wiki:Commentary/Packages/Concepts

Version 6 (modified by ezyang, 4 years ago) (diff)

--

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.

What problems do we need to solve?

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

[SYMBOL]
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.
[ABI]
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.
[SOURCE]
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].
[NIX]
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.
[TYPES]
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.)

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. By today's Hackage conventions, this can identify a [SOURCE]. Pre-GHC 7.10, it also identified [SYMBOL] and [TYPES].
Installed Package ID
Package name, package version, and the output of ghc --abi-hash. This identifies [ABI].
Package Key (new in 7.10)
Hash of package name, package version, the package keys of all direct dependencies the package was compiled against, and a a mapping of how Backpack holes in the package were instantiated. This now identifies [SYMBOL] and [TYPES]. It also identifies [NIX] if you assume that package IDs adequately identify a [SOURCE].

Furthermore, there have been some proposals for how things should further change in the future. Here are the few that I am aware of:

Installed Package ID (WITHOUT ABI)
In our conversations last summer, Duncan reported to me that he wanted to change Installed Package ID so that it was a hash of the source code and other relevant bits, serving the role of [NIX] hashes. I recorded this in this Cabal bug https://github.com/haskell/cabal/issues/2199
Package Key (WITHOUT KEYS OF INCLUDES)
While I was working on Backpack I discovered that our current definition of package key is circular (and thus undefined) in some use-cases which we might eventually want to support. The refinement I currently have in my working Git copy is that I have removed from the hash the package keys of the direct dependencies of the package. If you apply this change, package keys NO LONGER identify a [NIX].
Package KEY (WITH SOURCE HASH)
In this 2015 Haskell GSoC proposal https://gist.github.com/fugyk/37510958b52589737274 Vishal Agrawal proposes adding a hash of the source tarball to the package key. This would make it implement [NIX] in the strong sense. However, this proposal would make package key unsuitable for [SOURCE]

Features

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.
Views
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.

ezyang's proposal

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.