This is tricky design. Any instance in the composition which doesn't define showsPrecUnicode will ruin formatting of inner Strings. Maybe it's not as bad as in aeson* as most Show instances are derived. (But not Show1 etc.) * The problem with aeson is ToJSON class having toValue (old method), and toEncoding, which is fast but which default implementation is using slower toValue. Thus `instance ToJSON MyType` derived generally silently ruins the performance. It might be better to not define default implementation for showsPrecUnicode (cause most instances are derived), as though it will break explicitly written instances, but fixing them is straigh-forward. (GHC developers use head.hackage will hate this option though). I'm not sure this is any better then separate class, and whether two type-classes (either actually or two-in-one) is a good idea. - Oleg On 11.7.2021 7.01, Viktor Dukhovni wrote:
On Thu, Jul 08, 2021 at 06:11:28PM +0800, Kai Ma wrote:
It's proposed here to change the Show instance of String, to achieve the following output:
ghci> print "Hello, 世界” "Hello, 世界”
ghci> print "Hello, мир” "Hello, мир”
ghci> print "Hello, κόσμος” "Hello, κόσμος”
ghci> "Hello, 世界” “Hello, 世界”
ghci> "😀” “😀" Another possibility is to extend the `Show` class with two new methods and their default implementations:
class Show where ... showsPrecUnicode :: Int -> a -> ShowS showsPrecUnicode = showsPrec
showListUnicode :: [a] -> ShowS showListUnicode = showList
showUnicode :: Show a => a -> String showUnicode x = showsPrecUnicode 0 x ""
at which point a small number of classes can override `showUnicode` and `showListUnicode`:
instance Show a => Show [a] where showsPrec _ = showList showsPrecUnicode = showListUnicode
instance Show Char where showsPrecUnicode = ... -- Unicode char showListUnicode = ... -- Unicode string
instance Show Text where showsPrecUnicode = ... -- Unicode text
Once these are implemented, "ghci" can be modified to instead used `showUnicode`, rather than `show`, with no new incompatibilities elsewhere.
We can also introduce `uprint = putStrLn . showUnicode`, ...
This would still require explicit opt-in to use the Unicode show, but it would be available for all `Show` instances, and used by default in "ghci".
It would still be a good idea to implement `Render`, which is a related but separate concern.