# unification-fd

Simple generic unification algorithms.

https://wrengr.org/software/hackage.html

LTS Haskell 21.25: | 0.11.2@rev:1 |

Stackage Nightly 2023-06-21: | 0.11.2 |

Latest on Hackage: | 0.11.2@rev:1 |

**wren gayle romano**

**[email protected]**

`unification-fd-0.11.2@sha256:b2e11811b703b63682076ae326bd856fd47caf7db881e9e5b896a6342bceb51f,3736`

#### Module documentation for 0.11.2

- Control
- Control.Monad
- Control.Monad.EitherK
- Control.Monad.MaybeK
- Control.Monad.State

- Control.Unification

- Control.Monad
- Data
- Data.Functor

*(full list with versions)*:

# unification-fd

The unification-fd package offers generic functions for single-sorted
first-order structural unification (think of programming in Prolog,
or of the metavariables in type inference)[^1][^2]. The library
*is* sufficient for implementing higher-rank type systems à la
*Peyton Jones, Vytiniotis, Weirich, Shields*, but bear in mind that
unification variables are the metavariables of type inference— not
the type-variables.

## Build Warnings/Errors

This is a simple package and should be easy to install; however, on older setups you may encounter some of the following warnings/errors. If during building you see some stray lines that look like this:

```
mkUsageInfo: internal name? t{tv a7XM}
```

Feel free to ignore them. They shouldn’t cause any problems, even though they’re unsightly. This should be fixed in newer versions of GHC. For more details, see:

```
http://hackage.haskell.org/trac/ghc/ticket/3955
```

If you get a bunch of type errors about there being no `MonadLogic`

instance for `StateT`

, this means that your copy of the logict
library is not compiled against the same mtl that we’re using. To
fix this, update logict to use the same mtl.

## Portability

An effort has been made to make the package as portable as possible.
However, because it uses the `ST`

monad and the mtl-2 package it
can’t be H98 nor H2010. However, it only uses the following common
extensions which should be well supported[^3]:

- Rank2Types
- MultiParamTypeClasses
- FunctionalDependencies - Alas, necessary for type inference.
- FlexibleContexts - Necessary for practical use of MPTCs.
- FlexibleInstances - Necessary for practical use of MPTCs.
- UndecidableInstances - Needed for
`Show`

instances due to two-level types.

## Description

The unification API is generic in the type of the structures being unified and in the implementation of unification variables, following the two-level types pearl of Sheard (2001). This style mixes well with Swierstra (2008), though an implementation of the latter is not included in this package.

That is, all you have to do is define the functor whose fixed-point is the recursive type you’re interested in:

```
-- The non-recursive structure of terms
data S a = ...
-- The recursive term type
type PureTerm = Fix S
```

And then provide an instance for `Unifiable`

, where `zipMatch`

performs one level of equality testing for terms and returns the
one-level spine filled with pairs of subterms to be recursively
checked (or `Nothing`

if this level doesn’t match).

```
class (Traversable t) => Unifiable t where
zipMatch :: t a -> t b -> Maybe (t (a,b))
```

The choice of which variable implementation to use is defined by
similarly simple classes `Variable`

and `BindingMonad`

. We store
the variable bindings in a monad, for obvious reasons. In case it’s
not obvious, see Dijkstra et al. (2008) for benchmarks demonstrating
the cost of naively applying bindings eagerly.

There are currently two implementations of variables provided: one
based on `STRef`

s, and another based on a state monad carrying an
`IntMap`

. The former has the benefit of O(1) access time, but the
latter is plenty fast and has the benefit of supporting backtracking.
Backtracking itself is provided by the logict package and is described
in Kiselyov et al. (2005).

In addition to this modularity, unification-fd implements a number of optimizations over the algorithm presented in Sheard (2001)— which is also the algorithm presented in Cardelli (1987).

- Their implementation uses path compression, which we retain. Though we modify the compression algorithm in order to make sharing observable.
- In addition, we perform aggressive opportunistic observable sharing, a potentially novel method of introducing even more sharing than is provided by the monadic bindings. Basically, we make it so that we can use the observable sharing provided by the modified path compression as much as possible (without introducing any new variables).
- And we remove the notoriously expensive occurs-check, replacing it with visited-sets (which detect cyclic terms more lazily and without the asymptotic overhead of the occurs-check). A variant of unification which retains the occurs-check is also provided, in case you really need to fail fast.
- Finally, a highly experimental branch of the API performs
*weighted*path compression, which is asymptotically optimal. Unfortunately, the current implementation is quite a bit uglier than the unweighted version, and I haven’t had a chance to perform benchmarks to see how the constant factors compare. Hence moving it to an experimental branch.

These optimizations pass a test suite for detecting obvious errors. If you find any bugs, do be sure to let me know. Also, if you happen to have a test suite or benchmark suite for unification on hand, I’d love to get a copy.

## Notes and limitations

[^1]: At present the library does not appear amenable for implementing higher-rank unification itself; i.e., for higher-ranked metavariables, or higher-ranked logic programming. To be fully general we’d have to abstract over which structural positions are co/contravariant, whether the unification variables should be predicative or impredicative, as well as the isomorphisms of moving quantifiers around. It’s on my todo list, but it’s certainly non-trivial. If you have any suggestions, feel free to contact me.

[^2]: At present it is only suitable for single-sorted (aka untyped) unification, à la Prolog. In the future I aim to support multi-sorted (aka typed) unification, however doing so is complicated by the fact that it can lead to the loss of MGUs; so it will likely be offered as an alternative to the single-sorted variant, similar to how the weighted path-compression is currently offered as an alternative.

[^3]: With the exception of fundeps which are notoriously difficult to implement. However, they are supported by Hugs and GHC 6.6, so I don’t feel bad about requiring them. Once the API stabilizes a bit more I plan to release a unification-tf package which uses type families instead, for those who feel type families are easier to implement or use. There have been a couple requests for unification-tf, so I’ve bumped it up on my todo list.