RGB Terminal Colors with Haskell and Brick

Question

I know that the Brick and the VTY hackage do not support escape sequences. VTY only supports 240 colors.

Is there any workaround to use true RGB colors and not mess up the layout?

This is an example I made, but I can't get the border right:

module BrickTest where

import Brick                (simpleMain, Widget, str)
import Brick.Widgets.Border (border)
import Text.Printf          (printf)

main :: IO ()
main = simpleMain $ colorWidget (255, 0, 0)

type RGB = (Int, Int, Int)

colorWidget :: RGB -> Widget ()
colorWidget (r, g, b) = border $ str (prefix ++ "a" ++ postfix)
    where
        prefix = printf "\ESC[38;2;%d;%d;%dm" r g b
        postfix = "\ESC[0m"

output:

┌──────────────────┐
│a│
└──────────────────┘

Brick supports ANSI terminal escape codes, and Vty as well: hackage.haskell.org/package/vty-5.29/docs/… (this is used in Brick with hackage.haskell.org/package/brick-0.55/docs/…). The ISO colors are used exactly to get rid of the programmer specifying ANSI terminal escape sequences themselves. — Willem Van Onsem
@WillemVanOnsem but only 240 colors are supported and not 256^3 colors, which is supported by most modern terminal applications. — birneee
When you can add your own escape codes, you can tell the terminal to do whatever it supports. — Carl
@Carl escape codes can be inserted, but it always messes up the layout. Because the mentioned hackages do not support escape sequences. That's why I try to find a workaround. — birneee

birneee birneee · Accepted Answer · 2020-08-31T17:47:41

I found a workaround. I managed to implement a function zeroWidthStr that can print any string, and Brick handles it as if it has width 0. But I can't really explain how this is working internally, and it might have some other side effects.

module BrickTest where

import           Brick                       (Widget, raw, simpleMain, str,
                                              (<+>))
import           Brick.Widgets.Border        (border)
import           Data.List                   (intercalate)
import           Data.Text.Lazy              (pack)
import           Graphics.Vty                (defAttr)
import           Graphics.Vty.Image.Internal (Image (HorizText))
import           Text.Printf                 (printf)

main :: IO ()
main = simpleMain $ colorWidget (255, 0, 0)

type RGB = (Int, Int, Int)

colorWidget :: RGB -> Widget ()
colorWidget (r, g, b) = border $ prefix <+> str "a" <+> postfix
    where
        prefix = zeroWidthStr $ printf "\ESC[38;2;%d;%d;%dm" r g b
        postfix = zeroWidthStr $ "\ESC[0m"

zeroWidthStr :: String -> Widget ()
-- | workaround to print any string in terminal, and hackage Brick (vty) handles it as if it has width 0
zeroWidthStr str = raw image
    where
        image = HorizText defAttr (pack modStr) 0 0
        modStr = str ++ repeatN "\ESC\ESCa" (length str)
        repeatN :: String -> Int -> String
        repeatN str n = intercalate "" $ take n $ repeat str

output:

RGB Terminal Colors with Haskell and Brick

1 Answers