semigroupoids-do

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 = S.do
  ...

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 LICENSE.md file.