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It would be nice to have GHC-style colored error and warning messages:

Emitting these requires using an ANSI color library, for example ansi-terminal.

It would, however, be preferable to have color support directly in the pretty printing library rather than just including raw ANSI escape sequences as text. There are a bunch of options here, but none of them really are a fitting 'drop-in-replacement' for the currently used pretty printer:

• Hughes/PJ pretty printers:
  • pretty is currently used and does not support ANSI coloring
• Wadler/Leijen pretty printers:
  • ansi-wl-pprint has ANSI coloring support, but may get deprecated
  • ansi-pretty is an extension of ansi-wl-pprint
  • prettyprinter is modern and supports ANSI coloring, but uses Data.Text instead of String

Additionally, Wadler/Leijen pretty printers have a lot of subtle differences in their API compared to Hughes/PJ printers, thereby complicating a potential migration. This includes many of them providing their own Pretty class.