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diff --git a/_posts/2021-01-09-ginger.md b/_posts/2021-01-09-ginger.md deleted file mode 100644 index 3a97d7f..0000000 --- a/_posts/2021-01-09-ginger.md +++ /dev/null @@ -1,352 +0,0 @@ ---- -title: >- - Ginger -description: >- - Yes, it does exist. ---- - -This post is about a programming language that's been bouncing around in my head -for a _long_ time. I've tried to actually implement the language three or more -times now, but everytime I get stuck or run out of steam. It doesn't help that -everytime I try again the form of the language changes significantly. But all -throughout the name of the language has always been "Ginger". It's a good name. - -In the last few years the form of the language has somewhat solidified in my -head, so in lieu of actually working on it I'm going to talk about what it -currently looks like. - -## Abstract Syntax Lists - -_In the beginning_ there was assembly. Well, really in the beginning there were -punchcards, and probably something even more esoteric before that, but it was -all effectively the same thing: a list of commands the computer would execute -sequentially, with the ability to jump to odd places in the sequence depending -on conditions at runtime. For the purpose of this post, we'll call this class of -languages "abstract syntax list" (ASL) languages. - -Here's a hello world program in my favorite ASL language, brainfuck: - -``` -++++++++[>++++[>++>+++>+++>+<<<<-]>+>+>->>+[<]<-]>>.>---.+++++++..+++.>>.<-.<.++ -+.------.--------.>>+.>++. -``` - -(If you've never seen brainfuck, it's deliberately unintelligible. But it _is_ -an ASL, each character representing a single command, executed by the brainfuck -runtime from left to right.) - -ASLs did the job at the time, but luckily we've mostly moved on past them. - -## Abstract Syntax Trees - -Eventually programmers upgraded to C-like languages. Rather than a sequence of -commands, these languages were syntactically represented by an "abstract syntax -tree" (AST). Rather than executing commands in essentially the same order they -are written, an AST language compiler reads the syntax into a tree of syntax -nodes. What it then does with the tree is language dependent. - -Here's a program which outputs all numbers from 0 to 9 to stdout, written in -(slightly non-idiomatic) Go: - -```go -i := 0 -for { - if i == 10 { - break - } - fmt.Println(i) - i++ -} -``` - -When the Go compiler sees this, it's going to first parse the syntax into an -AST. The AST might look something like this: - -``` -(root) - |-(:=) - | |-(i) - | |-(0) - | - |-(for) - |-(if) - | |-(==) - | | |-(i) - | | |-(10) - | | - | |-(break) - | - |-(fmt.Println) - | |-(i) - | - |-(++) - |-(i) -``` - -Each of the non-leaf nodes in the tree represents an operation, and the children -of the node represent the arguments to that operation, if any. From here the -compiler traverses the tree depth-first in order to turn each operation it finds -into the appropriate machine code. - -There's a sub-class of AST languages called the LISP ("LISt Processor") -languages. In a LISP language the AST is represented using lists of elements, -where the first element in each list denotes the operation and the rest of the -elements in the list (if any) represent the arguments. Traditionally each list -is represented using parenthesis. For example `(+ 1 1)` represents adding 1 and -1 together. - -As a more complex example, here's how to print numbers 0 through 9 to stdout -using my favorite (and, honestly, only) LISP, Clojure: - -```clj -(doseq - [n (range 10)] - (println n)) -``` - -Much smaller, but the idea is there. In LISPs there is no differentiation -between the syntax, the AST, and the language's data structures; they are all -one and the same. For this reason LISPs generally have very powerful macro -support, wherein one uses code written in the language to transform code written -in that same language. With macros users can extend a language's functionality -to support nearly anything they need to, but because macro generation happens -_before_ compilation they can still reap the benefits of compiler optimizations. - -### AST Pitfalls - -The ASL (assembly) is essentially just a thin layer of human readability on top -of raw CPU instructions. It does nothing in the way of representing code in the -way that humans actually think about it (relationships of types, flow of data, -encapsulation of behavior). The AST is a step towards expressing code in human -terms, but it isn't quite there in my opinion. Let me show why by revisiting the -Go example above: - -```go -i := 0 -for { - if i > 9 { - break - } - fmt.Println(i) - i++ -} -``` - -When I understand this code I don't understand it in terms of its syntax. I -understand it in terms of what it _does_. And what it does is this: - -* with a number starting at 0, start a loop. -* if the number is greater than 9, stop the loop. -* otherwise, print the number. -* add one to the number. -* go to start of loop. - -This behavior could be further abstracted into the original problem statement, -"it prints numbers 0 through 9 to stdout", but that's too general, as there -are different ways for that to be accomplished. The Clojure example first -defines a list of numbers 0 through 9 and then iterates over that, rather than -looping over a single number. These differences are important when understanding -what code is doing. - -So what's the problem? My problem with ASTs is that the syntax I've written down -does _not_ reflect the structure of the code or the flow of data which is in my -head. In the AST representation if you want to follow the flow of data (a single -number) you _have_ to understand the semantic meaning of `i` and `:=`; the AST -structure itself does not convey how data is being moved or modified. -Essentially, there's an extra implicit transformation that must be done to -understand the code in human terms. - -## Ginger: An Abstract Syntax Graph Language - -In my view the next step is towards using graphs rather than trees for -representing our code. A graph has the benefit of being able to reference -"backwards" into itself, where a tree cannot, and so can represent the flow of -data much more directly. - -I would like Ginger to be an ASG language where the language is the graph, -similar to a LISP. But what does this look like exactly? Well, I have a good -idea about what the graph _structure_ will be like and how it will function, but -the syntax is something I haven't bothered much with yet. Representing graph -structures in a text file is a problem to be tackled all on its own. For this -post we'll use a made-up, overly verbose, and probably non-usable syntax, but -hopefully it will convey the graph structure well enough. - -### Nodes, Edges, and Tuples - -All graphs have nodes, where each node contains a value. A single unique value -can only have a single node in a graph. Nodes are connected by edges, where -edges have a direction and can contain a value themselves. - -In the context of Ginger, a node represents a value as expected, and the value -on an edge represents an operation to take on that value. For example: - -``` -5 -incr-> n -``` - -`5` and `n` are both nodes in the graph, with an edge going from `5` to `n` that -has the value `incr`. When it comes time to interpret the graph we say that the -value of `n` can be calculated by giving `5` as the input to the operation -`incr` (increment). In other words, the value of `n` is `6`. - -What about operations which have more than one input value? For this Ginger -introduces the tuple to its graph type. A tuple is like a node, except that it's -anonymous, which allows more than one to exist within the same graph, as they do -not share the same value. For the purposes of this blog post we'll represent -tuples like this: - -``` -1 -> } -add-> t -2 -> } -``` - -`t`'s value is the result of passing a tuple of two values, `1` and `2`, as -inputs to the operation `add`. In other words, the value of `t` is `3`. - -For the syntax being described in this post we allow that a single contiguous -graph can be represented as multiple related sections. This can be done because -each node's value is unique, so when the same value is used in disparate -sections we can merge the two sections on that value. For example, the following -two graphs are exactly equivalent (note the parenthesis wrapping the graph which -has been split): - -``` -1 -> } -add-> t -incr-> tt -2 -> } -``` - -``` -( - 1 -> } -add-> t - 2 -> } - - t -incr-> tt -) -``` - -(`tt` is `4` in both cases.) - -A tuple with only one input edge, a 1-tuple, is a no-op, semantically, but can -be useful structurally to chain multiple operations together without defining -new value names. In the above example the `t` value can be eliminated using a -1-tuple. - -``` -1 -> } -add-> } -incr-> tt -2 -> } -``` - -When an integer is used as an operation on a tuple value then the effect is to -output the value in the tuple at that index. For example: - -``` -1 -> } -0-> } -incr-> t -2 -> } -``` - -(`t` is `2`.) - -### Operations - -When a value sits on an edge it is used as an operation on the input of that -edge. Some operations will no doubt be builtin, like `add`, but users should be -able to define their own operations. This can be done using the `in` and `out` -special values. When a graph is used as an operation it is scanned for both `in` -and `out` values. `in` is set to the input value of the operation, and the value -of `out` is used as the output of the operation. - -Here we will define the `incr` operation and then use it. Note that we set the -`incr` value to be an entire sub-graph which represents the operation's body. - -``` -( in -> } -add-> out - 1 -> } ) -> incr - -5 -incr-> n -``` - -(`n` is `6`.) - -The output of an operation may itself be a tuple. Here's an implementation and -usage of `double-incr`, which increments two values at once. - -``` -( in -0-> } -incr-> } -> out - } - in -1-> } -incr-> } ) -> double-incr - -1 -> } -double-incr-> t -add-> tt -2 -> } -``` - -(`t` is a 2-tuple with values `2`, and `3`, `tt` is `5.) - -### Conditionals - -The conditional is a bit weird, and I'm not totally settled on it yet. For now -we'll use this. The `if` operation expects as an input a 2-tuple whose first -value is a boolean and whose second value will be passed along. The `if` -operation is special in that it has _two_ output edges. The first will be taken -if the boolean is true, the second if the boolean is false. The second value in -the input tuple, the one to be passed along, is used as the input to whichever -branch is taken. - -Here is an implementation and usage of `max`, which takes two numbers and -outputs the greater of the two. Note that the `if` operation has two output -edges, but our syntax doesn't represent that very cleanly. - -``` -( in -gt-> } -if-> } -0-> out - in -> } -> } -1-> out ) -> max - -1 -> } -max-> t -2 -> } -``` - -(`t` is `2`.) - -It would be simple enough to create a `switch` macro on top of `if`, to allow -for multiple conditionals to be tested at once. - -### Loops - -Loops are tricky, and I have two thoughts about how they might be accomplished. -One is to literally draw an edge from the right end of the graph back to the -left, at the point where the loop should occur, as that's conceptually what's -happening. But representing that in a text file is difficult. For now I'll -introduce the special `recur` value, and leave this whole section as TBD. - -`recur` is cousin of `in` and `out`, in that it's a special value and not an -operation. It takes whatever value it's set to and calls the current operation -with that as input. As an example, here is our now classic 0 through 9 printer -(assume `println` outputs whatever it was input): - -``` -// incr-1 is an operation which takes a 2-tuple and returns the same 2-tuple -// with the first element incremented. -( in -0-> } -incr-> } -> out - in -1-> } ) -> incr-1 - -( in -eq-> } -if-> out - in -> } -> } -0-> } -println-> } -incr-1-> } -> recur ) -> print-range - -0 -> } -print-range-> } -10 -> } -``` - -## Next Steps - -This post is long enough, and I think gives at least a basic idea of what I'm -going for. The syntax presented here is _extremely_ rudimentary, and is almost -definitely not what any final version of the syntax would look like. But the -general idea behind the structure is sound, I think. - -I have a lot of further ideas for Ginger I haven't presented here. Hopefully as -time goes on and I work on the language more some of those ideas can start -taking a more concrete shape and I can write about them. - -The next thing I need to do for Ginger is to implement (again) the graph type -for it, since the last one I implemented didn't include tuples. Maybe I can -extend it instead of re-writing it. After that it will be time to really buckle -down and figure out a syntax. Once a syntax is established then it's time to -start on the compiler! |