Robust prettyprinter for output of auto-generated Show instances

Version on this page:
LTS Haskell 14.27:
Stackage Nightly 2019-09-21:
Latest on Hackage:

See all snapshots show-prettyprint appears in

BSD-3-Clause licensed and maintained by David Luposchainsky <dluposchainsky (λ) google>
This version can be pinned in stack with:show-prettyprint-,1723

Module documentation for

Prettyprint Show output

Output of nested data structures by Show instances is often very hard to read. This package offers a simple function to insert line breaks and indentation into that ouput so that the semantics are unchanged, but makes it much easier to read.

The package does not rely on a parser for actual Haskell; instead, it merely reacts on parentheses, commas and the like. This makes it fairly robust even in the face of invalid Show instances, that may not produce valid Haskell code.



Hello Foo ("(Bar", Haha) (Baz (A { foo = C, bar = D, qux = (E,"He)llo World!",G,
    H,[A,B,c,d,e,Fghi]) } ) (B,C) [Baz A1 B2, (Baz A3 (B4)), (Baz A5 (B6)), (Baz
    (A7) B8)]) (Foo) (Bar) (Baz (A) (B))


Hello Foo ("(Bar",Haha)
          (Baz (A {foo = C
                  ,bar = D
                  ,qux = (E
                         ,"He)llo World!"
               [Baz A1 B2
               ,(Baz A3 (B4))
               ,(Baz A5 (B6))
               ,(Baz (A7) B8)])
          (Baz (A) (B))

Inspired by a real AST

Set  (fromList [(Name "A string with (parenthesis",Ann  (Entry (Quality 1 1)
    (Ann  False) (Ann  (Map [Ann  (Bound (Ann  (Id "lorem"))),Ann  (Variable
    (Ann  (Id "ipsum")))])))),(Name "string",Ann  (Entry (Quality 1 1) (Ann
    True) (Ann  (Internal (Ann  (Reduce (Ann  (Id "dolor")) (Ann  (Id "sit")))))
    ))),(Name "Another } here",Ann  (Entry (Quality 1 1) (Ann  (Or [Ann  (Not
    (Ann  (Is (Ann  Flagged) (Ann  Type) (Ann  (Multi [Ann  (Literal (Ann  One))
    ]))))),Ann  (Is (Ann  Flagged) (Ann  Type) (Ann  (Multi [Ann  (Literal (Ann
    Three))]))),Ann  (Is (Ann  Flagged) (Ann  Type) (Ann  (Multi [Ann  (Literal
    (Ann  Two))])))])) (Ann  (Internal (Ann  (Concat (Ann  (Id "amet"))))))))])


Set (fromList [(Name "A string with (parenthesis"
               ,Ann (Entry (Quality 1 1)
                           (Ann False)
                           (Ann (Map [Ann (Bound (Ann (Id "lorem")))
                                     ,Ann (Variable (Ann (Id "ipsum")))]))))
              ,(Name "string"
               ,Ann (Entry (Quality 1 1)
                           (Ann True)
                           (Ann (Internal (Ann (Reduce (Ann (Id "dolor"))
                                                       (Ann (Id "sit"))))))))
              ,(Name "Another } here"
               ,Ann (Entry (Quality 1 1)
                           (Ann (Or [Ann (Not (Ann (Is (Ann Flagged)
                                                       (Ann Type)
                                                       (Ann (Multi [Ann (Literal (Ann One))])))))
                                    ,Ann (Is (Ann Flagged)
                                             (Ann Type)
                                             (Ann (Multi [Ann (Literal (Ann Three))])))
                                    ,Ann (Is (Ann Flagged)
                                             (Ann Type)
                                             (Ann (Multi [Ann (Literal (Ann Two))])))]))
                           (Ann (Internal (Ann (Concat (Ann (Id "amet"))))))))])


  • Fix dependencies of Doctest suite


  • Fix lists not being prettified at the top level (#6)
  • Fix lists in nested data structures not being aligned (#6)


  • Fix escaping of backslashes (#5)
  • Fix a bug with ambiguous types on newer cabal new-repl runs (#4)


Add functions to prettify to Doc instead of just supporting String,

prettifyToDoc :: String -> Doc ann
prettyShowDoc :: Show a => a -> Doc ann

Tagged the wrong version as 0.2 on Github. Releasing a new version with an updated tag to remedy this.


Prettyprint based on the prettyprinter library, instead of ansi-wl-pprint. To support the Diagnostic module, the Trifecta-generated Doc has to be rendered still, so we cannot drop the dependency on ansi-wl-pprint just yet.