# `semigroupoids-do`
## What is this thing?
With GHC 9, we finally got the
[QualifiedDo](https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/qualified_do.html)
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`](https://hackage.haskell.org/package/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`](https://hackage.haskell.org/package/semigroupoids-5.3.5/docs/Data-Functor-Bind.html#t:Bind).
* With `ApplicativeDo`, in some cases, you can use do-notation when you only
have an
[`Apply`](https://hackage.haskell.org/package/semigroupoids-5.3.5/docs/Data-Functor-Bind.html#t:Apply),
subject to the same conditions as using `Applicative` for do-notation.
* You can use refutable patterns when you only have
[`Plus`](https://hackage.haskell.org/package/semigroupoids-5.3.5/docs/Data-Functor-Plus.html#t: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?
```haskell
{-# 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`](https://hackage.haskell.org/package/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`](https://spdx.org/licenses/Apache-2.0.html)). For more details,
please see the `LICENSE.md` file.