#669: negative indentation in Text.PrettyPrint.HughesPJ
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Reporter: [EMAIL PROTECTED] | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: hslibs/text | Version: 6.4.1
Severity: minor | Keywords:
Os: Unknown | Difficulty: Unknown
Architecture: Unknown |
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The pretty printing library has a long-standing bug:
it sometimes creates negative indentation.
Currently, the symptom is cured, but the cause is unknown.
I tried to dig a bit into the code, and here's what I found.
short summary: the law
{{{
(n6) x <> nest k y = x <> y, if x non-empty
}}}
seems broken by the implementation.
test case:
this seems OK:
{{{
*Main> text "foo" <> nest 20 ( nest 20 ( text "foo" ) $$ text "bar" )
Beside (TextBeside (Str "foo") 3 Empty) False
(Nest 40
(TextBeside (Str "foo") 3
(NilAbove (Nest -23 (TextBeside (Str "bar") 3
Empty)))))
}}}
but thes seems wrong (the Nest 40 is gone but the Nest -23 is kept,
this will lead to negative indentation later on)
{{{
*Main> reduceDoc $ text "foo" <> nest 20 ( nest 20 ( text "foo" ) $$ text
"bar" )
TextBeside (Str "foo") 3
(TextBeside (Str "foo") 3
(NilAbove (Nest -23 (TextBeside (Str "bar") 3
Empty))))
}}}
Now the formatter thinks that he has to indent by 3 + 3 - 23 = -17 spaces.
In the source, I found these problematic lines:
{{{
618:nilBeside g (Nest _ p) = nilBeside g p
657:sepNB g (Nest _ p) k ys = sepNB g p k ys
700:fillNB g (Nest _ p) k ys = fillNB g p k ys
862: lay2 k (Nest _ p) = lay2 k p
}}}
I think that no function (that produces a document)
is allowed to ignore the nesting level
because this adds to the total (relative) indentation
that must be kept in a consistent state.
At least that's my guess that these numbers represent relative indentation
-
the source is not exactly verbose on the meaning of all these numbers
(which are not in the original paper by John Hughes).
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/669>
GHC <http://www.haskell.org/ghc/>
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