Distribution.Types.UnitId

A unit identifier identifies a (possibly instantiated) package/component that can be installed the installed package database. There are several types of components that can be installed:

• A traditional library with no holes, so that unitIdHash is Nothing. In the absence of Backpack, UnitId is the same as a ComponentId.

• An indefinite, Backpack library with holes. In this case, unitIdHash is still Nothing, but in the install, there are only interfaces, no compiled objects.

• An instantiated Backpack library with all the holes filled in. unitIdHash is a Just a hash of the instantiating mapping.

A unit is a component plus the additional information on how the holes are filled in. Thus there is a one to many relationship: for a particular component there are many different ways of filling in the holes, and each different combination is a unit (and has a separate UnitId).

UnitId is distinct from OpenUnitId, in that it is always installed, whereas OpenUnitId are intermediate unit identities that arise during mixin linking, and don't necessarily correspond to any actually installed unit. Since the mapping is not actually recorded in a UnitId, you can't actually substitute over them (but you can substitute over OpenUnitId). See also Distribution.Backpack.FullUnitId for a mechanism for expanding an instantiated UnitId to retrieve its mapping.

Backwards compatibility note: if you need to get the string representation of a UnitId to pass, e.g., as a -package-id flag, use the display function, which will work on all versions of Cabal.

If you need backwards compatibility, consider using display instead, which is supported by all versions of Cabal.

A UnitId for a definite package. The DefUnitId invariant says that a UnitId identified this way is definite; i.e., it has no unfilled holes.

Unsafely create a DefUnitId from a UnitId. Your responsibility is to ensure that the DefUnitId invariant holds.

Create a unit identity with no associated hash directly from a ComponentId.

Make an old-style UnitId from a package identifier. Assumed to be for the public library

Returns library name prefixed with HS, suitable for filenames