Log 122009

After a number of changes, some small, some large, the to ml compiled compiler accepts its own system file again. It also cleanly compiles simple programs to C (fac, list manipulations). Various thoughts:

1. I simplified the type checker to a prover using solely antecedents and consequents. Both are stored in a list, giving quadratic behavior. It should become O(n x log(n)) here.
2. In order to avoid quadratic, or worse, behavior I tried to avoid alpha conversion on lambda terms during substitution. There have been/are too many bugs because of that. Should solve this.
3. I don't inline yet. I should do this at some point.
   
4. The method invocations are compiled down to combinators. Case statements are a linear collection of tests, that means linear behavior on lookups. Not sure this is a problem actually.
5. Everything not essential graph rewriting is done through FFI (libffi) where all functions are loaded from dynamic libraries (libdl). Every FFI call is evaluated each time. FFI is slow already, I could/should add memoization here.
6. FFI calls are dynamic in nature leading to runtime failures when types are not matched. Should look into swig to automate the glue.
7. I should allow unary arguments to FFI, now it is assumed all FFI symbols refer to functions.
   
8. I wonder how combinators will work out long-time. There might be just too much copying - should think over how spurious copying can be avoided.
   
9. I need serialization. This should be simple and obvious if all data is treated as trees.
10. I started on a simple ReST parser for documentation which I intend to be part of the compiler.
11. I should prove the type system correct?

Explanation in order why I use FFI even for simple addition: This is probably the reverse of what one would expect since tradional compilers often optimize heavily on native types. But, to me, the order of requirements was: 1) fast graph rewriting, 2) good simple interfacing with C, and 3) reasonably fast compiler. Hence, I considered thinking about integers a waste of time, or, a nice to have.