ginger conditionals errata

src/_posts/2021-03-04-conditionals-in-ginger-errata.md

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+---
+title: >-
+    Conditionals in Ginger, Errata
+description: >-
+    Too clever by half.
+series: ginger
+tags: tech
+---
+
+After publishing the last post in the series I walked away from my computer
+feeling that I was very clever and had made a good post. This was incorrect.
+
+To summarize [the previous post][prev], it's not obvious which is the best way
+to structure conditionals in a graphical programming language. My favorite
+solution looked something like this:
+
+```
+       in -> } -> } -if-> } -0-> } -add-> out
+in -1-> } -> }    }       } -1-> } -sub-> out
+in -0-> }         }
+                  }
+         in -lt-> }
+```
+
+Essentially an `if` operator which accepts a value and a boolean, and which has
+two output edges. If the boolean is true then the input value is sent along the
+first output edge, and if it's false it's sent along the second.
+
+This structure is not possible, given the properties of ginger graphs that have
+been laid out in [other posts in the series][other].
+
+## Nodes, Tuples, and Edges
+
+A ginger graph, as it has been presented so far, is composed of these three
+elements. A node has a value, and its value is unique to the graph; if two nodes
+have the same value then they are the same node. Edges connect two nodes or
+tuples together, and have a value and direction. Tuples are, in essence, a node
+whose value is its input edges.
+
+The `if` operation above lies on an edge, not a node or tuple. It cannot have
+multiple output edges, since it cannot have any edges at all. It _is_ an edge.
+
+So it's back to the drawing board, to some extent. But luckily I've got some
+more ideas in my back pocket.
+
+## Forks and Junctions
+
+In an older conception of ginger there was no tuple, but instead there were
+forks and junctions. A junction was essentially the same as a tuple, just named
+differently: a node whose value is its input edges. A fork was just the
+opposite, a node whose value is its output edges. Junctions and forks naturally
+complimented each other, but ultimately I didn't find forks to be useful for
+much because there weren't cases where it was necessary to have a single edge be
+split across multiple output edges directly; any case which appeared to require
+a fork could be satisfied by directing the edge into a 1-tuple and using the
+output edges of the 1-tuple.
+
+But now we have such a case. The 1-tuple won't work, because the `if` operator
+would only see the 1-tuple, not its edges. It could be supposed that the graph
+interpreter could say that an `if` operation must be followed by a 1-tuple, and
+that the 1-tuple's output edges have a special meaning in that circumstance. But
+making the output edges of a 1-tuple have different meaning in different
+circumstances isn't very elegant.
+
+So a fork might be just the thing here. For the example I will represent a
+fork as the opposite of a tuple: a vertical column of `{` characters.
+
+```
+       in -> } -> } -if-> { -0-> } -add-> out
+in -1-> } -> }    }       { -1-> } -sub-> out
+in -0-> }         }
+                  }
+         in -lt-> }
+```
+
+It _looks_ elegant, which is nice. I am curious though if there's any other
+possible use-case where a fork might be useful... if there's not then it seems
+odd to introduce an entire new element just to support a single operation. Why
+not just make that operation itself the new element?
+
+## Switch it Up
+
+In most conceptions of a flowchart that I've seen a conditional is usually
+represented as a node with a different shape than the other nodes (often a
+diamond). Ginger could borrow this idea for itself, and declare a new graph
+element, alongside nodes, tuples, and edges, called a switch.
+
+Let's say a switch is simply represented by a `-<>`, and acts like a node in all
+aspects except that it has no value and is not unique to the graph.
+
+The example presented in the [previous post][prev] would look something like
+this:
+
+```
+       in -> } -> } -<> -0-> } -add-> out
+in -1-> } -> }    }     -1-> } -sub-> out
+in -0-> }         }
+                  }
+         in -lt-> }
+```
+
+This isn't the _worst_. Like the fork it's adding a new element, but that
+element's existence is required and its usage is very specific to that
+requirement, whereas the fork's existence is required but ambiguously useful
+outside of that requirement.
+
+On the other hand, there are macros to consider...
+
+## Macrophillic
+
+Ginger will certainly support macros, and as alluded to in the last post I'd
+like even conditional operations to be fair game for those who want to construct
+their own more complex operators. In the context of the switch `-<>` element,
+would someone be able to create something like a pattern matching conditional?
+If the builtin conditional is implemented as a new graph element then it seems
+that the primary way to implement a custom conditional macro will also involve a
+new graph element.
+
+While I'm not flat out opposed to allowing for custom graph elements, I'm
+extremely skeptical that it's necessary, and would like it to be proven
+necessary before considering it. So if we can have a basic conditional, _and_
+custom conditional macros built on top of the same broadly useful element, that
+seems like the better strategy.
+
+So all of that said, it seems I'm leaning towards forks as the better strategy
+in this. But I'd like a different name. "Fork" was nice as being the compliment
+of a "junction", but I like "tuple" way more than "junction" because the term
+applies well both to the structural element _and_ to the transformation that
+element performs (i.e. a tuple element combines its input edges' values into a
+tuple value). But "tuple" and "fork" seem weird together...
+
+## Many Minutes Later...
+
+A brief search of the internet reveals no better word than "fork". A place
+where a tree's trunk splits into two separate trunks is called a "fork". A
+place where a river splits into two separate rivers is called a "fork".
+Similarly with roads. And that _is_ what's happening, from the point of view of
+the graph's structure: it is an element whose only purpose is to denote multiple
+outward edges.
+
+So "fork" it is.
+
+## Other considerations
+
+A 1-tuple is interesting in that it acts essentially as a concatenation of two
+edges. A 1-fork could, theoretically, do the same thing:
+
+```
+a -foo-> } -bar-> b
+
+c -far-> { -boo-> d
+```
+
+The top uses a tuple, the bottom a fork. Each is, conceptually, valid, but I
+don't like that two different elements can be used for the exact same use-case.
+
+A 1-tuple is an established concept in data structures, so I am loath to give it
+up.  A 1-fork, on the other hand, doesn't make sense structurally (would you
+point to any random point on a river and call it a "1-fork"?), and fork as a
+whole doesn't really have any analog in the realm of data structures. So I'm
+prepared to declare 1-forks invalid from the viewpoint of the language
+interpreter.
+
+Another consideration: I already expect that there's going to be confusion as to
+when to use a fork and when to use multiple outputs from a node. For example,
+here's a graph which uses a fork:
+
+```
+a -> { -op1-> foo
+     { -op2-> bar
+```
+
+and here's a graph which has multiple outputs from the same node:
+
+```
+a -op1-> foo
+  -op2-> bar
+```
+
+Each could be interpreted to mean the same thing: "set `foo` to the result of
+passing `a` into `op1`, and set `bar` to the result of passing `a` into `op2`."
+As with the 1-tuple vs 1-fork issue, we have another case where the same
+task might be accomplished with two different patterns. This case is trickier
+though, and I don't have as confident an answer.
+
+I think an interim rule which could be put in place, subject to review later, is
+that multiple edges from a node or tuple indicate that that same value is being
+used for multiple operations, while a fork indicates something specific to the
+operation on its input edge. It's not a pretty rule, but I think it will do.
+
+Stay tuned for next week when I realize that actually all of this is wrong and
+we start over again!
+
+[prev]: {% post_url 2021-03-01-conditionals-in-ginger %}
+[other]: {% post_url 2021-01-09-ginger %}