How DAML types are translated to DAML-LF

This page shows how types in DAML are translated into DAML-LF. It should help you understand and predict the generated client interfaces, which is useful when you’re building a DAML-based application that uses the Ledger API or client bindings in other languages.

For an introduction to DAML-LF, see DAML-LF.

Primitive types

Built-in data types in DAML have straightforward mappings to DAML-LF.

This section only covers the serializable types, as these are what client applications can interact with via the generated DAML-LF. (Serializable types are ones whose values can be written in a text or binary format. So not function types, Update and Scenario types, as well as any types built up from those.)

Most built-in types have the same name in DAML-LF as in DAML. These are the exact mappings:

DAML primitive type DAML-LF primitive type
Int Int64
Time Timestamp
() Unit
[] List
Decimal Decimal
Text Text
Date Date
Party Party
Optional Optional
ContractId ContractId

Be aware that only the DAML primitive types exported by the Prelude module map to the DAML-LF primitive types above. That means that, if you define your own type named Party, it will not translate to the DAML-LF primitive Party.

Tuple types

DAML tuple type constructors take types T1, T2, …, TN to the type (T1, T2, …, TN). These are exposed in the DAML surface language through the Prelude module.

The equivalent DAML-LF type constructors are daml-prim:DA.Types:TupleN, for each particular N (where 2 <= N <= 20). This qualified name refers to the package name (ghc-prim) and the module name (GHC.Tuple).

For example: the DAML pair type (Int, Text) is translated to daml-prim:DA.Types:Tuple2 Int64 Text.

Data types

DAML-LF has three kinds of data declarations:

  • Record types, which define a collection of data
  • Variant or sum types, which define a number of alternatives
  • Enum, which defines simplified sum types without type parameters nor argument.

Data type declarations in DAML (starting with the data keyword) are translated to record, variant or enum types. It’s sometimes not obvious what they will be translated to, so this section lists many examples of data types in DAML and their translations in DAML-LF.

Record declarations

This section uses the syntax for DAML records with curly braces.

DAML declaration DAML-LF translation
data Foo = Foo { foo1: Int; foo2: Text } record Foo ↦ { foo1: Int64; foo2: Text }
data Foo = Bar { bar1: Int; bar2: Text } record Foo ↦ { bar1: Int64; bar2: Text }
data Foo = Foo { foo: Int } record Foo ↦ { foo: Int64 }
data Foo = Bar { foo: Int } record Foo ↦ { foo: Int64 }
data Foo = Foo {} record Foo ↦ {}
data Foo = Bar {} record Foo ↦ {}

Variant declarations

DAML declaration DAML-LF translation
data Foo = Bar Int | Baz Text variant Foo ↦ Bar Int64 | Baz Text
data Foo a = Bar a | Baz Text variant Foo a ↦ Bar a | Baz Text
data Foo = Bar Unit | Baz Text variant Foo ↦ Bar Unit | Baz Text
data Foo = Bar Unit | Baz variant Foo ↦ Bar Unit | Baz Unit
data Foo a = Bar | Baz variant Foo a ↦ Bar Unit | Baz Unit
data Foo = Foo Int variant Foo ↦ Foo Int64
data Foo = Bar Int variant Foo ↦ Bar Int64
data Foo = Foo () variant Foo ↦ Foo Unit
data Foo = Bar () variant Foo ↦ Bar Unit
data Foo = Bar { bar: Int } | Baz Text variant Foo ↦ Bar Foo.Bar | Baz Text, record Foo.Bar ↦ { bar: Int64 }
data Foo = Foo { foo: Int } | Baz Text variant Foo ↦ Foo Foo.Foo | Baz Text, record Foo.Foo ↦ { foo: Int64 }
data Foo = Bar { bar1: Int; bar2: Decimal } | Baz Text variant Foo ↦ Bar Foo.Bar | Baz Text, record Foo.Bar ↦ { bar1: Int64; bar2: Decimal }
data Foo = Bar { bar1: Int; bar2: Decimal } | Baz { baz1: Text; baz2: Date } data Foo ↦ Bar Foo.Bar | Baz Foo.Baz, record Foo.Bar ↦ { bar1: Int64; bar2: Decimal }, record Foo.Baz ↦ { baz1: Text; baz2: Date }

Enum declarations

DAML declaration DAML-LF declaration
data Foo = Bar | Baz enum Foo ↦ Bar | Baz
data Color = Red | Green | Blue enum Color ↦ Red | Green | Blue

Banned declarations

There are two gotchas to be aware of: things you might expect to be able to do in DAML that you can’t because of DAML-LF.

The first: a single constructor data type must be made unambiguous as to whether it is a record or a variant type. Concretely, the data type declaration data Foo = Foo causes a compile-time error, because it is unclear whether it is declaring a record or a variant type.

To fix this, you must make the distinction explicitly. Write data Foo = Foo {} to declare a record type with no fields, or data Foo = Foo () for a variant with a single constructor taking unit argument.

The second gotcha is that a constructor in a data type declaration can have at most one unlabelled argument type. This restriction is so that we can provide a straight-forward encoding of DAML-LF types in a variety of client languages.

Banned declaration Workaround
data Foo = Foo data Foo = Foo {} to produce record Foo ↦ {} OR data Foo = Foo () to produce variant Foo ↦ Foo Unit
data Foo = Bar data Foo = Bar {} to produce record Foo ↦ {} OR data Foo = Bar () to produce variant Foo ↦ Bar Unit
data Foo = Foo Int Text Name constructor arguments using a record declaration, for example data Foo = Foo { x: Int; y: Text }
data Foo = Bar Int Text Name constructor arguments using a record declaration, for example data Foo = Bar { x: Int; y: Text }
data Foo = Bar | Baz Int Text Name arguments to the Baz constructor, for example data Foo = Bar | Baz { x: Int; y: Text }

Type synonyms

Type synonyms (starting with the type keyword) are eliminated during conversion to DAML-LF. The body of the type synonym is inlined for all occurrences of the type synonym name.

For example, consider the following DAML type declarations.

type Username = Text
data User = User { name: Username }

The Username type is eliminated in the DAML-LF translation, as follows:

record User ↦ { name: Text }

Template types

A template declaration in DAML results in one or more data type declarations behind the scenes. These data types, detailed in this section, are not written explicitly in the DAML program but are created by the compiler.

They are translated to DAML-LF using the same rules as for record declarations above.

These declarations are all at the top level of the module in which the template is defined.

Template data types

Every contract template defines a record type for the parameters of the contract. For example, the template declaration:

template Iou
  with
    issuer: Party
    owner: Party
    currency: Text
    amount: Decimal
  where

results in this record declaration:

data Iou = Iou { issuer: Party; owner: Party; currency: Text; amount: Decimal }

This translates to the DAML-LF record declaration:

record Iou ↦ { issuer: Party; owner: Party; currency: Text; amount: Decimal }

Similarly, the generic template definition

template Template a => Proposal a
  with
    proposal: a
    proposers: [Party]
    receivers: [Party]
  where

results in the record declaration:

data Proposal a = Proposal { proposal: a; proposers: [Party]; receivers: [Party] }

which translates to the DAML-LF record declaration:

record Proposal a ↦ { proposal: a; proposers: List Party; receivers: List Party }

Choice data types

Every choice within a contract template results in a record type for the parameters of that choice. For example, let’s suppose the earlier Iou template has the following choices:

    controller owner can
      nonconsuming DoNothing: ()
        do
          return ()

      Transfer: ContractId Iou
        with newOwner: Party
        do
          updateOwner newOwner

This results in these two record types:

data DoNothing = DoNothing {}
data Transfer = Transfer { newOwner: Party }

Whether the choice is consuming or nonconsuming is irrelevant to the data type declaration. The data type is a record even if there are no fields.

These translate to the DAML-LF record declarations:

record DoNothing ↦ {}
record Transfer ↦ { newOwner: Party }

Generic template instantiations

In the context of the templates Iou and Proposal above, the generic template instantiation

template instance IouProposal = Proposal Iou

does not produce a DAML data definition but only the DAML-LF record declaration:

record IouProposal ↦ { proposal: Iou; proposers: List Party; receivers: List Party }

Note that IouProposal is a copy of Proposal with the type variable a replaced by Iou on the right hand side.

In DAML, IouProposal can also be used as a type synonym for Proposal Iou.