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Programming language: Haskell
License: BSD 3-clause "New" or "Revised" License
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Latest version: v8.6.5-final

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This is the development site of the LiquidHaskell formal verification tool.

If you're a LiquidHaskell user (or just curious), you probably want to go to the documentation website instead.


This is an open-source project, and we love getting feedback (and patches)!

Reporting a Bug

If something doesn't work as it should, please consider opening a github issue to let us know. If possible, try to:

  • Try to use a descriptive title;
  • State as clearly as possible what is the problem you are facing;
  • Provide a small Haskell file producing the issue;
  • Write down the expected behaviour vs the actual behaviour;
  • If possible, let us know if you have used the [plugin](install.md) or the [executable](legacy.md) and which GHC version you are using.

Your first Pull Request

We are thrilled to get PRs! Please follow these guidelines, as doing so will increase the chances of having your PR accepted:

  • The main LH repo lives here
  • Please create pull requests against the develop branch.
  • Please be sure to include test cases that illustrate the effect of the PR
    • e.g. show new features that that are supported or how it fixes some previous issue
  • If you're making user-visible changes, please also add documentation
    • e.g. [options.md](docs/mkDocs/docs/options.md), [specifications.md](docs/mkDocs/docs/specifications.md), the [main tutorial](https:///github.com/ucsd-progsys/intro-refinement-types) (as relevant)

Pull requests don't just have to be about code: documentation can often be improved too!

Ask for Help

If you have further questions or you just need help, you can always reach out on our slack channel, google groups mailing list, GitHub issue tracker, or by emailing Ranjit Jhala, Niki Vazou.

General Development Guide

For those diving into the implementation of LiquidHaskell, here are a few tips:

Fast (re)compilation

When working on the liquidhaskell library, usually all we want is to make changes and quickly recompile only the bare minimum, to try out new ideas. Using a fully-fledged GHC plugin doesn't help in this sense, because packages like liquid-base or liquid-ghc-prim all have a direct dependency on liquidhaskell, and therefore every time the latter changes, an expensive rebuild of those packages is triggered, which might become tedious overtime. To mitigate this, we offer a faster, "dev-style" build mode which is based on the assumption that most changes to the liquidhaskell library do not alter the validity of already-checked libraries, and therefore things like liquid-base and liquid-ghc-prim can be considered "static assets", avoiding the need for a recompilation. In other terms, we explicitly disable recompilation of any of the liquid-* ancillary library in dev mode, so that rebuilds would also influence the liquidhaskell library.

Usage and recommended workflow

This is how you can use this:

  • To begin with, perform a full build of all the libraries, by doing either cabal v2-build or stack build, without specifying any extra environment variables from the command line. This is needed to ensure that we things like liquid-base and liquid-ghc-prim are compiled at least once, as we would need the refinements they contain to correctly checks other downstream programs;

  • At this point, the content of the liquid-* packages is considered "trusted" and "frozen", until you won't force another full, non-dev build;

  • In order to quickly test changes to the liquidhaskell library without recompiling the liquid-* packages, we need to start a build passing the LIQUID_DEV_MODE env var as part of the build command. Examples:

LIQUID_DEV_MODE=true stack build

If on NixOS

LIQUID_DEV_MODE=true stack --no-nix-pure build

With the above, stack will unregister and re-register the libraries, but hopefully it won't rebuild any modules.

LIQUID_DEV_MODE=true cabal v2-build

It's also possible (but not recommended) to add LIQUID_DEV_MODE to .bashrc or similar, but this would permanently disable building the liquid-* packages, and this might silently mask breaking changes to the liquidhaskell library that would manifest only when compiling these other packages.

If you wish to force building all the libraries again, it's sufficient to issue the same builds commands without the LIQUID_DEV_MODE.

How To Run Regression Tests

For documentation on the test-driver executable itself, please refer to the README.md in tests/ or run cabal run tests:test-driver -- --help or stack run test-driver -- --help

For a way of running the test suite for multiple GHC versions, consult the General Development FAQs. below

There are particular scripts for running LH in the different modes, e.g. for different compiler versions. These scripts are in:

$ ./scripts/test

So you can run all the tests for say the ghc-8.10 version by

$ ./scripts/test/test_810_plugin.sh

You can run a bunch of particular test-groups instead by

$ LIQUID_DEV_MODE=true ./scripts/test/test_810_plugin.sh <test-group-name1> <test-group-name2> ...

and you can list all the possible test options with

$ LIQUID_DEV_MODE=true ./scripts/test/test_810_plugin.sh --help

or get a list of just the test groups, one per line, with

$ LIQUID_DEV_MODE=true ./scripts/tests/test_810_plugin.sh --show-all

To pass in specific parameters and run a subset of the tests, you can invoke cabal directly with

$ LIQUID_DEV_MODE=true cabal build tests:<test-group-name> --ghc-options=-fplugin-opt=LiquidHaskell:--no-termination MySpecificTest

For example:

$ LIQUID_DEV_MODE=true cabal build tests:unit-neg --ghc-options=--fplugin-opt=LiquidHaskell:--no-termination AbsApp

Or your favorite number of threads, depending on cores etc.

You can directly extend and run the tests by modifying the files in


Parallelism in Tests

Most tests run in parallel, with a few module dependencies built sequentially in advance. Benchmarks are run sequentially after all other tests have finished. For details on adding tests, see note [Parallel_Tests] in tests/test.hs.

How to create performance comparison charts

When liquidhaskell tests run, we can collect timing information with

$ ./scripts/tests/test_810_plugin.sh --measure-timings

Measures will be collected in .dump-timings files. These can be converted to json data with

cabal v2-build ghc-timings
cabal v2-exec ghc-timings dist-newstyle

which will produce tmp/*.json files.

Then a csv report can be generated from this json files with

cabal v2-run benchmark-timings -- tmp/*.json --phase LiquidHaskell -o summary.csv

On each line, the report will contain the time taken by each test.

There is a script scripts/plot-performance/chart_perf.sh that can be used to generate comparison charts in svg and png formats. It requires gnuplot to run. The following command will produce two files perf.svg and perf.png in the current directory.

$ scripts/plot-performance/chart_perf.sh path_to_before_summary.csv path_to_after_summary.csv

The current formatting is optimized for comparing the outputs of running the benchmarks alone.

$ scripts/test/test_810_plugin.sh
    benchmark-stitch-lh \
    benchmark-bytestring \
    benchmark-cse230 \
    benchmark-esop2013 \
    benchmark-icfp15-pos \

How to Profile

  1. Build with profiling on

    $ stack build liquidhaskell --fast --profile
  2. Run with profiling

    $ stack exec -- liquid range.hs +RTS -hc -p
    $ stack exec -- liquid range.hs +RTS -hy -p

    Followed by this which shows the stats file

    $ more liquid.prof

    or by this to see the graph

    $ hp2ps -e8in -c liquid.hp
    $ gv liquid.ps


How to Get Stack Traces On Exceptions

  1. Build with profiling on

    $ stack build liquidhaskell --fast --profile
  2. Run with backtraces

    $ liquid +RTS -xc -RTS foo.hs
    stack exec -- liquid List00.hs +RTS -p -xc -RTS

Working With Submodules

To update the liquid-fixpoint submodule, run:

cd ./liquid-fixpoint
git fetch --all
git checkout <remote>/<branch>
cd ..

This will update liquid-fixpoint to the latest version on <branch> (usually master) from <remote> (usually origin). After updating liquid-fixpoint, make sure to include this change in a commit! Running:

git add ./liquid-fixpoint

will save the current commit hash of liquid-fixpoint in your next commit to the liquidhaskell repository. For the best experience, don't make changes directly to the ./liquid-fixpoint submodule, or else git may get confused. Do any liquid-fixpoint development inside a separate clone/copy elsewhere. If something goes wrong, run:

rm -r ./liquid-fixpoint
git submodule update --init

to blow away your copy of the liquid-fixpoint submodule and revert to the last saved commit hash.

Want to work fully offline? git lets you add a local directory as a remote. Run:

cd ./liquid-fixpoint
git remote add local /path/to/your/fixpoint/clone
cd ..

Then to update the submodule from your local clone, you can run:

cd ./liquid-fixpoint
git fetch local
git checkout local/<branch>
cd ..

Releasing on Hackage

NOTE: The following section is relevant only for few developers, i.e. the ones which are directly involved in the release process. Most contributors can skip this section.

We provide a convenience script to upload all the liquid-* packages (including liquid-fixpoint) on Hackage, in a lockstep fashion. To do so, it's possible to simply run the scripts/release_to_hackage.sh Bash script. The script doesn't accept any argument and it tries to determine the packages to upload by scanning the $PWD for packages named appropriately. It will ask the user for confirmation before proceeding, and stack upload will be used under the hood.

The GHC.API module

In order to allow LH to work with multiple GHC versions, we need a way to abstract over all the breaking changes of the ghc library, which might change substantially with every major GHC release. This is accomplished by the GHC.API module. The idea is that rather than importing multiple ghc modules, LH developers must import this single module in order to write future-proof code. This is especially important for versions of the compiler greater than 9, where the module hierarchy changed substantially, and using the GHC.API makes it easier to support new versions of GHC when they are released.

Fragile import strategy

import Predicate
import TyCoRep


-- This will break if 'isEqPrimPred' is (re)moved or the import hierarchy changes.
foo :: Type -> Bool
foo = isEqPrimPred

Recommended import strategy

import qualified Language.Haskell.Liquid.GHC.API as GHC


foo :: GHC.Type -> Bool
foo = GHC.isEqPrimPred -- OK.

GHC Plugin Development Guide

This code commentary describes the current architecture for the GHC Plugin that enables LiquidHaskell to check files as part of the normal compilation process. For the sake of this commentary, we refer to the code provided as part of the release/ branch, commit 9a2f8284c5fe5b18ed0410e842acd3329a629a6b.

GHC.Interface vs GHC.Plugin

The module GHC.Plugin is the main entrypoint for all the plugin functionalities. Whenever possible, this module is reusing common functionalities from the GHC.Interface, which is the original module used to interface LH with the old executable. Generally speaking, the GHC.Interface module is considered "legacy" and it's rarely what one wants to modify. It will probably be removed once the old executable stops being supported, with the functions now in use by the GHC.Plugin being moved into the latter.

The GhcMonadLike shim

Part of the tension in designing the plugin was trying to reuse as much code as possible from the original GHC.Interface shipped with LiquidHaskell. Unfortunately this was not possible from the get-go due to the fact most of the functions provided by that module were requiring a GhcMonad constraint or usually living in the Ghc monad, which is also the only concrete type which derives an instance for GhcMonad. While we could have run each and every function with runGhc, this was not very satisfactory due to the fact running the Ghc monad is fairly expensive as it requires a bit of extra state in order to run it.

However, most of the functions used by the Ghc.Interface didn't require anything specific from the underlying Ghc monad if not access to the HscEnv and the ability to grab the DynFlags, as well as doing IO. Therefore, the GhcMonadLike shim was born with the intent of replicating some of the functions used by the GHC.Interface but crucially making those polymorphic in a generic GhcMonadLike for which we can give instances for CoreM, TcM etc. We can do this because we do not require the extra ExceptionMonad constraint and we do not require to implement setHscEnv.

This allowed us to change ever so slightly the functions provided by the GHC.Interface, expose them and reuse them in the Plugin module.

Plugin architecture

Broadly speaking, the Plugin is organised this way: In the typechecking phase, we typecheck and desugar each module via the GHC API in order to extract the unoptimised core binds that are needed by LH to work correctly. This is due to a tension in the design space; from one side LH needs access to the "raw" core binds (binds where primitives types are not unboxed in the presence of a PRAGMA annotation, for example) but yet the user can specify any arbitrary optimisation settings during compilation and we do not want to betray the principle of least expectation by silently compiling the code with -O0. Practically speaking, this introduces some overhead and is far from ideal, but for now it allows us to iterate quickly. This phase is also responsible for:

  • Extracting the BareSpecs associated to any of the dependent modules;
  • Producing the LiftedSpec for the currently-compiled module;
  • Storing the LiftedSpec into an interface annotation for later retrieval;
  • Checking and verifying the module using LH's existing API.

The reason why we do everything in the typechecking phase is also to allow integrations with tools like ghcide. There are a number of differences between the plugin and the operations performed as part of the GHC.Interface, which we are going to outline in the next section.

Differences with the GHC.Interface

  • The GHC.Interface pre-processes the input files and calls into configureGhcTargets trying to build a dependency graph by discovering dependencies the target files might require. Then, from this list any file in the include directory is filtered out, as well as any module which has a "fresh" .bspec file on disk, to save time during checking. In the GHC.Plugin module though we don't do this and for us, essentially, each input file is considered a target, where we exploit the fact GHC will skip recompilation if unnecessary. This also implies that while the GHC.Interface calls into processTargetModule only for target files, the GHC.Plugin has a single, flat function simply called processModule that essentially does the same as GHC.Interface.processModule and GHC.Interface.processTargetModule fused together.

  • While the GHC.Interface sometimes "assembles" a BareSpec by mappending the commSpec (i.e. comment spec) with the LiftedSpec fetched from disk, if any, the Plugin doesn't do this but rather piggybacks on the SpecFinder (described later) to fetch dependencies' specs.

  • There is a difference in how we process LIQUID pragmas. In particular, for the executable they seems to be accumulated "in bulk" i.e. if we are refining a target module A that depends on B, B seems to inherit whichever flags we were using in the target module A. Conversely, the source plugin is "stateless" when it comes to LIQUID options, i.e. it doesn't have memory of past options, what it counts when compiling a module B is the global options and any option this module defines. The analogy is exactly the same as with GHC language extensions, they have either global scope (i.e. default-extensions in the cabal manifest) or local scope (i.e. {-# LANGUAGE ... #-}).

Finding specs for existing modules

This is all done by a specialised module called the SpecFinder. The main exported function is findRelevantSpecs which, given a list of Modules, tries to retrieve the LiftedSpecs associated with them. Typically this is done by looking into the interface files of the input modules, trying to deserialise any LiftedSpec from the interface file's annotations.

General Development FAQs

A new version of GHC is out. How do I support it?

Typically the first thing you might want to do is to run a "clean" cabal v2-build or stack build using the latest compiler and "check the damage". If you are lucky, everything works out of the box, otherwise compilation might fail with an error, typically because some ghc API function has been removed/moved/renamed. The way to fix it is to modify the GHC.API shim module and perform any required change, likely by conditionally compiling some code in a CPP block. For minor changes, it's usually enough to perform small changes, but for more tricky migrations it might be necessary to backport some GHC code, or create some patter synonym to deal with changes in type constructors. You can see an example of this technique in action by looking at the pattern synonym for FunTy.

Is there a way to run the testsuite for different versions of GHC?

Yes. The easiest way is to run one of the scripts inside the scripts/test directory. We provide scripts to run the testsuite for a variety of GHC versions, mostly using stack but also with cabal (e.g. test_810_plugin.sh). If run without arguments, the script will run the full testsuite. If an argument is given, only a particular pattern/test will be run. Running

./scripts/test/test_810_plugin.sh BST

will run all the tests which name matches "BST". In case the "fast recompilation" is desired, it's totally possibly to pass LIQUID_DEV_MODE to the script, for example:

LIQUID_DEV_MODE=true ./scripts/test/test_810_plugin.sh

GHC Plugin Development FAQs

Is it possible that the behaviour of the old executable and the new / the plugin differ?

It might happen, yes, but the surface area is fairly small. Both modules work by producing a TargetSrc that is passed to the internal LH API, which is shared by both modules. Therefore, any difference in behaviour has to be researched in the code path that produces such TargetSrc. For the GHC.Plugin this happens in the makeTargetSrc, whereas for the GHC.Interface this happens inside the [makeGhcSrc][] function.

Why is the GHC.Interface using slightly different types than the GHC.Plugin module?

Mostly for backward-compatibility and for historical reasons. Types like BareSpec used to be type alias rather than newtypes, and things were slightly renamed to reflect better purpose when the support for the plugin was added. While doing so we also added a compatibility layer in the form of some optics that can be used to map back and forth (sometimes in a partial way) between old and new data structures. When in doubt, consider the GHC.Plugin as the single source of truth, and prefer whichever data structure the latter is using.