I'd like to create a scanner for the ruby language, and to proceed
through test driven development.
As I understand it, test driven development means identifying a
before, an after, developing a test to determine whether the before
was successfully turned into the after, then writing the code to turn
the before into the after.
In the case of a ruby scanner, the before would be a string containing
ruby code, the after would be a Leo tree with the nodes partitioning
the ruby code into modules, classes, methods.
I'm concerned about two issues. The easiest, perhaps, first. The
scanner should take a string and return a portion of a Leo tree, and
my test should compare that Leo tree fragment to a pre-built Leo tree
fragment which is the expected result. Is there already a routine
that can do (deep) comparisons of trees, where equivalence is (a)
headers are identical (b) bodies are identical (c) children are
identical?
The harder problem may be ruby syntax. scanHelper() expects to be
able to find
whitespace
comments
strings
class definitions
function/method definitions
ids (names)
code blocks
"noise"
I'm worried about strings. Like /bin/sh and perl, Ruby supports
"<<here" documents, that is, strings which are introduced by "<<word",
and whose value is all the following lines of source up to the
occurrence of "word" on a line by itself. There are variations on
quoting and indentation, but those don't introduce difficult hurdles.
The real problem is that Ruby can also use "<<" as a method name, as
in the familiar left shift:
irb(main):002:0> 1<<8
=> 256
Thus to disambiguate the beginning of a string and the invocation of a
method, the parser needs to expect one or the other. I don't see how
to do this in cooperation with the current scanner class design. Does
anyone have an insight?
Hey, at least it's not perl, the parsing of which was recently show to
require solution of the halting problem:
http://www.perlmonks.org/?node_id=663393
- Stephen
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