144 lines
7.6 KiB
Plaintext
144 lines
7.6 KiB
Plaintext
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%p
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It is the
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%strong one
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thing i said i wasn’t going to do: Write a language.
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There are too many languages out there already, and just because i want to write a vm,
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doesn’t mean i want to add to the language jungle.
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%strong But
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…
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%h2#the-gap The gap
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%p
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As it happens in life, which is why they say never to say never, it happens just like it
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i didn’t want. It turns out the semantic gap of what i have is too large.
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%p
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There is the
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%strong register level
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, which is approximately assembler, and there is the
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%strong vm level
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which is more or less the ruby level. So my head hurts from trying to implement ruby in assembler,
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no wonder.
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%p
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Having run into this wall, which btw is the same wall that crystal ran into, one can see the sense
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in what others have done more clearly: Why rubinus uses c++ underneath. Why crystal does not
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implement ruby, but a statically typed language. And ultimately why there is no ruby compiler.
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The gap is just too large to bridge.
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%h2#the-need-for-a-language The need for a language
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%p
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As I have the architecture of passes, i was hoping to get by with just another layer in the
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architecture. A tried an tested approach after all. And while i won’t say that that isn’t a
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possibility, i just don’t see it. I think it may be one of those where hindsight will be perfect.
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%p
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I can see as far as this: If i implement a language, that will mean a parser, ast and compiler.
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The target will be my register layer. So a reasonable step up is a sort of object c, that has
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basic integer maths and object access. I’ll detail that more below, but the point is, if i have
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that, i can start writing a vm implementation in that language.
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%p
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Off course the vm implementation involves a parser, an ast and a compiler, unless we go to the free
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compilers (see below). And so implementing the vm in a new language is in essence swapping nodes of
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the higher level tree with nodes of the lower level (c-ish) one. Ie parsing should not strictly
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speaking be necessary. This node swapping is after all what the pass architecture was designed
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to do. But, as i said, i just can’t see that happening (yet?).
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%h3#trees-vs-blocks Trees vs. Blocks
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%p
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Speaking of the Pass architecture: I flopped. Well, maybe not so much with the actual Passes, but
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with the Method representation. Blocks holding Instructions, and being in essence a list.
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Misinformed copying from llvm, misinformed by the final outcome. Off course the final binary
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has a linear address space, but that is where the linearity ends. The natural structure of code
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is a tree, not a list, as demonstrated by the parse
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= succeed "." do
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%em tree
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%h2#soml---salama-object-machine-language Soml - Salama Object Machine Language
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%h3#typed Typed
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%p
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Quite a while before crystallizing into the idea of a new language, i already saw the need for a type
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system. Off course, and this dates back to the first memory layouts. But i mean the need for a
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%em strong typing
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system, or maybe it’s even clearer to call it compile time typing. The type that c
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and c++ have. It is essential (mentally, this is off course all for the programmer, not the computer)
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to be able to think in a static type system, and then extend that and make it dynamic.
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Or possibly use it in a dynamic way.
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%p
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This is a good example of this too big gap, where one just steps on quicksand if everything is
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all the time dynamic.
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%p
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The way i had the implementation figured was to have different versions of the same function. In
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each function we would have compile time types, everything known. I’ll probably still do that,
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just written in Soml.
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%h3#machine-language Machine language
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%p
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Soml is a machine language for the Salama machine. As i tried to implement without this layer, i was
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essentially implementing in assembler. Too much.
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%p
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There are two main feature we need from the machine language, one is typed a typed oo memory model,
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the other an oo call model.
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%h3#object-c Object c
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%p
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The language needs to be object based, off course. Just because it’s typed and not dynamic
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and closer to assembler, doesn’t mean we need to give up objects. In fact we mustn’t. Soml
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should be a little bit like c++, ie compile time known variable arrangement and types,
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objects. But no classes (or inheritance), more like structs, with full access to everything.
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So a struct.variable syntax would mean grab that variable at that address, no functions, no possible
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override, just get it. This is actually already implemented as i needed it for the slot access.
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%p So objects without encapsulation or classes. A lower level object orientation.
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%h3#whitequark Whitequark
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%p
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This new approach (and more experience) shed a new light on ruby parsing. The previous idea was to
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start small, write the necessary stuff in the parsable subset and with time expand that set.
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%p
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Alas . . ruby is a beast to parse, and because of the
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%strong semantic gap
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writing the system,
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even in a subset, is not viable. And it turns out the brave warriors of the ruby community have
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already produced a pure, production ready,
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= succeed "." do
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%a{:href => "https://github.com/whitequark/parser"} ruby parser
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%h3#interoperability Interoperability
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%p
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The system code needs to be callable from the higher level, and possibly the other way around.
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This probably means the same or compatible calling mechanism and data model. The data model is
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quite simple as the at the system level all is just machine words, but in object sized
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packets. As for the calling it will probably mean that the same message object needs to be used
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and what is now called calling at the machine level is supported. Sending off course won’t be.
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%h3#still-missing-a-piece Still missing a piece
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%p
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How the level below calling can be represented is still open. It is clear though that it does need
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to be present, as otherwise any kind of concurrency is impossible to achieve. The question ties
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in with the still open question of
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= succeed "." do
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%a{:href => "http://valerieaurora.org/synthesis/SynthesisOS/ch4.html"} Quajects
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%h3#start-small Start small
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%p The first next step is to wrap the functionality i have in the Passes as a language.
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%p Then to expand that language, by writing increasingly more complex programs in it.
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%p
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And then to re-attack ruby using the whitequark parser, that probably means jumping on the
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mspec train.
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%p All in all, no biggie :-)
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%h2#compilers-are-not-free Compilers are not free
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%p
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Oh and i re-read and re-watched Toms
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%a{:href => "http://codon.com/compilers-for-free"} compilers for free
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talk,
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which did make quite an impression on me the first time. But when i really thought about actually
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going down that road (who does’t enjoy a free beer), i got into the small print.
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%p
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The second biggest of which is that writing a partial evaluator is just about as complicated
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as writing a compiler.
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%p
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But the biggest problem is that the (free) compiler you could get, has the implementation language
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of the evaluator, as it’s
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= succeed "." do
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%strong output
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%em for
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c, not for ruby.
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%p
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Ok, maybe it is not quite as bad as that makes it sound. As i do have the register layer ready
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and will be writing a c-ish language, it may even be possible to write an interpreter
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= succeed "," do
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%strong in soml
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%strong for soml
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too.
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%p
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I will nevertheless go the straighter route for now, ie write a compiler, and maybe return to the
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promised freebie later. It does feel like a lot of what the partial evaluator is, would be called
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compiler optimization in another lingo. So may be road will lead there naturally.
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