Applying Filter to Contract Elements from a List of Contracts

Questions
1

Hi All,

I am working on a simple application as I learn DAML and ran into a roadblock. I have a list of contracts I’ve created and I want to be able to filter on elements embedded within the contracts.

I think the code below is failing due to the filter function looking at a list of TestData while the anon function is attempting to match on the 4 elements within TestData which I am unsure how to access. Is there a solution to have my filter access each TestData and pull the 4 fields out to allow me to filter based on the budget? In other words, I have a list of contracts each containing a budget element that I want to filter out if the budget is over a given value. I am unsure how to access the underlying elements as the filter traverses the list of contracts.

Appreciate any help on this!

template TestData
with
me: Party
rev: Int
exp: Int
budget: Int
where
signatory me

template TestTemp
with
me: Party
testList:[TestData]
where
signatory me
choice FilterList : ContractId TestTemp
controller me
do
let newList = filter ((,,_,a) → (a > 100)) testList <— fails here
create this with testList = newList

Error message:
c:/Users/barment/Documents/Blockchain/DAML/daml-test/daml/Test.daml:51:64: error:
• Couldn’t match type ‘TestData’ with ‘(a0, b0, c0, Int)’
Expected type: [(a0, b0, c0, Int)]
Actual type: [TestData]
• In the second argument of ‘filter’, namely ‘testList’
In the expression: filter (\ (_, _, , a) → (a > 100)) testList
In an equation for ‘newList’:
newList = filter (\ (, _, _, a) → (a > 100)) testList
• Relevant bindings include
newList : [(a0, b0, c0, Int)]
(bound at c:/Users/barment/Documents/Blockchain/DAML/daml-test/daml/Test.daml:51:21)

2

Hi @barment and welcome to the forum.

You can access the elements of a record type with the . operator.

You can filter for budget as follows:

template TestTemp
  with
    me: Party
    testList:[TestData]
  where
    signatory me

    choice FilterList : ContractId TestTemp
      controller me
      do
        let newList = filter (\e -> e.budget > 100) testList
        create this with testList = newList

Matteo

3

Thank you Matteo! I had tried the dot operator on the left side of the arrow with no luck but this works as intended. Appreciate the quick response.