Why must implementing Python be hard unlike Scheme?
nagle at animats.com
Thu Feb 21 18:28:53 CET 2008
seberino at spawar.navy.mil wrote:
> I'm learning Scheme and I am amazed how easy it is to start building a
> half baked Scheme implementation that somewhat works.
> After knowing Python for *years* I have no idea how to actually
> implement the darn thing.
Why? It's not very difficult. Get a parser for LALR(1) grammars,
like YACC or Bison, write a tokenizer that understands Python indentation,
hook up a dictionary, and parse the thing into a tree. This is all
covered in Compilers 101. Get the Dragon Book if you don't know this stuff.
Once you have a tree, figure out some way to encode the tree into a linear
form, and write an execution engine that reads byte codes and has the
big switch to call the function for each byte code.
The run-time data implementation is all dictionaries. In Python,
everything is a variable-sized hash. You don't even have to allocate
storage during compile time. The run-time environment is a tree of hashes.
The resulting implementation will be slow, but then, so is CPython.
Too much time goes into hash lookups.
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