Typeclasses

class GetField x r a where

GetField x r a provides the getter part of HasField

getField

: r -> a

class SetField x r a where

SetField x r a provides the setter part of HasField

setField

: a -> r -> r

Data Types

= (GetField x r a, SetField x r a)

HasField is a class synonym for GetField and SetField, which respectively give you getter and setter functions for each record field automatically.

In the vast majority of use-cases, plain Record syntax should be preferred:

daml> let a = MyRecord 1 "hello"
daml> a.foo
1
daml> a.bar
"hello"
daml> a { bar = "bye" }
MyRecord {foo = 1, bar = "bye"}
daml> a with foo = 3
MyRecord {foo = 3, bar = "hello"}
daml>

For more on Record syntax, see https://docs.digitalasset.com/build/3.4/reference/daml/stdlib/DA-Record.html.

GetField x r a and SetField x r a are typeclasses taking three parameters. The first parameter x is the field name, the second parameter r is the record type, and the last parameter a is the type of the field in this record. For example, if we define a type:

data MyRecord = MyRecord with
    foo : Int
    bar : Text

Then we get, for free, the following GetField and SetField instances:

GetField "foo" MyRecord Int
SetField "foo" MyRecord Int
GetField "bar" MyRecord Text
SetField "bar" MyRecord Text

If we want to get a value, we can use the getField method of class GetField:

getFoo : MyRecord -> Int
getFoo r = getField @"foo" r

getBar : MyRecord -> Text
getBar r = getField @"bar" r

Note that this uses the “type application” syntax ( f @t ) to specify the field name.

Likewise, if we want to set the value in the field, we can use the setField method of class SetField:

setFoo : Int -> MyRecord -> MyRecord
setFoo a r = setField @"foo" a r

setBar : Text -> MyRecord -> MyRecord
setBar a r = setField @"bar" a r