squeal

# squeal

![squeal-icon](https://raw.githubusercontent.com/morphismtech/squeal/dev/squeal.gif)

[![GithubWorkflowCI](https://github.com/morphismtech/squeal/actions/workflows/ci.yml/badge.svg)](https://github.com/morphismtech/squeal/actions/workflows/ci.yml)

[Github](https://github.com/morphismtech/squeal)

[Hackage](https://hackage.haskell.org/package/squeal-postgresql)

[Stackage](https://www.stackage.org/package/squeal-postgresql)

[YouTube](https://www.youtube.com/watch?v=rWfEQfAaNc4)

## introduction

Squeal is a deep embedding of SQL into Haskell. By "deep embedding", I am abusing the
term somewhat. What I mean is that Squeal embeds both SQL terms and SQL types
into Haskell at the term and type levels respectively. This leads to a very high level
of type-safety in Squeal.

Squeal embeds not just the structured query language of SQL but also the
data manipulation language and the data definition language; that's `SELECT`,
`INSERT`, `UPDATE`, `DELETE`, `WITH`, `CREATE`, `DROP`, and `ALTER` commands.

Squeal expressions closely match their corresponding SQL expressions so that
the SQL they actually generate is completely predictable. They are also highly
composable and cover a large portion of SQL.

## features

* generic encoding of Haskell tuples and records into query parameters
  and generic decoding of query results into Haskell records
  using [`generics-sop`](https://hackage.haskell.org/package/generics-sop)
* access to SQL alias system using the `OverloadedLabels` extension
* type-safe `NULL` and `DEFAULT`
* type-safe SQL constraints `CHECK`, `UNIQUE`, `PRIMARY KEY` and `FOREIGN KEY`
* type-safe aggregation
* escape hatches for writing raw SQL
* [`mtl`](https://hackage.haskell.org/package/mtl) compatible monad transformer
  for executing as well as preparing queries and manipulations
  and [Atkey](https://bentnib.org/paramnotions-jfp.pdf) indexed monad transformer
  for executing definitions.
* linear, pure or impure, one-way or rewindable migrations
* connection pools
* transactions
* views
* array, composite and enumerated types
* json functions and operations
* multischema support
* correlated subqueries
* window functions
* text search
* time functions
* ranges
* indexes
* inlining

## installation

`stack install squeal-postgresql`

## testing

Start postgres on localhost port `5432` and create a database named `exampledb`.
On macOS, you can create the database using `createdb exampledb`.

`stack test`

## contributing

We welcome contributors.
Please make pull requests on the `dev` branch instead of `master`.
The `Issues` page is a good place to communicate.

## usage

Let's see an example!

First, we need some language extensions because Squeal uses modern GHC
features.

```Haskell
>>> :set -XDataKinds -XDeriveGeneric -XOverloadedLabels -XFlexibleContexts
>>> :set -XOverloadedStrings -XTypeApplications -XTypeOperators -XGADTs
```

We'll need some imports.

```Haskell
>>> import Control.Monad.IO.Class (liftIO)
>>> import Data.Int