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pretty-simple

★ 260 Open GitHub ↗

pretty-printer for Haskell data types that have a Show instance

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Text.Pretty.Simple
==================

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`pretty-simple` is a pretty printer for Haskell data types that have a `Show`
instance.

For example, imagine the following Haskell data types and values:

```haskell
data Foo = Foo { foo1 :: Integer , foo2 :: [String] } deriving Show

foo :: Foo
foo = Foo 3 ["hello", "goodbye"]

data Bar = Bar { bar1 :: Double , bar2 :: [Foo] } deriving Show

bar :: Bar
bar = Bar 10.55 [foo, foo]
```

If you run this in `ghci` and type `print bar`, you'll get output like this:

```haskell
> print bar
Bar {bar1 = 10.55, bar2 = [Foo {foo1 = 3, foo2 = ["hello","goodbye"]},Foo {foo1 = 3, foo2 = ["hello","goodbye"]}]}
```

This is pretty hard to read.  Imagine if there were more fields or it were even
more deeply nested.  It would be even more difficult to read.

`pretty-simple` can be used to print `bar` in an easy-to-read format:

![example screenshot](https://raw.githubusercontent.com/cdepillabout/pretty-simple/master/img/pretty-simple-example-screenshot.png)

## Usage

`pretty-simple` can be easily used from `ghci` when debugging.

When using `stack` to run `ghci`, just append the `--package` flag to
the command line to load `pretty-simple`:

```sh
$ stack ghci --package pretty-simple
```

Or, with cabal:

```sh
$ cabal repl --build-depends pretty-simple
```

Once you get a prompt in `ghci`, you can use `import` to get `pretty-simple`'s
[`pPrint`](https://hackage.haskell.org/package/pretty-simple/docs/Text-Pretty-Simple.html#v:pPrint)
function in scope.

```haskell
> import Text.Pretty.Simple (pPrint)
```

You can test out `pPrint` with simple data types like `Maybe` or tuples.

```haskell
> pPrint $ Just ("hello", "goodbye")
Just
    ( "hello"
    , "goodbye"
    )
```

If for whatever reason you're not able to incur a dependency on the `pretty-simple` library, you can simulate its behaviour by using `process` to call out to the command line executable (see below for installation):
```hs
pPrint :: Show a => a -> IO ()
pPrint = putStrLn <=< readProcess "pretty-simple" [] . show
```

There's also a [web app](https://cdepillabout.github.io/pretty-simple), compiled with GHCJS, where you can play around with `pretty-simple` in your browser.

## Features

- Easy-to-read
    - Complex data types are simple to understand.
- Color
    - Prints in color using ANSI escape codes.
    - It is possible to print without color by using the
      [`pPrintNoColor`](https://hackage.haskell.org/package/pretty-simple/docs/Text-Pretty-Simple.html#v:pPrintNoColor)
      function.
- Rainbow Parentheses
    - Easy to understand deeply nested data types.
- Configurable
    - Indentation, compactness, colors and more are configurable with the
      [`pPrintOpt`](https://hackage.haskell.org/package/pretty-simple-1.0.0.6/docs/Text-Pretty-Simple.html#v:pPrintOpt)
      function.
- Fast
    - No problem pretty-printing data types thousands of lines long.
- Works with any data type with a `Show` instance
    - Some common Haskell data types have a `Show` instance that produces
      non-valid Haskell code.  `pretty-simple` will pretty-print even these
      data types.

## Why not `(some other package)`?

Other pretty-printing packages have some combination of these defects:

- No options for printing in color.
- No options for changing the amount of indentation
- Requires every data type to be an instance of some special typeclass (instead
  of just `Show`).
- Requires all `Show` instances to output valid Haskell code.

## Other Uses

### Pretty-print all GHCi output

The `pPrint` function can be used as the default output function in GHCi.

All you need to do is run GHCi with a command like one of these:

```sh
$ stack ghci --ghci-options "-interactive-print=Text.Pretty.Simple.pPrint" --package pretty-simple
```
```sh
$ cabal repl --repl-options "-interactive-print=Text.Pretty.Simple.pPrint" --build-depends pretty-simple
```

Now, whenever you make GHCi evaluate an expression, GHCi will pretty-print the
result using `pPrint`!  See
[here](https://downloads.haskell.org/%7Eghc/latest/docs/html/users_guide/ghci.html#using-a-custom-interactive-printing-function)
for more info on this neat feature in GHCi.

### Pretty-printing JSON

`pretty-simple` can be used to pretty-print any `String` that is similar to
Haskell data types.  The only requirement is that the `String` must correctly
use brackets, parenthese, and braces to indicate nesting.

For example, the
[`pString`](https://hackage.haskell.org/package/pretty-simple/docs/Text-Pretty-Simple.html#v:pString)
function can be used to pretty-print JSON.

Recall our example from before.

```haskell
data Foo = Foo { foo1 :: Integer , foo2 :: [String] } deriving Show

foo :: Foo
foo = Foo 3 ["hello", "goodbye"]

data Bar = Bar { bar1 :: Double , bar2 :: [Foo] } deriving Show

bar :: Bar
bar = Bar 10.55 [foo, foo]
```

You can use [`aeson`](https://hackage.haskell.org/package/aeson) to turn these
data types into JSON.  First, you must derive
[`ToJSON`](https://hackage.haskell.org/package/aeson/docs/Data-Aeson.html#t:ToJSON)
instances for the data types.  It is easiest to do this with Template Haskell:

```haskell
{-# LANGUAGE TemplateHaskell #-}

$(deriveJSON defaultOptions ''Foo)
$(deriveJSON defaultOptions ''Bar)
```

If you run this in `ghci` and type `encode bar`, you'll get output like this:

```haskell
> import Data.Aeson (encode)
> putLazyByteStringLn $ encode bar
{"bar1":10.55,"bar2":[{"foo1":3,"foo2":["hello","goodbye"]},{"foo1":3,"foo2":["hello","goodbye"]}]}
```

Just like Haskell's normal `print` output, this is pretty hard to read.

`pretty-simple` can be used to pretty-print the JSON-encoded `bar` in an
easy-to-read format:

![json example screenshot](https://raw.githubusercontent.com/cdepillabout/pretty-simple/master/img/pretty-simple-json-example-screenshot.png)

(You can find the `lazyByteStringToString`, `putLazyByteStringLn`,
and `putLazyTextLn` in the [`ExampleJSON.hs`](example/ExampleJSON.hs)
file.)

### Pretty-printing from the command line

`pretty-simple` includes a command line executable that can be used to
pretty-print anything passed in on stdin.

It can be installed to `~/.local/bin/` with the following command.

```sh
$ stack install pretty-simple
```

When run on the command line, you can paste in the Haskell datatype you want to
be formatted, then hit <kbd>Ctrl</kbd>-<kbd>D</kbd>:

![cli example screenshot](https://raw.githubusercontent.com/cdepillabout/pretty-simple/master/img/pretty-simple-cli-screenshot.png)

This is very useful if you accidentally print out a Haskell data type with
`print` instead of `pPrint`.

## Contributions

Feel free to open an
[issue](https://github.com/cdepillabout/pretty-simple/issues) or
[PR](https://github.com/cdepillabout/pretty-simple/pulls) for any
bugs/problems/suggestions/improvements.

### Testing

To run the test suite locally, one must install the executables `doctest` and
`cabal-doctest`, e.g. with
`cabal install --ignore-project doctest --flag cabal-doctest`.

Then run the command `cabal doctest`.

## Maintainers

- [@cdepillabout](https://github.com/cdepillabout)
- [@georgefst](https://github.com/georgefst)

pretty-simple

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