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Now the lib directory contains what will eventually end up on the
user's /usr/lib/csc. When I write make install, it will copy all of
the .csc files from lib into the destination lib directory. This means I
can start working on the standard library in lib/scheme.
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I was using the load procedure to load a program, but by a strict
reading of R7RS, load can only handle expressions and definitions, not
imports. So instead I'm defining each test as a library, and using the
environment procedure to load them at runtime.
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Global variables will be stored in an array which is kept in register 0.
The linker will later translate each library-ref into an integer, which
is used as an index into the globals table.
This design makes implementing eval quite straightforward, the eval
bytecode will accept a globals table and a bytevector of bytecode, save
the current set of registers, set register 0 to the new globals table,
and begin executing the given bytecode. When eval is finished, it will
restore the saved registers and return to the previous instruction
pointer. In this way, calling eval can create a call stack.
We could think about implementing function calls with eval, and it's
cool that it would work, but the CPS transformation mostly makes this
irrelevant. It's interesting to think about implementing an interpreter,
where a function call would simply eval the function's bytecode, and the
function call stack would be handled automatically.
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The point of argument conversion is to validate on each function call
that the right number of arguments were passed, and to ensure that no
function has more than 1 argument. This second condition makes CPS
slightly simpler, and ensures that the arguments will all fit in locals.
We take the strategy of allocating a vector for each function call.
Ideally we would optimize away most of these allocations, but for now I
just want it to work.
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I was hesitant to add a diff library, but it was surprisingly easy. A
straightforward application of dynamic programming.
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I desperately need a diffing library.
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Now variables can't be updated after they're created, so they can be
freely copied into closures.
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Goodbye soup. Thanks to "Compiling with Continuations" by Appel.
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Wow we actually finished CPS. Next is closure conversion, then codegen,
and then we should be able to run some end to end tests. Then we can
look at garbage collection, and from there continue building features
down the long road to self hosting.
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I improved the abstraction in to-cps.
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