https://github.com/sheyll/pretty-types

A small pretty printing DSL for complex types.
https://github.com/sheyll/pretty-types

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A small pretty printing DSL for complex types.

• Host: GitHub
• URL: https://github.com/sheyll/pretty-types
• Owner: sheyll
• License: other
• Created: 2016-08-11T03:26:15.000Z (over 9 years ago)
• Default Branch: master
• Last Pushed: 2021-02-04T12:28:09.000Z (about 5 years ago)
• Last Synced: 2025-10-21T13:38:35.469Z (5 months ago)
• Language: Haskell
• Size: 39.1 KB
• Stars: 8
• Watchers: 1
• Forks: 0
• Open Issues: 1
• Metadata Files:
  • Readme: README.md
  • Changelog: ChangeLog.md
  • License: LICENSE

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# Type Pretty Printing

A tiny eDSL, not unlike `ErrorMessage` for `TypeError`s to pretty print types,

especially uninhabited types.

This Example will render this table:

    +-------+-----+------------+

    |  col 1|col 2|       col 3|

    +-------+-----+------------+

    |   2423|  451|       21234|

    | 242322|   42|         n/a|

    |      0| 4351|      623562|

    |   4351|  n/a|         n/a|

    |      0| 4351|      623562|

    +-------+-----+------------+

From this code:

    module Main (main) where

    import Data.Type.Pretty

    import GHC.TypeLits

    import Data.Kind

    import Data.Proxy

    main :: IO ()

    main = putStrLn $ showPretty (Proxy :: Proxy TestTable)

    type TestTable =

      'MyTable         '[MyCol "col 1" 7, MyCol "col 2" 5, MyCol "col 3" 12]

              '[ MyRow '[2423           ,451             ,21234]

               , MyRow '[242322         ,42]

               , MyRow '[0              ,4351            ,623562]

               , MyRow '[4351]

               , MyRow '[0              ,4351            ,623562]

               ]

    data MyTable = MyTable [Type] [Type]

    data MyCol :: Symbol -> Nat -> Type

    data MyRow :: [Nat] -> Type

    type instance ToPretty ('MyTable cols rows) =

               PrettyManyIn (PutStr "+") (RowSepLine cols)

          <$$> PrettyManyIn (PutStr "|") (TableHeading cols)

          <$$> PrettyManyIn (PutStr "+") (RowSepLine cols)

          <$$> PrettyHigh   (TableRows cols rows)

          <$$> PrettyManyIn (PutStr "+") (RowSepLine cols)

    type family TableHeading (cols :: [Type]) :: [PrettyType]

    type instance TableHeading '[] = '[]

    type instance TableHeading (MyCol title width ': r) =

      PutStrW width title  ': TableHeading  r

    type family

       RowSepLine (cols :: [Type]) :: [PrettyType] where

       RowSepLine '[] = '[]

       RowSepLine (MyCol title width ': r) =

         PrettyOften width (PutStr "-") ': RowSepLine  r

    type family

      TableRows (cols :: [Type]) (rows :: [Type]) :: [PrettyType] where

      TableRows cols '[] = '[]

      TableRows cols (MyRow cells ': rest ) =

        PrettyManyIn (PutStr "|") (TableCells cols cells) ': TableRows cols rest

    type family

      TableCells (cols :: [Type]) (cells :: [Nat]) :: [PrettyType] where

      TableCells '[] cells = '[]

      TableCells (MyCol title width ': cols) (value ': cells) =

        PutNatW width value ':  TableCells cols cells

      TableCells (MyCol title width ': cols) '[] =

        PutStrW width "n/a" ':  TableCells cols '[]