?

Is there any way to do this?
I am having a type

public type DishType = { 
        name : Text;
        dishTypeId : Nat;
        parentDishTypeId : Nat;
        description: Text;

and my data is

record {3; record {parentDishTypeId=0; name="Main Course"; description=""; dishTypeId=3}};

Now if I want to update the name = "Main Course" field, what is the best way? How can i send only name to the backend? how can i make parameters optional? and how can i spread the name field to replace inside my api?

This is API my code

public func updateDishtype (dishType : Types.DishType) : async Result.Result<Types.DishType, Types.Error> {
        let existingDishType = dishTypeRepository.getDishType(dishType.dishTypeId);
        switch(existingDishType) {
            case null {

            case (? v) {
                dishTypeRepository.upsertDishtype(dishType.dishTypeId, dishType);


This is the DishTypeRepository code

public class DishTypeRepository() {
    let dishTypesHashMap = HashMap.HashMap<Types.DishTypeId, Types.DishType>(0, isDishTypeIdEq, Hash.hash);

    public func upsertDishtype (dishTypeId : Types.DishTypeId, dishType : Types.DishType) {
      dishTypesHashMap.put(dishTypeId, dishType);

    public func getDishType(dishTypeId : Types.DishTypeId): ?Types.DishType {
      return dishTypesHashMap.get(dishTypeId);

Since you’re using an opt returning collection, you could first make it real by using a predefined “invalid” record type with Option.get. Then do similarly for reach field of the record itself, ie for each optional parameter, use Option.get(newValue, existingValue) and then update the record in your collection with the “upserted” record. For instance,

private func persistUserProfileChanges(
   forUserId: Text,
   updatedEmailAddress: ?Text,
   updatedDisplayName: ?Text,
   updatedBioDescription: ?Text,
       ): Result.Result<Text, Text> {
   let exists = Option.get<UserProfile>.get(
   if (not (exists == INVALID_USER_PROFILE)) {
      let updatedUserProfile: UserProfile = {
         userId = forUserId;
         emailAddress = Option.get<Text>(
         preferredDisplayName = Option.get<Text>(
         bioDescription = Option.get<Text>(
      userId2UserMap.put(forUserId, updatedUserProfile);
      return #ok("Success: updated profile details for user id " # forUserId); 
   } else {
      // assert(true);
      #err("Could not update values of record that doesn't exist in id map for id " # forUserId);
   return #err("Error: unrecoverable intractable error while getting userProfile lose all hope ye who called upon this!");

However I feel like there’s a cleaner way to do this that does not need a predefined invalid type to be declared, but this is what has worked for me. Also an assertion at the beginning verifying the id maps to a given profile is probably “best practice”.

Note if you are using an iterable type, you could also use map, doing something similar to the above as far as the optional parameters “upserts” as a function of Option.get if the id of the record matches.

1 Like

Just for the sake of consistency, here’s the cleaner way (no arbitrary “invalid” declarations, uses opts as I believe they are intended to be implemented) to do this:

// note type UniqueId = Text;
// note idToProfilesMap is HashMap<UniqueId, UserProfile>
// where UserProfile 
//   = { epochCreationTimestamp, epochLastUpdateTimestamp, preferredDisplayName, emailAddress }

public func persistNewUserProfileDetails(
      forUniqueUserId: UniqueId,
      preferredDisplayNameIn: ?Text,
      emailAddressIn: ?Text,
      ): () {
      let exists = idToProfilesMap.get(forUniqueUserId);
      switch exists {
        case (null) { Prelude.unreachable(); };
        case (?exists) {
          idToProfilesMap.put(forUniqueUserId, {
            epochCreationTimestamp = exists.epochCreationTimestamp;
            epochLastUpdateTimestamp =;
            preferredDisplayName = Option.get<Text>(preferredDisplayNameIn, exists.preferredDisplayName);
            emailAddress =  Option.get<Text>(emailAddressIn, exists.emailAddress);
1 Like

Do UserProfile’s candid definition need any changes? I can use the same model class like this?

public type DishType = { 
        name : Text;
        dishTypeId : Nat;
        parentDishTypeId : Nat;
        description: Text;

This looks like the solution for now, I wanted to actually not define every parameters again that’s why i was curious about an operator like spread operator in Javascript, but this seems the best option we got, In my case I’m having a very big candid structure or model class and most of the things I just don’t want to change at all, I just need 2,3 parameters to change that too optional and just need to add rest of the parameters as it is, so here I have to assign it with exists.existingParameter for everything. Thank you for the help mate @ inviscidpixels

1 Like

It’s not the spread operator, but in addition to the above you can use destructuring for your function’s arguments so that the function call looks like:

public func someInboundCall({
  arg1: ?Text;
  arg2: ?Principal;
  arg3: ?Nat;
  arg4: ?CustomType
}) : async () { ... }

And then you don’t have to worry about the ordering or include each parameter when you make the call, (can even safely use the spread/rest operator from Javascript without having to enumerate each field as an argument), as null values will be handled (by the code of the language in the previous posts) fine.

Spread operator would be nice! One of these days… how long did take for Javscript to mature? Many of the ECMA standards are still fairly recent!

1 Like