bidirectional-instances: Make instance constraints bidirectional

[ language, library, mit ] [ Propose Tags ]

This package contains class for bidirectional instances and TH methods for generating instances for it. See for more information

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Dependencies base (>= && <4.15), template-haskell [details]
License MIT
Copyright Lev Dvorkin (c) 2022
Author Lev135
Category language
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Uploaded by lev_135 at 2022-09-04T13:47:46Z
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Readme for bidirectional-instances-

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This is a haskell package to provide wrapper class and useful template haskell instance-generator functions for bidirectional instances, i. e. for instance

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, related to this feature and an attempt to implement it as GHC extension. In the discussion about alternatives for that extension this solution was mentioned, 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.


Detailed documentation with examples can be found in the main module (Control.Bidirectional).


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:

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

and -XBidirectionalInstances to make all instances without OVERLAPS/OVERLAPPING/OVERLAPPABLE bidirectional by default.