carnac

Magical function memoization across program invocations.

Does It Work?

No, due to a subtle compiler bug with typed macros. This repository is in fact a canary with which we can monitor progress on said bug. The original bug surfaced in https://github.com/disruptek/cps but due to the complexity of that code, we need a simpler reproduction such as this.

How It Works

I mean, assuming it works, which it doesn't, this is how it works:

1. During compilation, we define a table holding the hash of inputs and their outputs for each func with the .carnac. pragma.
2. At runtime, we attempt to load the table from a file in $XDG_RUNTIME_DIR/.carnac, named according to the func signature.
3. We use the table as a cache for subsequent calls, updating it when necessary.
4. At program exit, we store the table to disk using frosty and supersnappy.

And, like, if you take the macro output and compile it indirectly, it does work. 😉

Demo

This example is from the test in the tests/ subdirectory. The program caches 1,125 bytes of data to disk.

import testes
import carnac

{.experimental: "strictFuncs".}

template test(fn: typed; x: int; r: int) =
  let y = fn(x)
  if y != r:
    raise newException(AssertionDefect, "compute")

testes:
  block:
    ## without carnac

    func fib1(x: int): int =
      case x
      of 0: 1
      of 1: 1
      else:
        fib1(x - 1) + fib1(x - 2)

    block ten:
      test fib1, 10, 89
    block forty_five:
      test fib1, 45, 1836311903

  block:
    ## with carnac

    func fib2(x: int): int {.carnac.} =
      case x
      of 0: 1
      of 1: 1
      else:
        # fib2(x - 1) + fib2(x - 2) does not work
        let q = x - 1
        let z = x - 2
        # fib2(q) + fib2(z) does not work
        let a = fib2(q) # x - 1 does not work
        let b = fib2(z) # x - 2 does not work
        a + b           # this works

    block ten:
      test fib2, 10, 89
    block forty_five:
      test fib2, 45, 1836311903

Installation

$ nimph clone carnac

or if you're still using Nimble like it's 2012,

$ nimble install https://github.com/disruptek/carnac

Documentation

The documentation is rebuilt during the CI process and hosted on GitHub.

License

MIT