@SubSection
    @Tag { horizontal }
    @Title { Horizontal galleys }
@Begin
@PP
There is a strong analogy between breaking a column of text into
page-sized pieces, and breaking a paragraph into line-sized pieces.  In
fact, the two differ only in direction:  vertical for body text,
horizontal for paragraphs.  In this section we define {@I{horizontal
galleys}}, and show how they provide an unlimited number of paragraph
breaking styles, as well as solve some other problems.  Regrettably,
lack of time has prevented their incorporation into the Basser Lout
interpreter.
@PP
Imagine a galley whose components are separated by horizontal
concatenation operators instead of vertical ones, perhaps indicated by a
@Code { horizontally into } clause.  Then all object breaking, including
paragraph breaking, could be replaced by galley component promotion like
this:
@ID @OneRow @Code {
"def @Paragraph right x"
"{"
"    def @LinePlace { @Galley }"
""
"    def @LineList"
"    {"
"        @HExpand @LinePlace"
"        //1vx  @LineList"
"    }"
""
"    def @Par horizontally into { @LinePlace&&preceding }"
"        right x"
"    { x }"
""
"    @LineList  //  @Par { 0.2i @Wide {}  &0i  x  &1rt }"
"}"
}
The @Code "@HExpand" operator, which is a primitive of Basser Lout,
horizontally expands the gaps in its right parameter until the result
fills the available space, thus implementing line adjustment, except
when the parameter contains tabulation gaps like {@Code "&1rt"}, which
cause the parameter to be already expanded.  The result of
@ID @Code "@Paragraph { A short paragraph of text. }"
would then be something like
@ID 1.5i @Wide {
0.2i @Wide {} & A short paragraph of text.
}
depending on the available horizontal space.  An unlimited range of
paragraph breaking styles could be defined, including ragged right, ragged
left, break-and-center, and so on.
@PP
In Basser Lout, indented paragraphs are produced by preceding them with
a horizontal concatenation operator, for example {@Code "|0.5i"}.  This
has the unfortunate effect of making an indented paragraph into a single
component of the enclosing galley, so that it will always be kept
together on one page.  Horizontal galleys solve this problem with a
simple change to {@Code "@LineList"}:
@ID @OneRow @Code {
"def @LineList"
"{"
"    |0.5i  @HExpand @LinePlace"
"    //1vx  @LineList"
"}"
}
showing the flexibility that comes from bringing the full power of the
Lout language to bear on paragraph layout.  It is easy to make provision
for a tag on the first line.
@PP
Although Basser Lout permits receptive symbols within paragraphs, they
are of little use, because their available width is calculated after
paragraph breaking, and the incoming galley cannot spread over more than
one line.  With horizontal galleys, such symbols would have infinite
available width, and we could easily produce a filled paragraph of
footnotes like this:
@ID 3.5i @Wide {
@OneRow { -2p @Font 1 ^/0.3vo } & See Jones and Saunders (1982). &2m
@OneRow { -2p @Font 2 ^/0.3vo } & Or so Jacobsen (1973) asserts. &2m
@OneRow { -2p @Font 3 ^/0.3vo } & {@I ibid}, p. 327.
}
based on an infinite horizontal sequence of @Code "@FootPlace" symbols
inside a horizontal galley.
@PP
When body text is placed on pages, the length of each column varies
depending on the available vertical space.  Horizontal galleys could
analogously produce lines of varying length, and so could fill
non-rectangular shapes.
@PP
An important theoretical benefit of horizontal galleys is that they
would permit horizontal and vertical to be treated in a perfectly
symmetrical way, whereas at present paragraph breaking is horizontal
only, and galley breaking is vertical only.  This must simplify the
treatment of non-European languages which fill in unusual directions,
although it is not itself sufficient to implement them.
@PP
There are a few minor problems with horizontal galleys.  First, the
syntactic overhead of enclosing each paragraph in @Code "@Paragraph { ... }"
or whatever is unacceptable.  Permitting user-defined operators to have lower
precedence than the white space between two words might help here.  Second,
the built-in paragraph breaker includes hyphenation, and it permits line
breaks in the input to determine line breaks in the output, if
desired.  These features must somehow be preserved.  Finally, we have
explained how the Basser Lout interpreter assigns equal width to the
wider columns of tables (Section {@NumberOf style}).  The equivalent
situation in vertical galleys occurs when two receptive symbols compete
for vertical space (e.g. @Code "@TextPlace" and {@Code "@FootSect"}),
and there it is conventional to grant as much as required to the first
arrival.  It is not clear to the author how these different approaches
can be reconciled.
@End @SubSection