Pretty-printingCommon needseditdelete

Pretty-printing is the process of formatting text files (ex. source code) following stylistic conventions which adjust the positioning, spacing, colour, and size of the text to make the content easier for people to view, read, and understand (wikipedia).

Examples

Formatting source code:

int main(){std::cout<<"Hello world!\n";}

int main() {
  std::cout << "Hello world!\n";
}

Formatting Haskell type signatures:

prettySig "example" ["Int", "Bool", "Char", "IO ()"]

-- Output for wide enough formats:
example :: Int -> Bool -> Char -> IO ()

-- Output for narrow formats:
example :: Int
        -> Bool
        -> Char
        -> IO ()

Recommendations

prettyprinter is the most modern pretty-printing library and has excellent documentation, uses text instead of Strings yielding great performance, and has good adoption. The pretty package is the most popular (by number of downloads and reverse deps) but uses String, lacks examples, and has a smaller ecosystem.

boxes is a pretty-printer which allows you to lay out content in 2 dimensions using a simple box model.

Pretty printers for programming languages

Pretty printers (and lexers, parsers, etc.) for a number of popular programming languages can be found in the language-LANG package (language-java for example).

Not included

wl-pprint is outdated and has name clashes with many things in base (<$> for example).

edit description

Pretty-printing is the process of formatting text files (ex. source code) following stylistic conventions which adjust the positioning, spacing, colour, and size of the text to make the content easier for people to view, read, and understand ([wikipedia](https://en.wikipedia.org/wiki/Prettyprint)).

**Examples**

_Formatting source code:_

```cpp
int main(){std::cout<<"Hello world!\n";}
```

```cpp
int main() {
  std::cout << "Hello world!\n";
}
```

_Formatting Haskell type signatures:_

```haskell
prettySig "example" ["Int", "Bool", "Char", "IO ()"]
```

```haskell
-- Output for wide enough formats:
example :: Int -> Bool -> Char -> IO ()

-- Output for narrow formats:
example :: Int
        -> Bool
        -> Char
        -> IO ()
```

# Recommendations

[prettyprinter](@hk) is the most modern pretty-printing library and has excellent documentation, uses [text](@hk) instead of `String`s yielding great performance, and has good adoption. The [pretty](@hk) package is the most popular (by number of downloads and reverse deps) but uses `String`, lacks examples, and has a smaller ecosystem.

[boxes](@hk) is a pretty-printer which allows you to lay out content in 2 dimensions using a simple box model.

# Pretty printers for programming languages

Pretty printers (and lexers, parsers, etc.) for a number of popular programming languages can be found in the `language-LANG` package ([language-java](@hk) for example).

# Not included

[wl-pprint](@hk) is outdated and has name clashes with many things in base (`<$>` for example).

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#

prettyprinter (Hackage)

Wadler/Leijen

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Summary

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Pretty printer loosely based on Wadler's "A Prettier Printer" paper.

Example

let prettyType = align . sep . zipWith (<+>) ("::" : repeat "->")
    prettySig name ty = pretty name <+> prettyType ty
in  prettySig "example" ["Int", "Bool", "Char", "IO ()"]

-- Output for wide enough formats:
example :: Int -> Bool -> Char -> IO ()

-- Output for narrow formats:
example :: Int
        -> Bool
        -> Char
        -> IO ()

Summary

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Pros

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Uses Text instead of String for better performance
Supports annotations (coloring, mouse-over hovers, adding URLs, etc.)
Excellent docs and tutorial
Has compat packages to help migrate from other pretty printing libraries

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Cons

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Ecosystem

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prettyprinter - the core package
prettyprinter-ansi-terminal - a renderer suitable for ANSI terminal output including colors
prettyprinter-compat-wl-pprint a drop-in compatibility layer for previous users of wl-pprint
prettyprinter-compat-ansi-wl-pprint - the same, but for previous users of ansi-wl-pprint
prettyprinter-compat-annotated-wl-pprint - the same, but for previous users of annotated-wl-pprint
prettyprinter-convert-ansi-wl-pprint - a converter, not a drop-in replacement, for documents generated by ansi-wl-pprint

See the README for more information.

Ecosystem

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pretty-simple (Hackage)

other

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Summary

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

Summary

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Pros

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can turn deriving(Show) into something readable automatically
includes a drop-in replacement for Debug.Trace

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Cons

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more limited than prettyprinter if you intend to write your own printers

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Ecosystem

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Ecosystem

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#

pretty (Hackage)

Wadler/Leijen

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Summary

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Pretty printer based on John Hughes' "The Design of a Pretty-printing Library" paper and improved by Simon Peyton Jones.

Summary

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Pros

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Few dependencies (and all of them ship with GHC)

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Cons

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Uses String instead of text which causes poor performance when formatting a large amount of output

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Ecosystem

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Ecosystem

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boxes (Hackage)

box model

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Summary

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Lays out text in two dimensions, using a simple box model.

Example

print_table :: [[String]] -> IO ()
print_table rows = printBox $ hsep 2 left (map (vcat left . map text) (transpose rows))

Mercoxit     335.67
Arkonor      276.15
Bistot       214.30

Summary

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Pros

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Few dependencies (base and split)
Simple model

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Cons

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Limited formatting configuration
No examples/tutorial

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Ecosystem

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Ecosystem

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