Exports the record machinery necessary to allow one to annotate code that is polymorphic in the underlying record type.


class HasField x r a where

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

For more on Record syntax, see https://docs.daml.com/daml/intro/3_Data.html#record.

HasField x r a is a typeclass that takes 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 HasField instances:

HasField "foo" MyRecord Int
HasField "bar" MyRecord Text

If we want to get a value using HasField, we can use the getField function:

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 function:

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

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