# How to Instantiate template with optional parameters

**URL:** <https://forum.canton.network/t/how-to-instantiate-template-with-optional-parameters/309>\
**Category:** App Development\
**Created:** [April 30, 2020, 5:53pm UTC](https://forum.canton.network/t/how-to-instantiate-template-with-optional-parameters/309 "2020-04-30T17:53:36Z")\
**Posts on this page:** 14\
**Page:** 1

<div class="post-metadata">

**Author:** ![Arvind\_Rao](https://yyz2.discourse-cdn.com/flex034/user_avatar/forum.canton.network/arvind_rao/32/176_2.png) [@Arvind\_Rao](https://forum.canton.network/u/Arvind_Rao)\
**Post date:** [April 30, 2020, 5:53pm UTC](https://forum.canton.network/t/how-to-instantiate-template-with-optional-parameters/309/1 "2020-04-30T17:53:37Z")

</div>

Hi, how can I instantiate a template & pass in the optional parameters?

eg in following ShipmentReceived template has customsInfo and fdaInfo as optional, but I am getting an error while calling the create ShipmentReceived as I do not have them to pass as arguments (Will set those later in workflow etc)

```haskell
  controller order.purchaseOrderTerms.buyerAgent can 
      ReceiveShipment : ContractId ShipmentReceived 
        with 
          receiptText : Text 
          dateReceived : Date 
        do 
          create ShipmentReceived with receiptText, dateReceived, shippingInfo, order

-- ShipmentReceived
template ShipmentReceived
  with 
    order : Order
    shippingInfo : ShippingInfo
    receiptText : Text 
    dateReceived : Date 
    customsInfo : Optional CustomsInfo
    fdaInfo : Optional FDAInfo
 where 
    signatory order.purchaseOrderTerms.buyerAgent 
    controller order.purchaseOrderTerms.buyerAgent can 
      AddCustomsInfo : ContractId ShipmentReceived 
        with 
          customsInfo1 : CustomsInfo
        do 
          create this with customsInfo = customsInfo1
      AddFDAInfo : ContractId ShipmentReceived

```

---

<div class="post-metadata">

**Author:** ![Arvind\_Rao](https://yyz2.discourse-cdn.com/flex034/user_avatar/forum.canton.network/arvind_rao/32/176_2.png) [@Arvind\_Rao](https://forum.canton.network/u/Arvind_Rao)\
**Post date:** [April 30, 2020, 7:21pm UTC](https://forum.canton.network/t/how-to-instantiate-template-with-optional-parameters/309/2 "2020-04-30T19:21:09Z")

</div>

Nevermind, Found solution is to use None

```haskell
create ShipmentReceived with receiptText, dateReceived, shippingInfo, order, customsInfo = None, fdaInfo = None

```

---

<div class="post-metadata">

**Author:** ![Arvind\_Rao](https://yyz2.discourse-cdn.com/flex034/user_avatar/forum.canton.network/arvind_rao/32/176_2.png) [@Arvind\_Rao](https://forum.canton.network/u/Arvind_Rao)\
**Post date:** [May 7, 2020, 2:59pm UTC](https://forum.canton.network/t/how-to-instantiate-template-with-optional-parameters/309/3 "2020-05-07T14:59:41Z")

</div>

Followup on this. Doesnt not look like the code is working. (ie. Optional field is NOT getting set)

Eg in the Scenario/init-script if I do

```haskell
  let 
    customsInfo1 = CustomsInfo with 
      reference = "CustomsInfoRef"
      date = DA.Date.date 2021 Jan 01
      name = "CustomsInfo"

  shipmentReceived1 <- submit buyerAgent1 do exerciseCmd shipmentReceived1 AddCustomsInfo with customsInfo1=customsInfo1

```

`shipmentReceived1` still shows customsInfo (None)

thoughts?

---

<div class="post-metadata">

**Author:** ![cocreature](https://yyz2.discourse-cdn.com/flex034/user_avatar/forum.canton.network/cocreature/32/36_2.png) [@cocreature](https://forum.canton.network/u/cocreature)\
**Post date:** [May 7, 2020, 7:13pm UTC](https://forum.canton.network/t/how-to-instantiate-template-with-optional-parameters/309/4 "2020-05-07T19:13:54Z")

</div>

Can you show the full code sample you are using now?

---

<div class="post-metadata">

**Author:** ![Arvind\_Rao](https://yyz2.discourse-cdn.com/flex034/user_avatar/forum.canton.network/arvind_rao/32/176_2.png) [@Arvind\_Rao](https://forum.canton.network/u/Arvind_Rao)\
**Post date:** [May 8, 2020, 2:38pm UTC](https://forum.canton.network/t/how-to-instantiate-template-with-optional-parameters/309/5 "2020-05-08T14:38:14Z")

</div>

Hope this helps…

```
----------------------------------------------------------
-- Shipment
----------------------------------------------------------
template ShipmentInitiated
  with 
    order : Order
    shippingInfo : ShippingInfo
  where 
    signatory order.quote.quoteInfo.seller 
    controller order.purchaseOrderTerms.buyerAgent can 
      ReceiveShipment : ContractId ShipmentReceived 
        with 
          receiptText : Text 
          dateReceived : Date 
        do 
          create ShipmentReceived with receiptText, dateReceived, shippingInfo, order, customsInfo = None, fdaInfo = None

----------------------------------------------------------
-- ShipmentReceived
----------------------------------------------------------
template ShipmentReceived
  with 
    order : Order
    shippingInfo : ShippingInfo
    receiptText : Text 
    dateReceived : Date 
    customsInfo : Optional CustomsInfo
    fdaInfo : Optional FDAInfo
 where 
    signatory order.purchaseOrderTerms.buyerAgent 
    controller order.purchaseOrderTerms.buyerAgent can 

      AddCustomsInfo : ContractId ShipmentReceived 
        with 
          customsInfoNew : CustomsInfo
        do 
          let
            this.customsInfo = customsInfoNew
          create ShipmentReceived with ..

      AddFDAInfo : ContractId ShipmentReceived 
        with 
          fdaInfoNew :FDAInfo
        do 
          let
            this.fdaInfo = fdaInfoNew
          create ShipmentReceived with ..

      ClearShipment : ContractId ShipmentCleared 
        with 
          reference : Text 
          dateCleared : Date 
        do 
          create ShipmentCleared with shipmentReceived=this, dateCleared, buyer = order.quote.quoteInfo.buyer, buyerAgent = order.purchaseOrderTerms.buyerAgent  

```

setup : Script ()

```
  buyerAgent1 <- allocatePartyWithHint "BuyerAgent1" $ PartyIdHint with partyIdHint = "BuyerAgent1"
  buyerAgentInvite1 <-submit buyer1 do exerciseCmd buyerRole1 InviteBuyerAgent with buyerAgent = buyerAgent1
  buyerAgentRole1 <- submit buyerAgent1 do exerciseCmd buyerAgentInvite1 AcceptBuyerAgentInvitation
  
let 
    purchaseOrderTerms1 = PurchaseOrderTerms with
      buyerAgent = buyerAgent1

-- Buyer accepts Quote from Seller 2
  purchaseOrder1 <- submit buyer1 do exerciseCmd quote2 AcceptQuote with purchaseOrderTerms = purchaseOrderTerms1

--Seller accepts PO and Order is created
  order1 <- submit seller2 do exerciseCmd purchaseOrder1 AcceptPurchaseOrder with acceptText = "Accepted"

-- Seller can InitiateShipment
  let
    shippingInfo1 = ShippingInfo with
      shippingDate = DA.Date.date 2021 Jan 01
      trackingNumber = "Text"  
  
  shipmentInitiated1 <- submit seller2 do exerciseCmd order1 InitiateShipment with shippingInfo = shippingInfo1
  
-- BuyersAgent can ReceiveShipment
  shipmentReceived1 <- submit buyerAgent1 do exerciseCmd shipmentInitiated1 ReceiveShipment with receiptText = "Received", dateReceived = DA.Date.date 2021 Jan 01

-- Add CustomsInfo
  let 
    customsInfo1 = CustomsInfo with 
      reference = "CustomsInfoRef"
      date = DA.Date.date 2021 Jan 01
      name = "CustomsInfo"

  shipmentReceived1 <- submit buyerAgent1 do exerciseCmd shipmentReceived1 AddCustomsInfo with customsInfoNew=customsInfo1

-- Add FDA Info
  let 
    fdaInfo1 = FDAInfo with 
      reference = "FDAInfoRef"
      date = DA.Date.date 2021 Jan 01
      name = "FDAInfo"

  shipmentReceived1 <- submit buyerAgent1 do exerciseCmd shipmentReceived1 AddFDAInfo with fdaInfoNew=fdaInfo1
```

---

<div class="post-metadata">

**Author:** ![cocreature](https://yyz2.discourse-cdn.com/flex034/user_avatar/forum.canton.network/cocreature/32/36_2.png) [@cocreature](https://forum.canton.network/u/cocreature)\
**Post date:** [May 8, 2020, 3:11pm UTC](https://forum.canton.network/t/how-to-instantiate-template-with-optional-parameters/309/6 "2020-05-08T15:11:40Z")

</div>

Let’s start with a very simplified version of your example to illustrate the problem:

```haskell
module Main where

import Daml.Script

data CustomsInfo = CustomsInfo
  deriving (Eq, Show)

template ShipmentReceived
  with
    p : Party
    customsInfo : Optional CustomsInfo
 where
    signatory p
    controller p can

      AddCustomsInfo : ContractId ShipmentReceived
        with
          customsInfoNew : CustomsInfo
        do
          let
            this.customsInfo = customsInfoNew
          create ShipmentReceived with ..

setup : Script ()
setup = do
  p <- allocatePartyWithHint "P" $ PartyIdHint with partyIdHint = "P"
  cid <- submit p (createCmd ShipmentReceived with p = p, customsInfo = None)
  shipments <- query @ShipmentReceived p
  debug shipments
  cid <- submit p (exerciseCmd cid (AddCustomsInfo CustomsInfo))
  shipments <- query @ShipmentReceived p
  debug shipments
  pure ()

```

As you said, this doesn’t work. We get the following output when running `daml script`:

```haskell
[DA.Internal.Prelude:540]: [(<contract-id>,ShipmentReceived {p = 'P', customsInfo = None})]
[DA.Internal.Prelude:540]: [(<contract-id>,ShipmentReceived {p = 'P', customsInfo = None})]

```

Now what is going wrong here?  
The crucial point is the following piece of code

```haskell
          let
            this.customsInfo = customsInfoNew
          create ShipmentReceived with ..

```

If you remove a bit of syntactic sugar this is equivalent to

```haskell
          let
            this.customsInfo = customsInfoNew
          create ShipmentReceived with p = p, customsInfo = customsInfo

```

Now the crucial questions is what does the `customsInfo` at the very end refer to.  
To understand that we have to understand what the following line does.

```haskell
let this.customsInfo = customsInfoNew

```

Remember that DAML is an immutable language. You cannot simply mutate a field in a template. This line isn’t mutating the `customsInfo` field so that `customsInfo` changes its meaning. `customsInfo` at the end still refers to the old value which is why you see the unchanged value.

To fix your code you have to actually change the value, e.g.,

```haskell
createCmd ShipmentReceived with p = p, customsInfo = Some customsInfoNew

```

or if you want to use a `let` and `..` you can use the following

```haskell
-- Note that this does not mutate customsInfo. It defines a new variable of this name that hides the one from your template.
let customsInfo = Some customsInfoNew
createCmd ShipmentReceived with ..

```

If you now rebuild and rerun your script you will see the expected output:

```haskell
[DA.Internal.Prelude:540]: [(<contract-id>,ShipmentReceived {p = 'P', customsInfo = None})]
[DA.Internal.Prelude:540]: [(<contract-id>,ShipmentReceived {p = 'P', customsInfo = Some CustomsInfo})]

```

Now there is one remaining piece of the puzzle. What does `let this.customsInfo = customsInfoNew` actually do? This is rather confusing: It defines an infix operator called `.` which accepts two arguments `this` and `customsInfo` and will return `customsInfoNew`. So you could call `"hello" . "world"` afterwards and it will give you back `customsInfoNew`.

---

<div class="post-metadata">

**Author:** ![Arvind\_Rao](https://yyz2.discourse-cdn.com/flex034/user_avatar/forum.canton.network/arvind_rao/32/176_2.png) [@Arvind\_Rao](https://forum.canton.network/u/Arvind_Rao)\
**Post date:** [May 8, 2020, 3:58pm UTC](https://forum.canton.network/t/how-to-instantiate-template-with-optional-parameters/309/7 "2020-05-08T15:58:56Z")

</div>

Thank you for the detailed explanation. (Code works as expected now 🙂 )

---

<div class="post-metadata">

**Author:** ![bartcant](https://yyz2.discourse-cdn.com/flex034/user_avatar/forum.canton.network/bartcant/32/823_2.png) [@bartcant](https://forum.canton.network/u/bartcant)\
**Post date:** [November 18, 2020, 11:16pm UTC](https://forum.canton.network/t/how-to-instantiate-template-with-optional-parameters/309/8 "2020-11-18T23:16:58Z")

</div>

> [@cocreature](#):
>
> `createCmd ShipmentReceived with p = p, customsInfo = Some customsInfoNew`

I have a variation of this problem

```haskell
template SurgicalEvent
  with
   operator : Party  
   hospital : Party
   emr: Text
   patientname: Text   
   patientdob : Date
   eventdate : Date
   eventdescription : Text
   surgeonname : Text
   surgicaleventproductdata : Optional [SurgicalEventProductdata]

  where
   signatory operator
   observer hospital 

   controller operator can
     nonconsuming AddSurgicalEventData : ContractId SurgicalEvent
      with
       newproductdata : SurgicalEventProductdata

      do
        archive self
        create this with surgicaleventproductdata = newproductdata :: surgicaleventproductdata

```

I tried to introduce “Some”

```haskell
 newproductdata : Some SurgicalEventProductdata

```

But errors occur

---

<div class="post-metadata">

**Author:** ![cocreature](https://yyz2.discourse-cdn.com/flex034/user_avatar/forum.canton.network/cocreature/32/36_2.png) [@cocreature](https://forum.canton.network/u/cocreature)\
**Post date:** [November 19, 2020, 7:38am UTC](https://forum.canton.network/t/how-to-instantiate-template-with-optional-parameters/309/9 "2020-11-19T07:38:32Z")

</div>

`Some` is not a type constructor. The type you are looking for is `Optional SurgicalEventProductData` which has the values `None` and `Some productData` for `productData : SurgicalEventProductData`.

---

<div class="post-metadata">

**Author:** ![bartcant](https://yyz2.discourse-cdn.com/flex034/user_avatar/forum.canton.network/bartcant/32/823_2.png) [@bartcant](https://forum.canton.network/u/bartcant)\
**Post date:** [November 19, 2020, 2:23pm UTC](https://forum.canton.network/t/how-to-instantiate-template-with-optional-parameters/309/10 "2020-11-19T14:23:46Z")

</div>

If I have this

```haskell
template SurgicalEvent
  with
   operator : Party  
   hospital : Party
   emr: Text
   patientname: Text   
   patientdob : Date
   eventdate : Date
   eventdescription : Text
   surgeonname : Text
   surgicaleventproductdata : Optional [SurgicalEventProductdata]
  where
   signatory operator
   observer hospital 
   controller operator can
     nonconsuming AddSurgicalEventData : ContractId SurgicalEvent
      with
       newproductdata: Optional SurgicalEventProductdata
      do
        archive self
        create this with surgicaleventproductdata = newproductdata :: surgicaleventproductdata

```

get this error code in last line

```haskell
C:\Users\bartc\Documents\Github\Loci\daml\SurgicalEvent.daml:30:9: error:
    • Couldn't match type ‘Optional [SurgicalEventProductdata]’
                     with ‘[Optional SurgicalEventProductdata]’
        arising from a functional dependency between:
          constraint ‘DA.Internal.Record.HasField
                        "surgicaleventproductdata"
                        SurgicalEvent
                        [Optional SurgicalEventProductdata]’
            arising from a use of ‘DA.Internal.Record.setField’
          instance ‘DA.Internal.Record.HasField
                      "surgicaleventproductdata"
                      SurgicalEvent
                      (Optional [SurgicalEventProductdata])’
            at <no location info>
    • In the first argument of ‘create’, namely
        ‘(DA.Internal.Record.setField
            @"surgicaleventproductdata"
            newproductdata :: surgicaleventproductdata
            this)’
      In a stmt of a 'do' block:
        create
          (DA.Internal.Record.setField
             @"surgicaleventproductdata"
             newproductdata :: surgicaleventproductdata
             this)
      In the expression:
        do archive self
           create
             (DA.Internal.Record.setField
                @"surgicaleventproductdata"
                newproductdata :: surgicaleventproductdata
                this)typecheck

```

---

<div class="post-metadata">

**Author:** ![cocreature](https://yyz2.discourse-cdn.com/flex034/user_avatar/forum.canton.network/cocreature/32/36_2.png) [@cocreature](https://forum.canton.network/u/cocreature)\
**Post date:** [November 19, 2020, 3:22pm UTC](https://forum.canton.network/t/how-to-instantiate-template-with-optional-parameters/309/11 "2020-11-19T15:22:53Z")

</div>

The issue is that you have an `Optional [a]` and you’re trying to add a `Optional a` to the front which doesn’t typecheck since `Optional [a]` is not a list. There are various options depending on your requirements. One solution would be to just change the type to `[Optional a]` which should make your example typecheck. Another would be to change it to `[a]` and do nothing if a `None` gets passed into `AddSurgicalEventData`. If you don’t want to change the types you have to define what you want to happen in the case where either the new or the old value is `None`.

Here is a solution for the second option:

```haskell
template SurgicalEvent
  with
   operator : Party
   hospital : Party
   surgicaleventproductdata : [SurgicalEventProductdata]
  where
   signatory operator
   observer hospital
   controller operator can
     nonconsuming AddSurgicalEventData : ContractId SurgicalEvent
      with
       optnewproductdata: Optional SurgicalEventProductdata
      do
        archive self
        case optnewproductdata of
          None -> pure self
          Some productdata -> create this with surgicaleventproductdata = productdata :: surgicaleventproductdata

```

Side note: Please use code blocks surrounded by triple backticks ``` instead of quote blocks where each line starts with `>`. The latter strips all indentation which makes code examples very hard to read.

Such as:

````
```
my code here
```
````

---

<div class="post-metadata">

**Author:** ![bartcant](https://yyz2.discourse-cdn.com/flex034/user_avatar/forum.canton.network/bartcant/32/823_2.png) [@bartcant](https://forum.canton.network/u/bartcant)\
**Post date:** [November 19, 2020, 5:03pm UTC](https://forum.canton.network/t/how-to-instantiate-template-with-optional-parameters/309/12 "2020-11-19T17:03:52Z")

</div>

> [@cocreature](#):
>
> `productdata :: surgicaleventproductdata`

Here is some updated template code where I keep the 2 data elements the same type

```haskell
template SurgicalEvent
  with
   operator : Party  
   hospital : Party
   emr: Text
   patientname: Text   
   patientdob : Date
   eventdate : Date
   eventdescription : Text
   surgeonname : Text
   surgicaleventproductdata : Optional [SurgicalEventProductdata]

  where
   signatory operator
   observer hospital 

   controller operator can
     nonconsuming AddSurgicalEventData : ContractId SurgicalEvent
      with
       newproductdata : Optional [SurgicalEventProductdata]

      do
        archive self
        case surgicaleventproductdata of
          None -> create this with surgicaleventproductdata = newproductdata
          Some surgicaleventproductdata -> 
           case newproductdata of
              None -> create this
              Some newproductdata -> create this with surgicaleventproductdata = newproductdata :: surgicaleventproductdata
          

```

This results in the following error on the last line…

```haskell
C:\Users\bartc\Documents\Github\Loci\daml\SurgicalEvent.daml:39:39: error:
    • Couldn't match type ‘Optional [SurgicalEventProductdata]’
                     with ‘[[SurgicalEventProductdata]]’
        arising from a functional dependency between:
          constraint ‘DA.Internal.Record.HasField
                        "surgicaleventproductdata"
                        SurgicalEvent
                        [[SurgicalEventProductdata]]’
            arising from a use of ‘DA.Internal.Record.setField’
          instance ‘DA.Internal.Record.HasField
                      "surgicaleventproductdata"
                      SurgicalEvent
                      (Optional [SurgicalEventProductdata])’
            at <no location info>
    • In the first argument of ‘create’, namely
        ‘(DA.Internal.Record.setField
            @"surgicaleventproductdata"
            newproductdata :: surgicaleventproductdata
            this)’
      In the expression:
        create
          (DA.Internal.Record.setField
             @"surgicaleventproductdata"
             newproductdata :: surgicaleventproductdata
             this)
      In a case alternative:
          Some newproductdata
            -> create
                 (DA.Internal.Record.setField
                    @"surgicaleventproductdata"
                    newproductdata :: surgicaleventproductdata
                    this)typecheck
Peek Problem (Alt+F8)
No quick fixes available

```

---

<div class="post-metadata">

**Author:** ![cocreature](https://yyz2.discourse-cdn.com/flex034/user_avatar/forum.canton.network/cocreature/32/36_2.png) [@cocreature](https://forum.canton.network/u/cocreature)\
**Post date:** [November 19, 2020, 5:27pm UTC](https://forum.canton.network/t/how-to-instantiate-template-with-optional-parameters/309/13 "2020-11-19T17:27:44Z")

</div>

`::` prepends a single element to a list. If you want to concatenate 2 lists, you need to use `++`. However, in your example, you don’t have 2 lists. You have 2 `Optional [a]`. That gives you 4 cases to considner:

| surgicaleventproductdata | newproductdata | result |
| --- | --- | --- |
| Some(xs) | Some(ys) | Some(xs ++ ys) |
| None | Some(ys) | What do you want here? |
| Some(xs) | None | What do you want here? |
| None | None | What do you want here? |

A common solution is to choose `None` as the answer for the last 3. You can get that as follows:

```haskell
liftA2 (++) surgicaleventproductdata newproductdata

```

However, looking at your example I expect you might have something else in mind?

It is also useful to check if there is really a difference between `None` and `Some []`. If not, there is no need to use `Optional` at all here.

---

<div class="post-metadata">

**Author:** ![Gary\_Verhaegen](https://yyz2.discourse-cdn.com/flex034/user_avatar/forum.canton.network/gary_verhaegen/32/81_2.png) [@Gary\_Verhaegen](https://forum.canton.network/u/Gary_Verhaegen)\
**Post date:** [November 20, 2020, 1:54pm UTC](https://forum.canton.network/t/how-to-instantiate-template-with-optional-parameters/309/14 "2020-11-20T13:54:21Z")

</div>

Elaborating on that last sentence, i.e. not using Optional at all and representing “no surgical data” as an empty list, you would write it as:

```haskell
module Main where

data SurgicalEventProductData = SurgicalEventProductData
  deriving (Eq, Show)

template SurgicalEvent
  with
   operator : Party  
   hospital : Party
   emr: Text
   patientname: Text   
   patientdob : Date
   eventdate : Date
   eventdescription : Text
   surgeonname : Text
   surgicaleventproductdata : [SurgicalEventProductData]
  where
   signatory operator
   observer hospital 
   controller operator can
     nonconsuming AddSurgicalEventData : ContractId SurgicalEvent
      with
       newproductdata: SurgicalEventProductData
      do
        archive self
        create this with surgicaleventproductdata = newproductdata :: surgicaleventproductdata

```

This typechecks, and could be what you want depending on the broader context.