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README.md

This started as a faithful implementation of Schorre's VM, starting with Simon Forman's code at http://comments.gmane.org/gmane.comp.lang.smalltalk.fonc/3642

There were some minor bugs in that code: erroring on trying to match tokens at the end-of-input.

There was a bug in the compiler/VM design. (Update: I think I was confused about the original design. It has a .EMPTY directive for you to use in rules like the below. I'm leaving the following remarks alone until I get around to figuring out how I misunderstood. If I ever do -- now the starting code linked above seems to be gone.) Consider this grammar:

THING = FAILURE | .OUT('hey') | .OUT('should not happen') .,
FAILURE = 'a rule I expect to fail' .,

Trying to match THING writes both 'hey' and 'should not happen'.

Schorre's compiler handles output expressions separately from other primary expressions, because they always succeed and hence needn't be followed by a test for failure. But there are a couple of other expression types with the same property. The new compiler takes advantage of these and also fixes the above bug, at the cost of making the output instructions set the success flag (which Schorre's VM did not bother doing).

(At first I tried to do this optimization without making the instructions set the flag, and spent no-kidding hours chasing a bug. This was when I added tracing, though actually it might've been more helpful to have added an ASSERT-WIN instruction in appropriate places (with identical behavior to WIN-OR-DIE, but meant to check that the assembly is correct, whereas WIN-OR-DIE is for reporting syntax errors in the input). Anyway, with context-free code generation it just didn't work out as significantly smaller unless everything touches the success flag.)

Various other aspects got modernized or streamlined along the way.

Differences from Schorre's design:

  • Separate value and call stacks make it Forthier. The value stack and its operations replace the LABEL1/LABEL2 instructions.
  • The call stack includes, for helpful backtrace on error, the destination label.
  • Changed all the instruction names.
  • No ADR or END instructions. We start at address 0 and end by returning. Just s/ADR/GOTO/ in your compiler.
  • Also no SET instruction; subsumed by new WRITE_Q and WIN_LOOP instructions.
  • The WRITE instructions set the success flag.
  • New READ_EOF and WRITE_Q instructions.
  • READ_DECIMAL (the old NUM) only matches integers now.
  • READ_ID allows underscores.
  • There's a trace mode. TODO: add an instruction to turn it on or off.
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