Version 10 (modified by ezyang, 4 years ago) (diff)

unit updates

Package keys, installed package IDs, ABI hashes, package names and versions, Nix-style hashes, ... there's so many different identifiers, what do they all mean? I think the biggest source of confusion (for myself included) is keeping straight not only what these terms mean, but also what people want them to mean in the future, and what we actually care about. So I want to help clarify this a bit, by clearly separating the problem you are trying to solve from how you are solving the problem.

The content here overlaps with wiki:Commentary/Packages but is looking at the latest iteration of the multi-instances and Backpack work.

Some relevant tickets: #10622

What problems do we need to solve?

When we come up with identification schemes for packages, we are trying to solve a few problems:

What symbol names should we put in the binary? (e.g., the "foozm0zi1" in "foozm0zi1_A_DZCF_closure")
  • It must be unique enough that for all libraries we would like to be able to link together, there should not be conflicts.
  • HOWEVER, it must be stable enough that if we make a minor source code change, we don't have to gratuitously recompile every dependency.
When can I swap out one compiled package with another WITHOUT recompiling, i.e. what is the ABI of the package? Equal ABIs implies equal symbols, though not vice versa. ABI is usually computed after compilation is complete.
  • ABI can serve as correctness condition: if we link against a specific ABI, we can be sure that anything with an equivalent ABI won't cause our package to segfault.
  • ABI can also serve as an indirection: we linked against an ABI, anything that is compatible can be hotswapped in without compilation. In practice, this capability is rarely used by users because it's quite hard to compile a package multiple times with the same ABI, because (1) compilation is nondeterministic, and (2) even if no types change, a change in implementation can cause a different exported unfolding, which is ABI relevant.
What is the unit of distribution? In other words, when a maintainer uploads an sdist to Hackage, how do you identify that source tarball?
  • On Hackage, a package name plus version uniquely identifies an sdist. This is enforced by community standards; in a local development environment, this may not hold since devs will edit code without updating the version number. Call this [WEAK SOURCE].
  • Alternately, a cryptographic hash of the source code uniquely identifies the stream of bytes. This is enforced by math. Call this [STRONG SOURCE].
When you build a library, you get an file. What identifies an OS level library?
What is the full set of source which I can use to reproduceably build a build product?
  • In today's Cabal, you could approximate this by taking [WEAK SOURCE] of a package, as well as all of its transitive dependencies. Call this [WEAK NIX].
  • The Nix approach is to ensure deterministic builds by taking the hash of the source [STRONG SOURCE] and also recursively including the [NIX] of each direct dependency. Call this [STRONG NIX].
  • Note that [ABI] does NOT imply [NIX]; a package might be binary compatible but do something different, and in a Nix model they should be recorded differently.
When are two types the same? If there are from differing packages, they are obviously different; if they are from the same package, they might still be different if the dependencies were different in each case.
  • Types show up in error message, so this is a USER VISIBLE notion. Many people have (cogently) argued that this should be AS SIMPLE as possible, because there's nothing worse than being told that Data.ByteString.ByteString is not equal to Data.ByteString.ByteString (because they were from different packages.)

Current mechanisms

Today, we have a lot of different MECHANISMS for identifying these:

Package Name
Something like "lens"
Package Version
Something like "0.1.2"
Package ID
Package name plus version. With Hackage today, this identifies a unit of distribution: given a package ID you can download a source tarball [SOURCE] of a package (but not build it). Pre-GHC 7.10, the package ID was used for library identification, symbols and type-checking ([LIBRARY], [SYMBOL] and [TYPES]), but this is no longer the case.
Installed Package ID
Package name, package version, and the output of ghc --abi-hash. This is currently used to uniquely identify a built package, although technically it only identifies [ABI].
Package Key (new in 7.10)
Hash of package name, package version, the package keys of all textual dependencies the package included, and in Backpack a mapping from hole name to module by package key. In GHC 7.10 this is used for library identification, symbols and type-checking ([LIBRARY], [SYMBOL] and [TYPES]). Because it includes package keys of textual dependencies, it also distinguishes between different dependency resolutions, ala [WEAK NIX].

Proposed mechanisms

Here are the changes to the mechanisms which we have in flight.

First, some new concepts:

Backpack introduces the concept of a unit, which is a package-like code organization principle which allows multiple modules to be structured together. A package contains one or more units, one of which is a distinguished unit that is publicly available to other packages; most packages contain exactly one unit.
Indefinite/definite unit
An indefinite unit is a single unit which hasn't been instantiated; a definite unit is one that has an instantiation of its holes. Units without holes are both definite and indefinite (they can be used for both contexts).
Indefinite unit record
An indefinite unit record is the most general result of type-checking a unit without any of its holes instantiated. It is identified by an installed indefinite unit ID, and consists of the types of the modules in the unit (ModIfaces) as well as the source code of the unit (so that it can be recompiled into a definite unit). Indefinite unit records can be installed in the "indefinite unit database."
Definite unit record (previously installed package record)
A definite unit record is a fully-instantiated unit with its associated library. It is identified by an installed definite unit ID, and consists of the types and objects of the compiled unit; they also contain metadata for their associated package. Definite unit records can be installed in the "definite unit database" (previously known as the "installed package database.") If the unit is the distinguished unit for the package, this record also contains all of the metadata about the containing package.

And now the identifiers:

Unit name
Something like "p", a unit name is a source-level identifier which distinguishes the multiple units in a package; e.g. unit p where ... defines a unit with name p. The unit name that is the same as the containing package is special: it is the "public" unit that is externally accessible.
Unit occurrence
Something like the "p" in include p, a unit occurrence is a string which occurs in a Backpack file and is intended to refer to a indefinite unit. If the Backpack file has a unit named p, it refers to that unit; otherwise, the unit occurrence must be renamed to point to a specific installed unit ID (e.g., by some flags to GHC).
(Installed) indefinite unit ID
Installed package ID and unit name which identifies the (transitive) source code of an indefinite unit.
Installed (definite) unit ID (previously package key)
Indefinite unit ID and a mapping from holes to modules (installed definite unit ID plus module name). In the absence of Backpack, installed indefinite unit IDs, installed definite unit IDs, and installed package IDs are all the same. [LIBRARY, TYPES, ABI, SYMBOL] With the , this also subsumes what we previously referred to as "library names" or "version hashes"
Installed Package ID (not based off of ABI)
Hash of source code sdist tarball, selected Cabal flags (not the command line flags), GHC flags, IPIDs of direct dependencies. This brings IPIDs more inline with the concept of a [NIX] hash, which lets us truly uniquely identify builds of packages (whereas a package key may conclude two builds are the same, although their source has changed.) This is especially important if all sandboxed builds are being installed to the same location. This proposal is in Cabal bug and subject to today's GSOC.


There are a number of enhancements proposed for how Cabal handles packages, which have often been conflated together. I want to clearly separate them out here:

Non-destructive installs
If I have package foo-0.2 compiled against bar-0.1, and a different build compiled against bar-0.2, I should be able to put them in the same installed package database. THIS IS HIGH PRIORITY.
If I have package foo compiled against bar-0.1, and baz compiled against bar-0.2, these two packages aren't usable together (modulo private dependencies, see below). Views are a UI paradigm making it easier for users to work in a universe where foo is available, or a universe where baz is available, but not both simultaneously. Cabal sandboxes are views but without a shared installed package database. This is lower priority, because if you use cabal-install to get a coherent dependency set, you'll never see both foo and baz at the same time; the primary benefit of this is to assist with direct use of GHC/GHCi, however, it is generally believed that non-destructive installs will make it difficult to use GHC/GHCi by itself.
Private dependencies
If I have a package foo-0.2 which depends on a library bar-0.1, but not in any externally visible way, it should be allowed for a client to separately use bar-0.2. This is LOW priority; amusingly, in 7.10, this is already supported by GHC, but not by Cabal.
Hot swappable libraries
If I install a library and it's assigned ABI hash 123abc, and then I install a number of libraries that depend on it, hot swappable library means that I can replace that installed library with another version with the same ABI hash, and everything will keep working. This feature is accidentally supported by GHC today, but no one uses it (because ABIs are not stable enough); we are willing to break this mode of use to support other features.


For an implementer, it is best if each problem is solved separately. However, Simon has argued strongly it is best if we REDUCE the amount of package naming concepts. You can see this in pre-7.10 GHC, where the package ID (package name + version) was used fulfill many functions: linker symbols, type identity as well as being a unit of distribution.

So the way I want to go about arguing for the necessity of a given identifier is by showing that it is IMPOSSIBLE (by the intended functions) for a single identifier to serve both roles. Here are the main constraints:

  • [SYMBOL] and [STRONG NIX]/[STRONG SOURCE] are incompatible. If you modify your source code, a [STRONG NIX/SOURCE] identifier must change; if this means [SYMBOL] changes too, you will have to recompile everything. Not good.
  • [SOURCE] and [TYPES] are incompatible under non-destructive installs and private dependencies. With private dependencies (which GHC supports!), I may link against the multiple instances of the same source but compiled against different dependencies; we MUST NOT consider these types to be the same. Note: GHC used to use package ID for both of these; so coherence was guaranteed by requiring destructive installs.
  • [NIX] and [TYPES] are incompatible under Backpack. In Backpack, a library author may distribute a package with the explicit intent that it may be used in the same client multiple times with different instantiations of its holes; these types must be kept distinct.