BSD-3-Clause licensed by Dan Doel
Maintained by Andrew Lelechenko
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Provides support for logic-based evaluation. Logic-based programming uses a technique known as backtracking to consider alternative values as solutions to logic statements, and is exemplified by languages such as Prolog and Datalog.

Logic-based programming replaces explicit iteration and sequencing code with implicit functionality that internally “iterates” (via backtracking) over a set of possible values that satisfy explicitly provided conditions.

This package adds support for logic-based programming in Haskell using the continuation-based techniques adapted from the paper Backtracking, Interleaving, and Terminating Monad Transformers by Oleg Kiselyov, Chung-chieh Shan, Daniel P. Friedman, Amr Sabry. This paper extends previous research into using MonadPlus functionality—where mplus is used to specify value alternatives for consideration and mzero use used to specify the lack of any acceptable values—to add support for fairness and pruning using a set of operations defined by a new MonadLogic class.


In a typical example for Prolog logic programming, there are a set of facts (expressed as unconditional statements):

parent(sarah, john).
parent(arnold, john).
parent(john, anne).

and a set of rules that apply if their conditions (body clause) are satisfied:

grandparent(Person, Grandchild) :- parent(Person, X), parent(X, Grandchild).

Execution of a query for this rule grandparent(G, anne) would result in the following “values”:

grandparent(sarah, anne).
grandparent(arnold, anne).

For this query execution, Person and X are “free” variables where Grandchild has been fixed to anne. The Prolog engine internally “backtracks” to the parent(Person, X) statement to try different known values for each variable, executing forward to see if the values satisfy all the results and produce a resulting value.

Haskell logict Package

The Haskell equivalent for the example above, using the logict package might look something like the following:

import Control.Applicative
import Control.Monad.Logic

parents :: [ (String, String) ]
parents = [ ("Sarah",  "John")
          , ("Arnold", "John")
          , ("John",   "Anne")

grandparent :: String -> Logic String
grandparent grandchild = do (p, c) <- choose parents
                            (c', g) <- choose parents
                            guard (c == c')
                            guard (g == grandchild)
                            pure p

choose = foldr ((<|>) . pure) empty

main = do let grandparents = observeAll (grandparent "Anne")
          putStrLn $ "Anne's grandparents are: " <> show grandparents

In this simple example, each of the choose calls acts as a backtracking choice point where different entries of the parents array will be generated. This backtracking is handled automatically by the MonadLogic instance for Logic and does not need to be explicitly written into the code. The observeAll function collects all the values “produced” by Logic, allowing this program to display:

Anne's grandparents are: ["Sarah","Arnold"]

This example is provided as the grandparents executable built by the logict package so you can run it yourself and try various experimental modifications.

The example above is very simplistic and is just a brief introduction into the capabilities of logic programming and the logict package. The logict package provides additional functionality such as:

  • Fair conjunction and disjunction, which can help with potentially infinite sets of inputs.

  • A LogicT monad stack that lets logic operations be performed along with other monadic actions (e.g. if the parents sample was streamed from an input file using the IO monad).

  • A MonadLogic class which allows other monads to be defined which provide logic programming capabilities.

Additional Notes

The implementation in this logict package provides the backtracking functionality at a lower level than that defined in the associated paper. The backtracking is defined within the Alternative class as <|> and empty, whereas the paper uses the MonadPlus class and the mplus and mzero functions; since Alternative is a requirement (constraint) for MonadPlus, this allows both nomenclatures to be supported and used as appropriate to the client code.

More details on using this package as well as other functions (including fair conjunction and disjunction) are provided in the Haddock documentation.


  • Breaking change: do not re-export Control.Monad and Control.Monad.Trans from Control.Monad.Logic.
  • Generalize instance Traversable (LogicT Identity) to instance (Traversable m, Monad m) => Traversable (LogicT m).
  • Add conversion functions fromLogicT and fromLogicTWith to facilitate interoperation with list-t and logict-sequence packages.
  • Add hoistLogicT and embedLogicT to convert LogicT computations from one underlying monad to another.

  • Improve documentation.
  • Breaking change: relax superclasses of MonadLogic to Monad and Alternative instead of MonadPlus.

  • Support GHC 9.0.

  • Add Safe pragmas.

  • Fix MonadReader r (LogicT m) instance again.

  • Remove unlawful MonadLogic (Writer T w m) instances.
  • Fix MonadReader r (LogicT m) instance.
  • Move lnot into MonadLogic class.

  • Comply with MonadFail proposal.