353 lines
12 KiB
Markdown
353 lines
12 KiB
Markdown
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---
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title: >-
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Ginger
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description: >-
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Yes, it does exist.
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---
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This post is about a programming language that's been bouncing around in my head
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for a _long_ time. I've tried to actually implement the language three or more
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times now, but everytime I get stuck or run out of steam. It doesn't help that
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everytime I try again the form of the language changes significantly. But all
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throughout the name of the language has always been "Ginger". It's a good name.
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In the last few years the form of the language has somewhat solidified in my
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head, so in lieu of actually working on it I'm going to talk about what it
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currently looks like.
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## Abstract Syntax Lists
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_In the beginning_ there was assembly. Well, really in the beginning there were
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punchcards, and probably something even more esoteric before that, but it was
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all effectively the same thing: a list of commands the computer would execute
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sequentially, with the ability to jump to odd places in the sequence depending
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on conditions at runtime. For the purpose of this post, we'll call this class of
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languages "abstract syntax list" (ASL) languages.
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Here's a hello world program in my favorite ASL language, brainfuck:
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```
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++++++++[>++++[>++>+++>+++>+<<<<-]>+>+>->>+[<]<-]>>.>---.+++++++..+++.>>.<-.<.++
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+.------.--------.>>+.>++.
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```
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(If you've never seen brainfuck, it's deliberately unintelligible. But it _is_
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an ASL, each character representing a single command, executed by the brainfuck
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runtime from left to right.)
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ASLs did the job at the time, but luckily we've mostly moved on past them.
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## Abstract Syntax Trees
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Eventually programmers upgraded to C-like languages. Rather than a sequence of
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commands, these languages were syntactically represented by an "abstract syntax
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tree" (AST). Rather than executing commands in essentially the same order they
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are written, an AST language compiler reads the syntax into a tree of syntax
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nodes. What it then does with the tree is language dependent.
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Here's a program which outputs all numbers from 0 to 9 to stdout, written in
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(slightly non-idiomatic) Go:
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```go
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i := 0
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for {
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if i == 10 {
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break
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}
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fmt.Println(i)
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i++
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}
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```
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When the Go compiler sees this, it's going to first parse the syntax into an
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AST. The AST might look something like this:
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```
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(root)
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|-(:=)
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| |-(i)
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| |-(0)
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|-(for)
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|-(if)
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| |-(break)
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|-(fmt.Println)
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|-(++)
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|-(i)
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```
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Each of the non-leaf nodes in the tree represents an operation, and the children
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of the node represent the arguments to that operation, if any. From here the
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compiler traverses the tree depth-first in order to turn each operation it finds
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into the appropriate machine code.
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There's a sub-class of AST languages called the LISP ("LISt Processor")
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languages. In a LISP language the AST is represented using lists of elements,
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where the first element in each list denotes the operation and the rest of the
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elements in the list (if any) represent the arguments. Traditionally each list
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is represented using parenthesis. For example `(+ 1 1)` represents adding 1 and
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1 together.
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As a more complex example, here's how to print numbers 0 through 9 to stdout
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using my favorite (and, honestly, only) LISP, Clojure:
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```clj
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(doseq
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[n (range 10)]
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(println n))
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```
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Much smaller, but the idea is there. In LISPs there is no differentiation
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between the syntax, the AST, and the language's data structures; they are all
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one and the same. For this reason LISPs generally have very powerful macro
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support, wherein one uses code written in the language to transform code written
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in that same language. With macros users can extend a language's functionality
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to support nearly anything they need to, but because macro generation happens
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_before_ compilation they can still reap the benefits of compiler optimizations.
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### AST Pitfalls
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The ASL (assembly) is essentially just a thin layer of human readability on top
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of raw CPU instructions. It does nothing in the way of representing code in the
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way that humans actually think about it (relationships of types, flow of data,
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encapsulation of behavior). The AST is a step towards expressing code in human
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terms, but it isn't quite there in my opinion. Let me show why by revisiting the
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Go example above:
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```go
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i := 0
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for {
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if i > 9 {
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break
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}
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fmt.Println(i)
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i++
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}
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```
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When I understand this code I don't understand it in terms of its syntax. I
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understand it in terms of what it _does_. And what it does is this:
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* with a number starting at 0, start a loop.
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* if the number is greater than 9, stop the loop.
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* otherwise, print the number.
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* add one to the number.
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* go to start of loop.
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This behavior could be further abstracted into the original problem statement,
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"it prints numbers 0 through 9 to stdout", but that's too general, as there
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are different ways for that to be accomplished. The Clojure example first
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defines a list of numbers 0 through 9 and then iterates over that, rather than
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looping over a single number. These differences are important when understanding
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what code is doing.
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So what's the problem? My problem with ASTs is that the syntax I've written down
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does _not_ reflect the structure of the code or the flow of data which is in my
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head. In the AST representation if you want to follow the flow of data (a single
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number) you _have_ to understand the semantic meaning of `i` and `:=`; the AST
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structure itself does not convey how data is being moved or modified.
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Essentially, there's an extra implicit transformation that must be done to
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understand the code in human terms.
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## Ginger: An Abstract Syntax Graph Language
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In my view the next step is towards using graphs rather than trees for
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representing our code. A graph has the benefit of being able to reference
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"backwards" into itself, where a tree cannot, and so can represent the flow of
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data much more directly.
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I would like Ginger to be an ASG language where the language is the graph,
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similar to a LISP. But what does this look like exactly? Well, I have a good
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idea about what the graph _structure_ will be like and how it will function, but
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the syntax is something I haven't bothered much with yet. Representing graph
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structures in a text file is a problem to be tackled all on its own. For this
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post we'll use a made-up, overly verbose, and probably non-usable syntax, but
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hopefully it will convey the graph structure well enough.
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### Nodes, Edges, and Tuples
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All graphs have nodes, where each node contains a value. A single unique value
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can only have a single node in a graph. Nodes are connected by edges, where
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edges have a direction and can contain a value themselves.
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In the context of Ginger, a node represents a value as expected, and the value
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on an edge represents an operation to take on that value. For example:
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```
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5 -incr-> n
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```
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`5` and `n` are both nodes in the graph, with an edge going from `5` to `n` that
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has the value `incr`. When it comes time to interpret the graph we say that the
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value of `n` can be calculated by giving `5` as the input to the operation
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`incr` (increment). In other words, the value of `n` is `6`.
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What about operations which have more than one input value? For this Ginger
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introduces the tuple to its graph type. A tuple is like a node, except that it's
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anonymous, which allows more than one to exist within the same graph, as they do
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not share the same value. For the purposes of this blog post we'll represent
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tuples like this:
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```
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1 -> } -add-> t
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2 -> }
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```
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`t`'s value is the result of passing a tuple of two values, `1` and `2`, as
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inputs to the operation `add`. In other words, the value of `t` is `3`.
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For the syntax being described in this post we allow that a single contiguous
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graph can be represented as multiple related sections. This can be done because
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each node's value is unique, so when the same value is used in disparate
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sections we can merge the two sections on that value. For example, the following
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two graphs are exactly equivalent (note the parenthesis wrapping the graph which
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has been split):
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```
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1 -> } -add-> t -incr-> tt
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2 -> }
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```
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```
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(
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1 -> } -add-> t
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2 -> }
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t -incr-> tt
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)
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```
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(`tt` is `4` in both cases.)
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A tuple with only one input edge, a 1-tuple, is a no-op, semantically, but can
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be useful structurally to chain multiple operations together without defining
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new value names. In the above example the `t` value can be eliminated using a
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1-tuple.
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```
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1 -> } -add-> } -incr-> tt
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2 -> }
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```
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When an integer is used as an operation on a tuple value then the effect is to
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output the value in the tuple at that index. For example:
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```
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1 -> } -0-> } -incr-> t
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2 -> }
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```
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(`t` is `2`.)
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### Operations
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When a value sits on an edge it is used as an operation on the input of that
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edge. Some operations will no doubt be builtin, like `add`, but users should be
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able to define their own operations. This can be done using the `in` and `out`
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special values. When a graph is used as an operation it is scanned for both `in`
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and `out` values. `in` is set to the input value of the operation, and the value
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of `out` is used as the output of the operation.
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Here we will define the `incr` operation and then use it. Note that we set the
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`incr` value to be an entire sub-graph which represents the operation's body.
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```
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( in -> } -add-> out
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1 -> } ) -> incr
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5 -incr-> n
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```
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(`n` is `6`.)
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The output of an operation may itself be a tuple. Here's an implementation and
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usage of `double-incr`, which increments two values at once.
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```
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( in -0-> } -incr-> } -> out
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}
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in -1-> } -incr-> } ) -> double-incr
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1 -> } -double-incr-> t -add-> tt
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2 -> }
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```
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(`t` is a 2-tuple with values `2`, and `3`, `tt` is `5.)
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### Conditionals
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The conditional is a bit weird, and I'm not totally settled on it yet. For now
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we'll use this. The `if` operation expects as an input a 2-tuple whose first
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value is a boolean and whose second value will be passed along. The `if`
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operation is special in that it has _two_ output edges. The first will be taken
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if the boolean is true, the second if the boolean is false. The second value in
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the input tuple, the one to be passed along, is used as the input to whichever
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branch is taken.
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Here is an implementation and usage of `max`, which takes two numbers and
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outputs the greater of the two. Note that the `if` operation has two output
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edges, but our syntax doesn't represent that very cleanly.
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```
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( in -gt-> } -if-> } -0-> out
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in -> } -> } -1-> out ) -> max
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1 -> } -max-> t
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2 -> }
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```
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(`t` is `2`.)
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It would be simple enough to create a `switch` macro on top of `if`, to allow
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for multiple conditionals to be tested at once.
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### Loops
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Loops are tricky, and I have two thoughts about how they might be accomplished.
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One is to literally draw an edge from the right end of the graph back to the
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left, at the point where the loop should occur, as that's conceptually what's
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happening. But representing that in a text file is difficult. For now I'll
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introduce the special `recur` value, and leave this whole section as TBD.
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`recur` is cousin of `in` and `out`, in that it's a special value and not an
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operation. It takes whatever value it's set to and calls the current operation
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with that as input. As an example, here is our now classic 0 through 9 printer
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(assume `println` outputs whatever it was input):
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```
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// incr-1 is an operation which takes a 2-tuple and returns the same 2-tuple
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// with the first element incremented.
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( in -0-> } -incr-> } -> out
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in -1-> } ) -> incr-1
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( in -eq-> } -if-> out
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in -> } -> } -0-> } -println-> } -incr-1-> } -> recur ) -> print-range
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0 -> } -print-range-> }
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10 -> }
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```
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## Next Steps
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This post is long enough, and I think gives at least a basic idea of what I'm
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going for. The syntax presented here is _extremely_ rudimentary, and is almost
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definitely not what any final version of the syntax would look like. But the
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general idea behind the structure is sound, I think.
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I have a lot of further ideas for Ginger I haven't presented here. Hopefully as
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time goes on and I work on the language more some of those ideas can start
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taking a more concrete shape and I can write about them.
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The next thing I need to do for Ginger is to implement (again) the graph type
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for it, since the last one I implemented didn't include tuples. Maybe I can
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extend it instead of re-writing it. After that it will be time to really buckle
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down and figure out a syntax. Once a syntax is established then it's time to
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start on the compiler!
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