Self-edges only work on top or bottom

[DusterTheFirst]

When using an edge to point from a node to itself, with bend: 130deg the bend will come either out of the top of the bottom (depending on if bend is negative) however there does not seem to be a way to make the bend come out of the side, either left or right.

[Jollywatt]

There is a hack you can use which is why I've not bothered to add a new controller for this:

#import "@preview/fletcher:0.4.5": diagram, node, edge

#diagram({
  node((0,0), [Hello], stroke: 1pt, shape: circle)

  edge((0,-.1), (0,+.1), "-|>", bend: 120deg)

  let polar(r, theta) = (r*calc.cos(theta), r*calc.sin(theta))
  let theta = 45deg
  edge(polar(0.1, theta), polar(-0.1, theta), "-|>", bend: 130deg)
})

By adjusting the edge‘s vertices by small amounts, you change the angle of the line that connects them; this line is then bent into the final arc that you see.

I haven't thought of a good interface for this. We could add a loop-angle option to edge, but then it wouldn't make sense for non-loop edges, and edge() already has lots of options. I want to keep it simple. (Ideas are welcome!) At the very least, there should be some examples in the manual, though.

[EnzoMoser]

Unless I'm interpreting it wrong, the hack for rotating the angle using polar coordinates seems to only work for (0,0).

#import "@preview/fletcher:0.5.1" as fletcher: diagram, node, edge

#let diagram = diagram.with(
  node-shape: circle,
  node-stroke: 1pt,
) 

#let edge = edge.with(
  marks: "-|>"
)

#diagram(
  let polar(r, theta) = (r*calc.cos(theta), r*calc.sin(theta)),
  let theta = 45deg,

  node((0,0), [0,0]),
  edge(polar(0.1, theta), polar(-0.1, theta), bend: 120deg),

  node((1,0), [1,0]),
  edge(polar(1.1, theta), polar(0.9, theta), bend: 120deg),

  node((-1,0), [-1,0]),
  edge(polar(-0.9, theta), polar(-1.1, theta), bend: 120deg),
)

I also prefer being able to use node names.

I do think a proper option for changing the angle of a self loop is warranted. We can have it throw an error if both the value is not none and the first and last coordinates are not the same.

[Jollywatt]

@EnzoMoser You can use relative coordinates such as (rel: polar(0.1, theta), to: (x, y)) to create polar displacements about (x, y) instead of the origin (0, 0). You could wrap this in a function like so:

#import "@preview/fletcher:0.5.1" as fletcher: diagram, node, edge

#let diagram = diagram.with(
  node-shape: circle,
  node-stroke: 1pt,
) 

#let edge = edge.with(
  marks: "-|>"
)

#let polar-nudge(origin, theta, r: 0.1) = (
  (rel: (+r*calc.cos(theta), +r*calc.sin(theta)), to: origin),
  (rel: (-r*calc.cos(theta), -r*calc.sin(theta)), to: origin),
)

#diagram(
  let theta = 45deg,

  node((0,0), [0,0]),
  edge(..polar-nudge((0,0), theta), bend: 120deg),

  node((1,0), [1,0]),
  edge(..polar-nudge((1,0), theta), bend: 120deg),

  node((-1,0), [-1,0]),
  edge(..polar-nudge((-1,0), theta), bend: 120deg),
)

I do agree with you, though, that a loop-angle option is warranted. And I think it's a good idea to make it error when it is specified if the edge is not a loop.

I'm afraid I'm not very available to contribute much to fletcher at the moment, though. If you have the capacity, feel free to have a go at this feature.