prettyprinter-ansi-terminal: There should be a way to reset a color/style to terminal default

[fmthoma]

Currently, it's possible to keep some colors and override others (e.g. keep foreground color, but change background color), but it's impossible to revert to terminal default for one or the other. This is particularly inconvenient when trying to use a »default« color like black or white in a color scheme that is required to work for both dark and light backgrounds.

This behavior is encoded in the type AnsiStyle:

data AnsiStyle = SetAnsiStyle
    { ansiForeground  :: Maybe (Intensity, Color) -- ^ Set the foreground color, or keep the old one.
    , ansiBackground  :: Maybe (Intensity, Color) -- ^ Set the background color, or keep the old one.
    , ansiBold        :: Maybe Bold               -- ^ Switch on boldness, or don’t do anything.
    , ansiItalics     :: Maybe Italicized         -- ^ Switch on italics, or don’t do anything.
    , ansiUnderlining :: Maybe Underlined         -- ^ Switch on underlining, or don’t do anything.
    } deriving (Eq, Ord, Show)

Instead of Maybe a, this should be a ternary Keep | Default | SetTo a just like vty's Attr does it with its MaybeDefault type.

[quchen]

Makes sense!

[fmthoma]

Actually, I just realized that due to the design of the prettyprinter, this is not even necessary: The behaivor is actually to reset any style before applying the new style, so mempty is already the default style.

So it's really the converse that's missing, the possibility to keep a style from the previous annotation. Which is probably not even desirable, as it drags around context we probably don't want to have most of the time.

So the real issue I just had is with documentation:

ansiForeground :: Maybe (Intensity, Color) -- ^ Set the foreground color, or keep the old one.

really should be »[…] or keep the default one«.

[quchen]

»Or keep the old one (or the default if there was none)« sounds a bit verbose though, no? Deleting the »old« would swallow the fact that blue <> red is blue.

[fmthoma]

The fact that blue <> red == blue also somehow contradicts the »set the foreground color« part – adding red on top of blue does not set the color to red, it will stay blue.

[fmthoma]

The terminology of »keeping« and »setting« a color lives on two different levels: The level of AnsiStyle's Semigroup instance (where mempty keeps whatever was set previously or after), and the level of styling a Doc (where mempty actually reverts everything to default).

[quchen]

Closing since we resolved this by talking about it, or am I mistaken?

[banacorn]

I think the documentation could be rephrased better, perhaps with the word "reset" mentioned somewhere.

Update: I came across this issue because I want to find a way to replace the semantics for the Reset constructor in the ansi-terminal package.