semigroupoids-do: Support for QualifiedDo with semigroupoids classes.

[ apache, library, semigroupoids ] [ Propose Tags ]

Provides support for QualifiedDo using the more generalized type classes from the semigroupoids package.

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Versions [RSS] 1.0
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Dependencies base (>=4.15 && <5), semigroupoids (>=5.3.5 && <5.4) [details]
License Apache-2.0
Copyright (C) Koz Ross 2021
Author Koz Ross
Category Semigroupoids
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Uploaded by koz_ross at 2021-09-26T05:09:46Z
Distributions NixOS:1.0
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Readme for semigroupoids-do-1.0

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What is this thing?

With GHC 9, we finally got the QualifiedDo extension, allowing us to overload do-notation in a convenient way. This has a range of uses; in our case, we are interested in being able to use do-notation in a more general context than would normally be possible.

semigroupoids as a package is designed to generalize certain common type classes, such as Applicative and Monad. This can be convenient: for example, something could be an Apply or Bind, but not an Applicative or Monad if no sensible definition for pure exists. However, without the convenience of do-notation, such instances mean significantly less, as they are awkward to use. This is an unnecessary restriction at least in this example, as nothing about do-notation requires pure.

In this package, we provide a set of definitions suitable for use with QualifiedDo, generalizing all the necessary operations to their semigroupoids equivalents. Specifically:

  • It is now possible to use do-notation when you only have a Bind.
  • With ApplicativeDo, in some cases, you can use do-notation when you only have an Apply, subject to the same conditions as using Applicative for do-notation.
  • You can use refutable patterns when you only have Plus.

What are the goals of this project?

Our only goal is to provide QualifiedDo support for semigroupoid generalizations; no more, no less.

How do I use this?

{-# LANGUAGE QualifiedDo #-}

module MyModule where

import Data.Functor.Bind (Bind)
import qualified Semigroupoids.Do as S

foo :: (Bind m) => m a
foo =

Wait, haven't I seen this somewhere before?

You are probably thinking of semigroupoids-syntax. It is true that we take inspiration from it. However, we differ from it in a couple of ways:

  • We only support the subset of syntax modification allowed by QualifiedDo; semigroupoids-syntax goes beyond this, supplying a range of RebindableSyntax capabilities.
  • semigroupoids-syntax hasn't been updated since 2014.
  • semigroupoids-syntax has support going back to much older GHCs; we limit ourselves only to GHCs that have QualifiedDo.

In short, this is a maintained, small, and simple package, to do only one task.

What does this run on?

We support (and CI check) only those versions of GHC which support QualifiedDo. Currently, that list is:

  • 9.0.1

We check on the following platforms:

  • Windows
  • Linux
  • MacOS

What can I do with this?

The project is licensed Apache 2.0 (SPDX code Apache-2.0). For more details, please see the file.