An extra blank line in a vcat in the wl-pprint package

Question

I use wl-pprint package, because the standard PrettyPrinter lacks functionality. All is good, except an empty doc in a vcat function (the same thing with <$> combinator).

Correct behavior:

import Text.PrettyPrint
> vcat[text "a", empty, text "b"]   
a
b

wl-pprint shows an extra blank line:

import Text.PrettyPrint.Leijen
> vcat[text "a", empty, text "b"]   
a

b

So what can I do? It is imposible to filter vcat list, because there is no Eq instance for Doc.

Hesta

looks like the lib could use an isEmpty function, or somesuch thing. Contact the package author, and possibly send him a patch that will work for you. – Fayola

Answer 1

Because I hadn't have any better ideas, I made the following changes in the source

(<$$>) :: Doc -> Doc -> Doc
x <$$> Empty    = x                   -- <<< added
Empty <$$> y    = y                   -- <<< added
x <$$> y        = x <> linebreak <> y

Hesta

Answer 2

define vcatSoft = fold <//>

The docs say "The document (vcat xs) concatenates all documents xs vertically with ().", and looking at <$$>, it says it "concatenates document x and y with a linebreak in between." But notice the next function, <//>, which uses a softbreak instead. And looking at the defn of vcat, it's simply vcat = fold <$$>, so define a function = fold <//>.

Kawasaki

This function already exists (fillCat), but it returns “ab” inline – Hesta

Argh, I knew I should've tested this. Reading more closely: Although empty has no content, it does have a 'height' of 1 and behaves exactly like (text "") (and is therefore not a unit of <$>). – Kawasaki