Optional spaces and multiple alternatives in wl-pprint-extras

问题

Is there a good/accepted way to get more than two alternative layouts, specifically in a way which supports optional spaces?

In particular, I might want to print a list in one of three ways:

[1, 2, 3, 4, 5]

[1,2,3,4,5]

[ 1
, 2
, 3
, 4
, 5
]

in that order of preference.

回答1:

You can try one alternatives before another with group and flatAlt.

(<|>) :: Doc e -> Doc e -> Doc e
a <|> b = group $ flatAlt b a

infixl 5 <|>

We'll define your three examples.

{-# LANGUAGE OverloadedStrings #-}

import Text.PrettyPrint.Free
import System.IO

items = map char ['1'..'5']

widest = brackets . hcat . punctuate ", " $ items
wide   = brackets . hcat . punctuate ","  $ items
skinny = brackets . hcat . punctuate "," . map (\x -> " " <> x <> linebreak) $ items

The widest of these that fits on the screen, widestFits, can be defined in terms of <|> which we defined above.

widestFits = widest <|> wide <|> skinny

We will try pretty printing these on three documents, one wide enough to hold the widest example, one only wide enough to hold the wide example, and one that can't hold either.

main = do
    displayIO stdout $ renderPretty 1.0 15 widestFits
    putStrLn ""
    displayIO stdout $ renderPretty 1.0 14 widestFits   
    putStrLn "" 
    displayIO stdout $ renderPretty 1.0 10 widestFits
    putStrLn "

This results in the desired output.

[1, 2, 3, 4, 5]
[1,2,3,4,5]
[ 1
, 2
, 3
, 4
, 5
]

来源：https://stackoverflow.com/questions/28439182/optional-spaces-and-multiple-alternatives-in-wl-pprint-extras