@Section
    @Tag { dia_link }
    @Title { Links }
@Begin
@PP
@Code "@Diag" has one basic symbol for creating links, called
link. @Index { @Code "@Link" symbol from @Code "@Diag" }
{@Code "@Link"}.  It draws a link between two points or nodes
given by {@Code from} and {@Code to} options, along a path
given by a {@Code path} option:
@ID @Code {
"@Link"
"    path { ... }"
"    from { ... }"
"    to { ... }"
}
Unlike {@Code "@Node"}, {@Code "@Link"} has no following object.
@PP
The @Code "path" option may be used to produce a link of any shape, as
Section {@NumberOf dia_defi} explains.  There are also values
that produce standard paths.  These are listed in full in the summary
(Section {@NumberOf dia_summ}); here is a sample:
@ID @Tab
    @Fmta { @Col @Code { path "{" A "}" } ! @Col ! @Col B }
{

@Rowa
    A { line }
    B {
@Diag {
A:: @Circle //1c ||2c B:: @Circle
// @Link from { A } to { B } path { line } arrow { yes }
}
}

@Rowa
    A { acurve }
    B {
@Diag {
A:: @Circle //1c ||2c B:: @Circle
// @Link from { A } to { B } path { acurve } arrow { yes }
}
}

@Rowa
    A { ccurve }
    B {
@Diag {
A:: @Circle //1c ||2c B:: @Circle
// @Link from { A } to { B } path { ccurve } arrow { yes }
}
}

@Rowa
    A { rvlcurve }
    B {
@Diag {
A:: @Circle //1c ||2c B:: @Circle
// @Link from { A } to { B } path { rvlcurve } arrow { yes }
}
}

}
The name of the last one is a reminder that it goes right, then vertically,
then left, with curved corners.  The @Code acurve and @Code ccurve values
produce circular arcs, anticlockwise and clockwise respectively, lying on
the circle passing through the endpoints, or through the centres of the
endpoints when they are tags denoting nodes.  There is also @Code "curve"
which is an abbreviation for {@Code "acurve"}.  All these standard paths
are defined in a way that makes sense no matter where the two nodes are
relative to each other, except that no promise of a sensible result is
made for two nodes very close together.
@PP
@Code "@Link" has two options, @Code bias and {@Code radius}, that may be
used to fine-tune the path.  The @Code "bias" option determines the
maximum distance that a curve is permitted to stray:
@CD @Tab
    @Fmta { @Col A ! @Col ! @Col B }
{

@Rowa

    A { @Diag vstrut { no } margin { 0.5c } {
A:: @Circle //1.5c ||2c B:: @Circle
//
LA:: @Line pathstyle { cdashed } from { A } to { B }
LB:: @Curve from { A } to { B }
@Line arrow { both } from { LA@LMID } to { LB@LMID }
    ylabel { @I bias } # ylabeladjust { 0.15c 0 }
} }

    B { @Diag vstrut { no } margin { 0.5c } {
A:: @Circle //1.5c ||2c B:: @Circle
//
LA:: @RVLCurve from { A } to { B }
LB:: @Line pathstyle { cdashed } from { B@E } to { B@E ++ {0 2.5c} }
@Line arrow { both } from { LB@LMID } to { LA@LMID }
     ylabel { @I bias } ylabeladjust { 0 0.05c }
} }

}
The @Code radius option does @I not apply to @Code acurve and
{@Code ccurve}; rather, it determines the radius of the arcs at
the corners of @Code rvlcurve and its kin.  A very large radius will be
reduced to the largest reasonable value, which provides a way to get
a semicircle at the right in an {@Code rvlcurve}.
@PP
Lout has no idea where the path is wandering, and cannot take it into
account when placing a diagram on the page:
@ID {
@Code {
"@Link"
"    path { ccurve }"
"    bias { 2c }"
}
||7ct
@Diag vstrut { no } {
A:: @Circle &3c B:: @Circle
//
@Link path { ccurve } bias { 2c } from { A } to { B }
}
}
In such cases you have to arrange for the extra space yourself, by adding
an extra paragraph symbol, blank row or column in a table, or whatever.
@PP
As with the options of {@Code "@Node"}, the options of {@Code "@Link"}
may all be given to {@Code "@Diag"} as well, where they apply to every
link in the diagram, unless overridden in the usual way.  They also appear
in the setup file, where they apply to every link in every diagram of the
document, unless overridden.
@PP
There are {@Code pathstyle}, {@Code pathdashlength} and {@Code pathwidth}
options which affect the appearance of the path in the same way as the
{@Code outlinestyle}, {@Code outlinedashlength} and {@Code outlinewidth}
options of {@Code "@Node"} affect the outline.  When {@Code pathstyle}
contains just one value (as opposed to a sequence of values) @Code "@Diag"
tries to divide the path into fewer segments than it would otherwise, to
make dashed and dotted paths look as good as possible.  There is also
a {@Code pathgap} option which affects only @Code doubleline paths; it
determines the gap between the centres of the two lines.
@PP
The @Code "@Link" symbol has an @Code arrow option, which adds an
arrow. @Index { arrows }
arrowhead to the end of the link:
@ID {
@Code {
"@Link"
"    arrow { yes }"
}
||7ct
@Diag {
1c @High 3c @Wide
//
@Link
    from { 0,0 }
    to { 1,1 }
    arrow { yes }
}
}
Its value may be {@Code no} (the default), {@Code yes}, {@Code forward}
(which is the same as {@Code yes}), {@Code back}, or {@Code both}:
@ID {
@Code {
"@Link"
"    arrow { both }"
}
||7ct
@Diag {
1c @High 3c @Wide
//
@Link
    from { 0,0 }
    to { 1,1 }
    arrow { both }
}
}
@Code "@Link" has three options for controlling the appearance of
arrowheads:  {@Code arrowstyle}, {@Code arrowwidth}, and
{@Code arrowlength}.  Although every link symbol has these options, for
consistency it is almost always better to set the corresponding options
to the @Code "@Diag" symbol, which applies them to every arrow in the
diagram:
@ID @Code {
"@Diag"
"    arrowstyle { solid }"
"    arrowwidth { 0.3f }"
"    arrowlength { 0.5f }"
"{"
"    ..."
"}"
}
This shows the default values:  a solid arrowhead like the ones above,
@Code "0.3f" wide (across) and @Code "0.5f" long.  The @Code "arrowwidth"
and @Code "arrowlength" options may be any length; it may be necessary to
decrease @Code "arrowwidth" when many arrows enter one node.  The full list
of possible values for @Code "arrowstyle" is
@ID @Tab
    @Fmta { @Col @Code { "arrowstyle {" A "}" } ! @Col B  }
    vmargin { 1.0vx }
{
@Rowa
    A { solid }
    B { @Diag vstrut { no } { A:: @Circle  |2c B:: @Circle 
	// @Link from { A } to { B } arrow { yes } arrowstyle { solid } } }
@Rowa
    A { halfopen }
    B { @Diag vstrut { no } { A:: @Circle  |2c B:: @Circle 
	// @Link from { A } to { B } arrow { yes } arrowstyle { halfopen } } }
@Rowa
    A { open }
    B { @Diag vstrut { no } { A:: @Circle  |2c B:: @Circle 
	// @Link from { A } to { B } arrow { yes } arrowstyle { open } } }
@Rowa
    A { curvedsolid }
    B { @Diag vstrut { no } { A:: @Circle  |2c B:: @Circle 
	// @Link from { A } to { B } arrow { yes } arrowstyle { curvedsolid } } }
@Rowa
    A { curvedhalfopen }
    B { @Diag vstrut { no } { A:: @Circle  |2c B:: @Circle 
	// @Link from { A } to { B } arrow { yes } arrowstyle { curvedhalfopen } } }
@Rowa
    A { curvedopen }
    B { @Diag vstrut { no } { A:: @Circle  |2c B:: @Circle 
	// @Link from { A } to { B } arrow { yes } arrowstyle { curvedopen } } }
@Rowa
    A { circle }
    B { @Diag vstrut { no } { A:: @Circle  |2c B:: @Circle 
	// @Link from { A } to { B } arrow { yes } arrowstyle { circle } } }
@Rowa
    A { box }
    B { @Diag vstrut { no } { A:: @Circle  |2c B:: @Circle 
	// @Link from { A } to { B } arrow { yes } arrowstyle { box } } }
}
The reader is invited to admire the beautifully sharp points on these
arrowheads.
@FootNote {
The outlines of all nodes and arrowheads are drawn on the inside of the
geometrical curve defining them, not centred over the curve as is common in
PostScript documents.  Hence, the arrowheads and node outlines intersect at
a true geometrical point; they do not overlap by one line width.  Furthermore,
the standard link paths terminate at the base of the arrowhead, not at
the point; the arrowhead itself is responsible for continuing the link
path, at the appropriate width (although never dashed or dotted), from its
base to its point, and hence can and does ensure that the link path does
not overstrike and thicken the point of the arrow.
}
@PP
It is possible to place an arbitrary object at the beginning or
end of a link, using the @Code "fromlabel" and @Code "tolabel" options
of Section {@NumberOf dia_labe}.
@PP
To save time in common cases, @Code "@Diag" provides link symbols,
each of which is just @Code "@Link" with one of the standard paths
already set:  {@Code "@Line"}, {@Code "@Curve"}, {@Code "@CCurve"},
{@Code "@RVLCurve"}, and so on.  There are also symbols in which
the @Code "arrow" option is set to @Code yes in addition:  {@Code "@Arrow"},
{@Code "@CurveArrow"}, {@Code "@CCurveArrow"}, {@Code "@RVLCurveArrow"},
and so on.  See the summary (Section {@NumberOf dia_summ}) for the
full list of these symbols.  You will still need the @Code "arrow" option
to get backward arrows and double-ended arrows.
@End @Section
