Brian Picciano
3 years ago
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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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|
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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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|
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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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|
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## Abstract Syntax Lists |
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|
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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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|
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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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``` |
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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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|
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ASLs did the job at the time, but luckily we've mostly moved on past them. |
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|
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## Abstract Syntax Trees |
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|
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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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|
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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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|
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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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|
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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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``` |
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(root) |
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|-(:=) |
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| |-(i) |
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| |-(0) |
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| |
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|-(for) |
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|-(if) |
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| |-(==) |
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| | |-(i) |
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| | |-(10) |
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| | |
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| |-(break) |
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| |
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|-(fmt.Println) |
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| |-(i) |
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| |
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|-(++) |
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|-(i) |
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``` |
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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## Ginger: An Abstract Syntax Graph Language |
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|
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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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|
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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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|
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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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|
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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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( |
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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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|
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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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|
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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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