Hi,

    https://jyp.github.io/posts/towards-the-prettiest-printer.html

It says:

Wadler’s design fares somewhat better. It does not suffer from the above problem… by default. That is, it lacks the capability to express that sub-documents should be vertically aligned — compositionally.

• Objection 2: Leijen’s extension of Wadler’s design solves the issue: it provides an align combinator.

A package based on (a later version of) the design in this article is available here:

    https://hackage.haskell.org/package/pretty-compact

This claims to be more ideal ("Prettiest") than either the Hughes ("Pretty") or Wadler ("Prettier") printers.  I think it uses dynamic programming to avoid being too slow.  If I understand correctly, GHC internally uses a version of the Hughes pretty printer, not the Wadler-Leijen one.

2. Doesn't the wl-print package already have a `nest` combinator?

    https://hackage.haskell.org/package/wl-pprint-1.2.1/docs/Text-PrettyPrint-Leijen.html

It also has the `align` combinator.  If I remember correctly, these are part of the Leijen extension to Wadler.  Are these not enough to get the behavior that you want?

3. Have you seen hindent?  It has a module called HIndent.Pretty that might be relevant to laying out Haskell source.

-BenRI

Dear Cafe,

I was looking for a way to pretty-print Haskell literals
(with lists, tuples, records with named and positional notation)
like this example

( Leftist
    { tree = Branch
        { left = Branch { left = Leaf, key = 4, right = Leaf }
        , key = 3
        , right = Leaf
        }
    , refs = listToFM [ ( Ref 13, [ 0 ] ), ( Ref 17, [ ] ) ]
    }
, [ Any, Any ]
)

for each sub-structure, the indentation level
(for the following lines) should increase - by a _fixed_ amount.
in the above example: line break after "tree = Branch".
But (missing from this example), line break _before_
the list starts in "{ foo = [ 42 , ... ] ... }".

I found this impossible to do with wl-pprint
but perhaps I did not try hard enough.


Instead, I "invented" combinators `nest` and `skip`
and made this prototypical implementation
https://gitlab.imn.htwk-leipzig.de/autotool/all0/-/blob/master/todoc/src/Text/PrettyPrint/Dent.hs  (it has some explanatory text at the top)
see also https://gitlab.imn.htwk-leipzig.de/autotool/all0/-/issues/960

but certainly this cannot be a new idea.


While I do like the semantics (in the context of my application),
I don't like the performance of my implementation.
What am I doing wrong?
It's just updating indentation level and current position,
this should not take any time at all?

Of course, it would be best if I don't need the implementation at all -
if the effect could be achieved via some combinators in
established libraries (that have optimized implementation).

Any pointers appreciated.


Best regards - J.
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