[Haskell-cafe] Re: Meaning of "ribbonsPerLine" at Text.PrettyPrint.HughesPJ ?

[Simon Peyton-Jones]

Benedikt

Despite the name, neither I nor John Hughes are actively maintaining this library, so if you have got some better implementations of 'cat' and 'sep', do please submit a patch. That's how it'll improve.

Do test carefully!  Preferably add some Quickcheck tests too.

Thanks

Simon

| -----Original Message-----
| From: haskell-cafe-bounces at haskell.org [mailto:haskell-cafe-
| bounces at haskell.org] On Behalf Of Benedikt Huber
| Sent: 19 June 2008 13:31
| To: Evan Laforge
| Cc: haskell
| Subject: [Haskell-cafe] Re: Meaning of "ribbonsPerLine" at
| Text.PrettyPrint.HughesPJ ?
|
| Evan Laforge schrieb:
| >> byorgey: fons: I can't explain it, all I know is that you must set it
| >> to 1 or else it does bizarre things
| >> fons: hahah, ok
| >> fons: byorgey: that's funny considering its default value is 1.5
| >> byorgey: if you set it to 1 then lineLength means what you think it should
| >> byorgey: fons: EXACTLY
| >
| > Excellent, thanks for solving a nagging problem I couldn't be bothered
| > to track down.  I was wondering why my pretty printing was a little
| > messed up and slightly too wide.
| >
| > And isn't 100 columns a bit non-standard for a default?  I thought 80
| > columns had more traction?  I know that's what my terminals are at...
|
| Hi,
|
| The "ribbon length" is used when choosing the most beautiful layout:
| I'll just summarize the relevant section from John Hughes paper
| (http://www.cs.chalmers.se/~rjmh/Papers/pretty.ps), which explains it
| very nicely:
|
| "... Using [the criterion whether the text fits on the page] alone tends
| to produce layouts such as
|
|  > for i = 1 to 100; for j=1 to 100; for k=1 to 100; a[i][j][k]:=0;
|
| which fits on a page {==> line-width} but cannot be described as pretty.
| We therefore impose an additional constraint limiting the number of
| characters on each line [==> ribbon-width} [...]
|
|  > for i = 1 to 100
|  >     for j = 1 to 100
|  > ...
| "
|
| So the pretty printer tries to avoid sequences (ribbons) of characters
| which are longer than ribbon_length, when using auto layout stuff like
| `sep'.
|
| In the source code, we have (paraphrased)
|
|  > ribbon_length = line_length / ribbonsPerLine
|
| and
|
|  > choose_nicest_layout indent p q =
|  >   if p + indent fits into line_length and p fits into ribbon_length
|  >    then p
|  >    else q
|
| Working example below.
|
| I'm not sure 80 characters is still standard when _pretty_-printing -
| the longest line in Text.PrettyPrint.HughesPJ is 109 characters wide ;)
|
| Setting the ribbon ratio to 1 essentially disables the ribbon feature.
|
| Btw: while studying the source code, I also found the cat (and sep) can
| be implemented in a more space efficient way (currently, cat needs to
| evaluate every document in a list to yield some output). Does this make
| sense (see below) ?
|
| cheers,
|
| benedikt
|
|
| -- * ribbon example
|
|  > -- lineLength = 26, ribbonsPerLine = 1.5 ==> ribbonLength = 17
|  > -- therefore, we have a line break if width-indent > 17 or width > 26
|  > testStyle = Style { mode = PageMode,
|  >                    lineLength = 26,
|  >                    ribbonsPerLine = 1.5 }
|  > ribbonTest = renderStyle testStyle $
|  >
|  >   -- use hsep as width == 17 <= ribbonLength
|  >       sep [ txt 5, txt 11 ]
|  >
|  >   -- linebreak, as width-indent = width = 18 > ribbonLength
|  >   $+$ sep [ txt 5, txt 12 ]
|  >
|  >   -- use hsep, as width - indent == 17, and width == 22 < lineLength
|  >   $+$ sep (map (nest 5) $ [txt 5, txt 11] )
|  >
|  >   -- linebreak, as width would be 27 > lineLength
|  >   $+$ sep (map (nest 10) $ [txt 5, txt 11] )
|  >
|  > txt :: Int -> Doc
|  > txt 0 = text ""
|  > txt k = text $
|  >  let ks = show k in
|  >   (replicate (k - (length ks)) '_') ++ ks
|
|
| -- * lazy variants of vcat and hcat
| --   you need the constructors from the HughesPJ module
|
|  > vcat' = foldAbove . foldr vcomp2 empty
|  > hcat' = foldBeside . foldr hcomp2 empty
|  >
|  > foldAbove :: Doc -> Doc
|  > foldAbove (Above Empty _ d2) = d2
|  > foldAbove (Above d1 f d2) = Above d1 f $ foldAbove d2
|  > foldAbove doc = doc
|  >
|  > vcomp2 :: Doc -> Doc -> Doc
|  > vcomp2 d1 Empty = d1
|  > -- do not match `vcomp2 Empty d1' !
|  > vcomp2 d1 d2 = Above d1 False d2
|  >
|  > foldBeside :: Doc -> Doc
|  > foldBeside (Beside Empty _ d2) = d2
|  > foldBeside (Beside d1 f d2) = Beside d1 f $ foldBeside d2
|  > foldBeside doc = doc
|  >
|  > hcomp2 :: Doc  -> Doc -> Doc
|  > hcomp2 p Empty = p
|  > hcomp2 p q = Beside p False q
|
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