4 ms·

I wrote a parser/interpreter for VBA a few years back, and even though the core concepts were straightforward, conforming to a language that had been in develop

by 13of40 3y ago

I wrote a parser/interpreter for VBA a few years back, and even though the core concepts were straightforward, conforming to a language that had been in development for decades turned out to be brain-wreckingly tedious. One of the biggest parts of it was a block of thousands(?) of conditional statements to figure out the result type of every combination of type+operator+type. At the end of the ordeal, though, I threw the engine into a test app that looked like GWBASIC, and got a little nostalgia kick from being able to run BASIC like it was 1983.

sancarn 3y ago
Is that really needed? You can use VBA to do all the heavy lifting. That's what we do here: https://github.com/sancarn/stdVBA/blob/master/src/stdLambda.cls#L939-L962 https://github.com/sancarn/stdVBA/blob/master/src/stdLambda....
tomcam 3y ago
That and there were never official grammars published for any version of any basic from Microsoft afaik
- makeset 3y ago
  I doubt they ever existed. I had to write a VBScript parser some 25 years ago (to syntax-check ASP files before they would crash live) and to get it to parse valid syntax correctly, I ended up with a messy patchwork of logic which was also letting through some really pathological edge cases. While trying to fix it, to my utter delight, I found that Microsoft’s parsers also allowed the exact same cases which looked nothing like valid code otherwise. I don’t recall specific examples anymore, but they were far from obvious without looking at the parser code, and I made and won bets with other programmers on whether those snippets would throw syntax errors or not.
  - jakobson14 3y ago
    > I doubt they ever existed. Probably right, since Microsoft's BASIC interpreter is probably derived from the one that Bill Gates wrote as a teenage slacker. He wrote it, tried to sell it at the homebrew computer club, and then threw a hissy fit when people kept copying his paper tapes for their friends. Thus, Microsoft was born. (He got lucky later when Microsoft were chosen by IBM to develop a DOS, then bought a working DOS from somebody else. The rest as they say is history)
    - vasac 3y ago
      By the time Microsoft secured a deal with IBM, they were already a multimillion-dollar company, which was quite significant in 1980. Judging by their future moves, it's evident that luck wasn't the only factor contributing to their massive success.
      - adrian_b 3y ago
        Microsoft was the most important vendor of programming language compilers and interpreters for the CP/M operating system, for computers based on Intel 8080 or Zilog Z80. They had compilers for many programming languages, but before buying the operating system that they have rebranded proudly as MS-DOS and which they have resold to IBM at an overprice, they had nothing to do with operating systems. So the deal had nothing to do with their previous technical competence, but they happened to know the right information about who is selling and who is buying, which allowed them to gain the most as the middlemen.
pests 3y ago
Did you need thousands of conditionals? Couldn't you map types and operators into an efficient representation for a single lookup?!
ww520 3y ago
That's amazing to implement a full interpreter for VBA. I would imagine a lot of the type resolution are redundant. E.g. +, -, *, and / would have the same type resolution for numbers. Those can reuse the same type resolver. May be + also work on string. Then call the string type resolver on it in addition to see if it can be resolved. Anyway, kudos for the amazing work.
- sancarn 3y ago
  I have an open source compiler-interpreter for VBA here: https://github.com/sancarn/stdVBA/blob/master/src/stdLambda.cls https://github.com/sancarn/stdVBA/blob/master/src/stdLambda.... In case you were interested :P (I say compiler, it compiled to byte code, not asm.)
userbinator 3y ago
One of the biggest parts of it was a block of thousands(?) of conditional statements to figure out the result type of every combination of type+operator+type This is not something uncommon for a dynamically typed language; I think the simplest approach is a multidimensional array lookup.
luismedel 3y ago
Same boat here. I wrote a VB to C++ transpiler to make VB6-like IDE for PalmOS (basically a clone of the existing Handheld Basic++). The type system was a mess. After one year of development on my free time I had the compiler and a form editor, but lots of out of memory errors when running the code on the simulator (and real devices) :-| At the same time, Palm sold the OS licenses to ACCESS inc., IIRC, and the future of the platform was sealed. Long story short, I shelved the project and I lost it during one of my cyclical "move backups to to a bigger disk" migration.

by 13of40 3y ago

sancarn 3y ago

Is that really needed? You can use VBA to do all the heavy lifting. That's what we do here: https://github.com/sancarn/stdVBA/blob/master/src/stdLambda.cls#L939-L962 https://github.com/sancarn/stdVBA/blob/master/src/stdLambda....

tomcam 3y ago

That and there were never official grammars published for any version of any basic from Microsoft afaik

makeset 3y ago
I doubt they ever existed. I had to write a VBScript parser some 25 years ago (to syntax-check ASP files before they would crash live) and to get it to parse valid syntax correctly, I ended up with a messy patchwork of logic which was also letting through some really pathological edge cases. While trying to fix it, to my utter delight, I found that Microsoft’s parsers also allowed the exact same cases which looked nothing like valid code otherwise. I don’t recall specific examples anymore, but they were far from obvious without looking at the parser code, and I made and won bets with other programmers on whether those snippets would throw syntax errors or not.
- jakobson14 3y ago
  > I doubt they ever existed. Probably right, since Microsoft's BASIC interpreter is probably derived from the one that Bill Gates wrote as a teenage slacker. He wrote it, tried to sell it at the homebrew computer club, and then threw a hissy fit when people kept copying his paper tapes for their friends. Thus, Microsoft was born. (He got lucky later when Microsoft were chosen by IBM to develop a DOS, then bought a working DOS from somebody else. The rest as they say is history)
  - vasac 3y ago
    By the time Microsoft secured a deal with IBM, they were already a multimillion-dollar company, which was quite significant in 1980. Judging by their future moves, it's evident that luck wasn't the only factor contributing to their massive success.
    - adrian_b 3y ago
      Microsoft was the most important vendor of programming language compilers and interpreters for the CP/M operating system, for computers based on Intel 8080 or Zilog Z80. They had compilers for many programming languages, but before buying the operating system that they have rebranded proudly as MS-DOS and which they have resold to IBM at an overprice, they had nothing to do with operating systems. So the deal had nothing to do with their previous technical competence, but they happened to know the right information about who is selling and who is buying, which allowed them to gain the most as the middlemen.

pests 3y ago

Did you need thousands of conditionals? Couldn't you map types and operators into an efficient representation for a single lookup?!

ww520 3y ago

That's amazing to implement a full interpreter for VBA. I would imagine a lot of the type resolution are redundant. E.g. +, -, *, and / would have the same type resolution for numbers. Those can reuse the same type resolver. May be + also work on string. Then call the string type resolver on it in addition to see if it can be resolved. Anyway, kudos for the amazing work.

sancarn 3y ago
I have an open source compiler-interpreter for VBA here: https://github.com/sancarn/stdVBA/blob/master/src/stdLambda.cls https://github.com/sancarn/stdVBA/blob/master/src/stdLambda.... In case you were interested :P (I say compiler, it compiled to byte code, not asm.)

userbinator 3y ago

One of the biggest parts of it was a block of thousands(?) of conditional statements to figure out the result type of every combination of type+operator+type This is not something uncommon for a dynamically typed language; I think the simplest approach is a multidimensional array lookup.

luismedel 3y ago

Same boat here. I wrote a VB to C++ transpiler to make VB6-like IDE for PalmOS (basically a clone of the existing Handheld Basic++). The type system was a mess. After one year of development on my free time I had the compiler and a form editor, but lots of out of memory errors when running the code on the simulator (and real devices) :-| At the same time, Palm sold the OS licenses to ACCESS inc., IIRC, and the future of the platform was sealed. Long story short, I shelved the project and I lost it during one of my cyclical "move backups to to a bigger disk" migration.