# Reference: Daml packages¶

## Building Daml archives¶

When a Daml project is compiled, the compiler produces a Daml archive. These are platform-independent packages of compiled Daml code that can be uploaded to a Daml ledger or imported in other Daml projects.

Daml archives have a .dar file ending. By default, when you run daml build, it will generate the .dar file in the .daml/dist folder in the project root folder. For example, running daml build in project foo with project version 0.0.1 will result in a Daml archive .daml/dist/foo-0.0.1.dar.

You can specify a different path for the Daml archive by using the -o flag:

daml build -o foo.dar


For details on how to upload a Daml archive to the ledger, see the deploy documentation. The rest of this page will focus on how to import a Daml package in other Daml projects.

## Inspecting DARs¶

To inspect a DAR and get information about the packages inside it, you can use the daml damlc inspect-dar command. This is often useful to find the package id of the project you just built.

You can run daml damlc inspect-dar /path/to/your.dar to get a human-readable listing of the files inside it and a list of packages and their package ids. Here is a (shortened) example output:

$daml damlc inspect-dar .daml/dist/create-daml-app-0.1.0.dar DAR archive contains the following files: create-daml-app-0.1.0-29b501bcf541a40e9f75750246874e0a35de72e00616372da435e4b69966db5d/create-daml-app-0.1.0-29b501bcf541a40e9f75750246874e0a35de72e00616372da435e4b69966db5d.dalf create-daml-app-0.1.0-29b501bcf541a40e9f75750246874e0a35de72e00616372da435e4b69966db5d/daml-prim-75b070729b1fbd37a618493652121b0d6f5983b787e35179e52d048db70e9f15.dalf create-daml-app-0.1.0-29b501bcf541a40e9f75750246874e0a35de72e00616372da435e4b69966db5d/daml-stdlib-0.0.0-a535cbc3657b8df953a50aaef5a4cd224574549c83ca4377e8219aadea14f21a.dalf create-daml-app-0.1.0-29b501bcf541a40e9f75750246874e0a35de72e00616372da435e4b69966db5d/daml-stdlib-DA-Internal-Template-d14e08374fc7197d6a0de468c968ae8ba3aadbf9315476fd39071831f5923662.dalf create-daml-app-0.1.0-29b501bcf541a40e9f75750246874e0a35de72e00616372da435e4b69966db5d/data/create-daml-app-0.1.0.conf create-daml-app-0.1.0-29b501bcf541a40e9f75750246874e0a35de72e00616372da435e4b69966db5d/User.daml create-daml-app-0.1.0-29b501bcf541a40e9f75750246874e0a35de72e00616372da435e4b69966db5d/User.hi create-daml-app-0.1.0-29b501bcf541a40e9f75750246874e0a35de72e00616372da435e4b69966db5d/User.hie META-INF/MANIFEST.MF DAR archive contains the following packages: create-daml-app-0.1.0-29b501bcf541a40e9f75750246874e0a35de72e00616372da435e4b69966db5d "29b501bcf541a40e9f75750246874e0a35de72e00616372da435e4b69966db5d" daml-stdlib-DA-Internal-Template-d14e08374fc7197d6a0de468c968ae8ba3aadbf9315476fd39071831f5923662 "d14e08374fc7197d6a0de468c968ae8ba3aadbf9315476fd39071831f5923662" daml-prim-75b070729b1fbd37a618493652121b0d6f5983b787e35179e52d048db70e9f15 "75b070729b1fbd37a618493652121b0d6f5983b787e35179e52d048db70e9f15" daml-stdlib-0.0.0-a535cbc3657b8df953a50aaef5a4cd224574549c83ca4377e8219aadea14f21a "a535cbc3657b8df953a50aaef5a4cd224574549c83ca4377e8219aadea14f21a"  In addition to the human-readable output, you can also get the output as JSON. This is easier to consume programmatically and it is more robust to changes across SDK versions: $ daml damlc inspect-dar --json .daml/dist/create-daml-app-0.1.0.dar
{
"packages": {
"29b501bcf541a40e9f75750246874e0a35de72e00616372da435e4b69966db5d": {
"path": "create-daml-app-0.1.0-29b501bcf541a40e9f75750246874e0a35de72e00616372da435e4b69966db5d/create-daml-app-0.1.0-29b501bcf541a40e9f75750246874e0a35de72e00616372da435e4b69966db5d.dalf",
"name": "create-daml-app",
"version": "0.1.0"
},
"name": null,
"version": null
},
"75b070729b1fbd37a618493652121b0d6f5983b787e35179e52d048db70e9f15": {
"path": "create-daml-app-0.1.0-29b501bcf541a40e9f75750246874e0a35de72e00616372da435e4b69966db5d/daml-prim-75b070729b1fbd37a618493652121b0d6f5983b787e35179e52d048db70e9f15.dalf",
"name": "daml-prim",
"version": "0.0.0"
},
"name": "daml-stdlib",
"version": "0.0.0"
}
},
"main_package_id": "29b501bcf541a40e9f75750246874e0a35de72e00616372da435e4b69966db5d",
"files": [
"create-daml-app-0.1.0-29b501bcf541a40e9f75750246874e0a35de72e00616372da435e4b69966db5d/create-daml-app-0.1.0-29b501bcf541a40e9f75750246874e0a35de72e00616372da435e4b69966db5d.dalf",
"create-daml-app-0.1.0-29b501bcf541a40e9f75750246874e0a35de72e00616372da435e4b69966db5d/daml-prim-75b070729b1fbd37a618493652121b0d6f5983b787e35179e52d048db70e9f15.dalf",
"create-daml-app-0.1.0-29b501bcf541a40e9f75750246874e0a35de72e00616372da435e4b69966db5d/data/create-daml-app-0.1.0.conf",
"create-daml-app-0.1.0-29b501bcf541a40e9f75750246874e0a35de72e00616372da435e4b69966db5d/User.daml",
"create-daml-app-0.1.0-29b501bcf541a40e9f75750246874e0a35de72e00616372da435e4b69966db5d/User.hi",
"create-daml-app-0.1.0-29b501bcf541a40e9f75750246874e0a35de72e00616372da435e4b69966db5d/User.hie",
"META-INF/MANIFEST.MF"
]
}


Note that name and version will be null for packages in Daml-LF < 1.8.

## Importing Daml packages¶

There are two ways to import a Daml package in a project: via dependencies, and via data-dependencies. They each have certain advantages and disadvantages. To summarize:

• dependencies allow you to import a Daml archive as a library. The definitions in the dependency will all be made available to the importing project. However, the dependency must be compiled with the same SDK version, so this method is only suitable for breaking up large projects into smaller projects that depend on each other, or to reuse existing libraries.
• data-dependencies allow you to import a Daml archive (.dar) or a Daml-LF package (.dalf), including packages that have already been deployed to a ledger. These packages can be compiled with any previous SDK version. On the other hand, not all definitions can be carried over perfectly, since the Daml interface needs to be reconstructed from the binary.

The following sections will cover these two approaches in more depth.

### Importing a Daml package via dependencies¶

A Daml project can declare a Daml archive as a dependency in the dependencies field of daml.yaml. This lets you import modules and reuse definitions from another Daml project. The main limitation of this method is that the dependency must be built for the same SDK version as the importing project.

Let’s go through an example. Suppose you have an existing Daml project foo, located at /home/user/foo, and you want to use it as a dependency in a project bar, located at /home/user/bar.

To do so, you first need to generate the Daml archive of foo. Go into /home/user/foo and run daml build -o foo.dar. This will create the Daml archive, /home/user/foo/foo.dar.

Next, we will update the project config for bar to use the generated Daml archive as a dependency. Go into /home/user/bar and change the dependencies field in daml.yaml to point to the created Daml archive:

dependencies:
- daml-prim
- daml-stdlib
- ../foo/foo.dar


The import path can also be absolute, for example, by changing the last line to:

- /home/user/foo/foo.dar


When you run daml build in the bar project, the compiler will make the definitions in foo.dar available for importing. For example, if foo exports the module Foo, you can import it in the usual way:

import Foo


By default, all modules of foo are made available when importing foo as a dependency. To limit which modules of foo get exported, you may add an exposed-modules field in the daml.yaml file for foo:

exposed-modules:
- Foo


### Importing a Daml archive via data-dependencies¶

You can import a Daml archive (.dar) or Daml-LF package (.dalf) using data-dependencies. Unlike dependencies, this can be used when the SDK versions do not match.

For example, you can import foo.dar as follows:

dependencies:
- daml-prim
- daml-stdlib
data-dependencies:
- ../foo/foo.dar


When importing packages this way, the Daml compiler will try to reconstruct the original Daml interface from the compiled binaries. However, to allow data-dependencies to work across SDK versions, the compiler has to abstract over some details which are not compatible across SDK versions. This means that there are some Daml features that cannot be recovered when using data-dependencies. In particular:

1. Export lists cannot be recovered, so imports via data-dependencies can access definitions that were originally hidden. This means it is up to the importing module to respect the data abstraction of the original module. Note that this is the same for all code that runs on the ledger, since the ledger does not provide special support for data abstraction.
2. If you have a dependency that limits the modules that can be accessed via exposed-modules, you can get an error if you also have a data-dependency that references something from the hidden modules (even if it is only reexported). Since exposed-modules are not available on the ledger in general, we recommend to not make use of them and instead rely on naming conventions (e.g., suffix module names with .Internal) to make it clear which modules are part of the public API.
3. Prior to Daml-LF version 1.8, typeclasses could not be reconstructed. This means if you have a package that is compiled with an older version of Daml-LF, typeclasses and typeclass instances will not be carried over via data-dependencies, and you won’t be able to call functions that rely on typeclass instances. This includes the template functions, such as create, signatory, and exercise, as these rely on typeclass instances.
4. Starting from Daml-LF version 1.8, when possible, typeclass instances will be reconstructed by re-using the typeclass definitions from dependencies, such as the typeclasses exported in daml-stdlib. However, if the typeclass signature has changed, you will get an instance for a reconstructed typeclass instead, which will not interoperate with code from dependencies. Furthermore, if the typeclass definition uses the FunctionalDependencies language extension, this may cause additional problems, since the functional dependencies cannot be recovered. So this is something to keep in mind when redefining typeclasses and when using FunctionalDependencies.
5. Certain advanced type system features cannot be reconstructed. In particular, DA.Generics and DeriveGeneric cannot be reconstructed. This may result in certain definitions being unavailable when importing a module that uses these advanced features.

Because of their flexibility, data-dependencies are a tool that is recommended for performing Daml model upgrades. See the upgrade documentation for more details.

### Referencing Daml packages already on the ledger¶

Daml packages that have been uploaded to a ledger can be imported as data dependencies, given you have the necessary permissions to download these packages. To import such a package, add the package name and version separated by a colon to the data-dependencies stanza as follows:

ledger:
host: localhost
port: 6865
dependencies:
- daml-prim
- daml-stdlib
data-dependencies:
- foo:1.0.0


If your ledger runs at the default host and port (localhost:6865), the ledger stanza can be omitted. This will fetch and install the package foo-1.0.0. A daml.lock file is created at the root of your project directory, pinning the resolved packages to their exact package ID:

dependencies:
name: foo
version: 1.0.0


The daml.lock file needs to be checked into version control of your project. This assures that package name/version tuples specified in your data dependencies are always resolved to the same package ID. To recreate or update your daml.lock file, delete it and run daml build again.

## Handling module name collisions¶

Sometimes you will have multiple packages with the same module name. In that case, a simple import will fail, since the compiler doesn’t know which version of the module to load. Fortunately, there are a few tools you can use to approach this problem.

The first is to use package qualified imports. Supposing you have packages with different names, foo and bar, which both expose a module X, you can select which one you want with a package qualified import.

To get X from foo:

import "foo" X


To get X from bar:

import "bar" X


To get both, you need to rename the module as you perform the import:

import "foo" X as FooX
import "bar" X as BarX


Sometimes, package qualified imports will not help, because you are importing two packages with the same name. For example, if you’re loading different versions of the same package. To handle this case, you need the --package build option.

Suppose you are importing packages foo-1.0.0 and foo-2.0.0. Notice they have the same name foo but different versions. To get modules that are exposed in both packages, you will need to provide module aliases. You can do this by passing the --package build option. Open daml.yaml and add the following build-options:

build-options:
- '--package'
- 'foo-1.0.0 with (X as Foo1.X)'
- '--package'
- 'foo-2.0.0 with (X as Foo2.X)'


This will alias the X in foo-1.0.0 as Foo1.X, and alias the X in foo-2.0.0 as Foo2.X. Now you will be able to import both X by using the new names:

import qualified Foo1.X
import qualified Foo2.X


It is also possible to add a prefix to all modules in a package using the module-prefixes field in your daml.yaml. This is particularly useful for upgrades where you can map all modules of version v of your package under V\$v. For the example above you can use the following:

module-prefixes:
foo-1.0.0: Foo1
foo-2.0.0: Foo2


That will allow you to import module X from package foo-1.0.0 as Foo1.X and X from package foo-2.0.0 as Foo2.

You can also use more complex module prefixes, e.g., foo-1.0.0: Foo1.Bar which will make module X available under Foo1.Bar.X.