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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