Commit 5d57087e authored by thomie's avatar thomie

Pretty: fix a broken invariant (#10735)

This is a backport of a bug fix from
6cfbd0444981c074bae10a3cf72733bcb8597bef in libraries/pretty:

    Fix a broken invariant
    Patch from #694,  for the problem "empty is an identity for <> and $$" is
    currently broken by eg. isEmpty (empty<>empty)"
parent bcfae08c
......@@ -623,12 +623,17 @@ union_ = Union
-- * @(x '$$' y) '<>' z = x '$$' (y '<>' z)@, if @y@ non-empty.
--
($$) :: Doc -> Doc -> Doc
p $$ q = Above p False q
p $$ q = above_ p False q
-- | Above, with no overlapping.
-- '$+$' is associative, with identity 'empty'.
($+$) :: Doc -> Doc -> Doc
p $+$ q = Above p True q
p $+$ q = above_ p True q
above_ :: Doc -> Bool -> Doc -> Doc
above_ p _ Empty = p
above_ Empty _ q = q
above_ p g q = Above p g q
above :: Doc -> Bool -> RDoc -> RDoc
above (Above p g1 q1) g2 q2 = above p g1 (above q1 g2 q2)
......@@ -679,12 +684,17 @@ nilAboveNest g k q | not g && k > 0 -- No newline if no overlap
-- | Beside.
-- '<>' is associative, with identity 'empty'.
(<>) :: Doc -> Doc -> Doc
p <> q = Beside p False q
p <> q = beside_ p False q
-- | Beside, separated by space, unless one of the arguments is 'empty'.
-- '<+>' is associative, with identity 'empty'.
(<+>) :: Doc -> Doc -> Doc
p <+> q = Beside p True q
p <+> q = beside_ p True q
beside_ :: Doc -> Bool -> Doc -> Doc
beside_ p _ Empty = p
beside_ Empty _ q = q
beside_ p g q = Beside p g q
-- Specification: beside g p q = p <g> q
beside :: Doc -> Bool -> RDoc -> RDoc
......
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