GenericPretty. A Generic, Derivable, Haskell Pretty Printer
===============================================================================
GenericPretty is a haskell library that provides support for automatic
derivation of pretty printing functions on user defined data types.
The output provided by the library functions is identical to that of Prelude.show,
except it has extra whitespace.
I find examples are the best aid in understanding. So, here is a possible
haskell source file, called 'SimpleTest.hs'
----------------------------------------------------
{-# LANGUAGE DeriveGeneric #-}
import Text.PrettyPrint.GenericPretty
data Tree a = Leaf a | Node (Tree a) (Tree a) deriving (Generic)
instance (Out a) => Out (Tree a)
tree1 :: Tree Int
tree1 = Node (Node (Leaf 333333) (Leaf (-555555)))(Node (Node(Node(Leaf 888888)
(Leaf 57575757))(Leaf (-14141414)))(Leaf 7777777))
main = pp tree1
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The flag DeriveGeneric must be given to ghc. This can be done as above,
in a 'LANGUAGE' pragma, or manually by compiling with 'ghc -XDeriveGeneric'.
As can be seen, to use the library one must simply import it, derive 'Generic'
on the custom data type, and write an empty instance of 'Out'.
Then one can use the pretty printing functions, such as 'pp' and 'prettyP'.
Compiling and running the file is simple and gives the following result.
-----------------------------
$ ghc SimpleTest.hs
[1 of 1] Compiling Main ( SimpleTest.hs, SimpleTest.o )
Linking SimpleTest.exe ...
$ SimpleTest
Node (Node (Leaf 333333) (Leaf (-555555)))
(Node (Node (Node (Leaf 888888) (Leaf 57575757))
(Leaf (-14141414)))
(Leaf 7777777))
---------------------------
If we replaced the main function with 'main = prettyP 30 1 tree1',
the result would instead be:
Node (Node (Leaf 333333)
(Leaf (-555555)))
(Node (Node (Node (Leaf 888888)
(Leaf 57575757))
(Leaf (-14141414)))
(Leaf 7777777))
In this case the output tries to remain under 30 characters/line if possible,
while maintaining correct indentation.
------Note---------
The defined pretty printing functions work with any type that implements
'Out' OR 'Outputable'.
This is so that if the data type that is using the deriving mechanism
relies on some other data type that can't, if the inner type implements
'Outputable', then the deriving still works.
-------------------
The above 'Tree' example can be found in 'TestSuite/SimpleTest.hs'.
More involved examples integrated with QuickCheck can be found in 'TestSuite/Tests.hs'.
Further information can be found in the haddock generated API at dist/doc/html
and in the source code itself.
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Please send any questions/suggestions to:
Razvan Ranca <ranca.razvan@gmail.com>