Text.Pretty.Simple
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:
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:
> 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:
Usage
pretty-simple
can be easily used from ghci
when debugging.
When using stack
to run ghci
, just append append the --package
flag to
the command line to load pretty-simple
.
$ stack ghci --package pretty-simple
Once you get a prompt in ghci
, you can use import
to get pretty-simple
’s
pPrint
function in scope.
> import Text.Pretty.Simple (pPrint)
You can test out pPrint
with simple data types like Maybe
or tuples.
> pPrint $ Just ("hello", "goodbye")
Just
( "hello"
, "goodbye"
)
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
function.
- Rainbow Parentheses
- Easy to understand deeply nested data types.
- Configurable Indentation
- Amount of indentation is configurable with the
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 like this:
$ stack ghci --ghci-options "-interactive-print=Text.Pretty.Simple.pPrint" --package pretty-simple
Now, whenever you make GHCi evaluate an expression, GHCi will pretty-print the
result using pPrint
! See
here
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
function can be used to pretty-print JSON.
Recall our example from before.
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
to turn these
data types into JSON. First, you must derive
ToJSON
instances for the data types. It is easiest to do this with Template 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:
> 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:
(You can find the lazyByteStringToString
, putLazyByteStringLn
,
and putLazyTextLn
in the 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. Note that you
must enable the buildexe
flag, since it will not be built by default:
$ stack install pretty-simple-2.2.0.1 --flag pretty-simple:buildexe
When run on the command line, you can paste in the Haskell datatype you want to
be formatted, then hit Ctrl-D:
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 or
PR for any
bugs/problems/suggestions/improvements.