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:
We check on the following platforms:
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.
