https://github.com/lev135/bidirectional-instances

This package contains class for bidirectional instances and TH methods for generating instances for it.
https://github.com/lev135/bidirectional-instances

haskell language

Last synced: about 2 months ago
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This package contains class for bidirectional instances and TH methods for generating instances for it.

• Host: GitHub
• URL: https://github.com/lev135/bidirectional-instances
• Owner: Lev135
• License: mit
• Created: 2022-09-01T18:31:11.000Z (over 2 years ago)
• Default Branch: master
• Last Pushed: 2022-09-04T16:45:36.000Z (over 2 years ago)
• Last Synced: 2024-04-23T22:55:22.673Z (10 months ago)
• Topics: haskell, language
• Language: Haskell
• Homepage:
• Size: 46.9 KB
• Stars: 2
• Watchers: 1
• Forks: 0
• Open Issues: 1
• Metadata Files:
  • Readme: README.md
  • Changelog: CHANGELOG.md
  • License: LICENSE

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README

        
bidirectional-instances

===

This is a haskell package to provide wrapper class and useful template haskell

instance-generator functions for bidirectional instances, i. e. for instance

```hs

instance (A a, B b) => C (Foo a b)

```

deduce not only `forall a b. (A a, B b) => C (Foo a b)`, but also

`forall a b. C (Foo a b) => A a` and `forall a b. C (Foo a b) => B b`.

This behavior doesn't implemented in modern GHC version, but its very intuitive

(and, of course, sound) if there are no overlapping instances for `C (Foo a b)`.

There was a [paper](https://arxiv.org/pdf/1906.12242.pdf), related to this 

feature and an attempt to 

[implement it as GHC extension](https://github.com/KoenP/ghc-proposals/blob/patch-1/proposals/0000-bidirectional-instances.md).

In the discussion about alternatives for that extension this solution 

[was mentioned](https://github.com/KoenP/ghc-proposals/blob/patch-1/proposals/0000-bidirectional-instances.md#using-constraint-kinds),

but was discarded as "decidedly nontrivial, and some boilerplate is still 

required, which needs to be written for every instance".

To reduce these downsides, instances from this package are made more general

and template haskell function for generating boilerplate code are provided.

I suppose that implementation this as an compiler extension probably would be 

better for performance, but I don't have enough qualification to implement it.

Documentation

---

Detailed documentation with examples can be found in the main module 

*(Control.Bidirectional)*.

Usage

---

All function and classes you need to work with are exported from 

`Control.Bidirectional`.

To be implemented somewhere in the future

---

I'll try to make a GHC plugin to improve this solution in two directions:

- no need for template haskell

- more intuitive syntax

More precisely, I'd like to have the following syntax for declaring 

bidirectional instances:

```hs

instance {-# BIDIRECTIONAL #-} (A a, B b) => C (Foo a b)

```

and `-XBidirectionalInstances` to make all instances without 

`OVERLAPS`/`OVERLAPPING`/`OVERLAPPABLE` bidirectional by default.