Distribution.Backpack

An OpenUnitId describes a (possibly partially) instantiated Backpack component, with a description of how the holes are filled in. Unlike OpenUnitId, the ModuleSubst is kept in a structured form that allows for substitution (which fills in holes.) This form of unit cannot be installed. It must first be converted to a UnitId.

In the absence of Backpack, there are no holes to fill, so any such component always has an empty module substitution; thus we can lossily represent it as a 'DefiniteUnitId uid'.

For a source component using Backpack, however, there is more structure as components may be parametrized over some signatures, and these "holes" may be partially or wholly filled.

OpenUnitId plays an important role when we are mix-in linking, and is recorded to the installed packaged database for indefinite packages; however, for compiled packages that are fully instantiated, we instantiate OpenUnitId into UnitId.

For more details see the Backpack spec https://github.com/ezyang/ghc-proposals/blob/backpack/proposals/0000-backpack.rst

Get the set of holes (ModuleVar) embedded in a UnitId.

Safe constructor from a UnitId. The only way to do this safely is if the instantiation is provided.

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.

Create a DefUnitId from a ComponentId and an instantiation with no holes.

Unlike a Module, an OpenModule is either an ordinary module from some unit, OR an OpenModuleVar, representing a hole that needs to be filled in. Substitutions are over module variables.

Get the set of holes (ModuleVar) embedded in a Module.

An explicit substitution on modules.

NB: These substitutions are NOT idempotent, for example, a valid substitution is (A -> B, B -> A).

Pretty-print the entries of a module substitution, suitable for embedding into a OpenUnitId or passing to GHC via --instantiate-with.

Pretty-print a single entry of a module substitution.

Inverse to dispModSubst.

Inverse to dispModSubstEntry.

Get the set of holes (ModuleVar) embedded in a OpenModuleSubst. This is NOT the domain of the substitution.

When typechecking, we don't demand that a freshly instantiated IndefFullUnitId be compiled; instead, we just depend on the installed indefinite unit installed at the ComponentId.

Take a module substitution and hash it into a string suitable for UnitId. Note that since this takes Module, not OpenModule, you are responsible for recursively converting OpenModule into Module. See also Distribution.Backpack.ReadyComponent.