Couldn't match expected type ‘Optional Decimal’ with actual type ‘Decimal’

Questions
1

Hey, DAML’ers! I am adjusting the Contract.daml for a scenario using the ex-cdm-swaps model, and need to specify the initialRate of the constructor FloatingRateSpecification.

initialValue is type ‘Decimal’. It accepts a decimal value without throwing an error. However, I get an error when I write a decimal for initialRate which is of type ‘Optional Decimal’.

What is the difference? How can I specify the decimal value without throwing an error? I noticed in the Utils.daml file here that there are helper functions to deal with types that require meta. Could I add something to that file?

Thanks for the help!

C:\Users\James Jaworski\OneDrive\Optimis\betaApp\third-test\Bloqbook-demo\daml\DA\swaps\Test\ExampleData\Contract.daml:285:59: error:
• Couldn't match expected type ‘Optional Decimal’
              with actual type ‘Decimal’
• In the ‘initialRate’ field of a record
  In the first argument of ‘Some’, namely
    ‘(FloatingRateSpecification
        {averagingMethod = None, capRateSchedule = [],
         finalRateRounding = None,
         floatingRateIndex = fieldWithEmptyMeta
                               FloatingRateIndexEnum_USD_LIBOR_ISDA,
         floatingRateMultiplierSchedule = None, floorRateSchedule = [],
         id = None, indexTenor = None, negativeInterestRateTreatment = None,
         rateTreatment = None,
         spreadSchedule = [SpreadSchedule
                             {id = None, initialValue = 0.005000, step = [], _type = None}],
         initialRate = 0.032753})’
  In the ‘floatingRate’ field of a record

                                rateSpecification =
                                RateSpecification with
                                    inflationRate = None
                                    floatingRate = Some
                                      (FloatingRateSpecification with
                                        averagingMethod = None
                                        capRateSchedule = []
                                        finalRateRounding = None
                                        floatingRateIndex = fieldWithEmptyMeta FloatingRateIndexEnum_USD_LIBOR_ISDA
                                        floatingRateMultiplierSchedule = None
                                        floorRateSchedule = []
                                        id = None
                                        indexTenor = None
                                        negativeInterestRateTreatment = None
                                        rateTreatment = None                                            
                                        spreadSchedule =  
                                          [SpreadSchedule with
                                            id = None
                                            initialValue = 0.005000
                                            step = []
                                            _type = None]
                                        initialRate = 0.032753
                                      )
                                    fixedRate = None
2

Optional Decimal is like a “wrapper” around Decimal that can have either Some value or None, where value would be of type Decimal. This allows you to clearly distinguish if a value is present or not.

In other languages this is often done using null, but that’s less explicit and brings its own set of problem (like NullPointerExceptions etc.).

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3

Okay cool. So I would specify an Optional Decimal like this initial value = Some 0.023756. Thanks for the clarification!

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4

Worth mentioning there are some helper functions defined in the DA.Optional module (docs here). You can use those via:

import DA.Optional
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5

Okay, neat. Thanks @georg!

1 Like
6

If you need to write DAML code which consumes an Optional value, the most general way of doing this is by pattern matching, introduced using the case keyword. For example:

myFunction : Optional Decimal -> Text
myFunction opt =
  case opt of
    None -> "I got nothing"
    Some value -> "I got a value: " <> show value

Even more succinctly, a pattern matching function can be written like this:

myFunction None = "I got nothing"
myFunction (Some value) = "I got a value: " <> show value
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