Compile Time D Expression Parser?

[Philippe Sigaud]

> Almost the complete language is available in CTFE, therefore classes
could be used to implement the parse tree representation. However, a
limitation that does exist is that classes that were created in CTFE cannot
yet be stored in static variables or enums. How will the interface to your
library look like?

I recently wrote a parsing expression grammar module in D, also to create
grammars and parsers at compile-time and parse inputs at CT.

(PEG: http://en.wikipedia.org/wiki/Parsing_expression_grammar)

Usage is like this:

mixin Grammar(
      "JSON <- '{' ( Pair ( ',' Pair )* )? '}'"
    , "Pair <- String ':' Value"
    , "Value <- String / Number / JSON / Array / True / False / Null"
    , `True <- "true"`
(..., rest of JSON grammar)
);

enum result = JSON.parse(
`{ "Hello":42,
    "World!":
    {
        "a":[0,1,2,3]
    }
}`);

I deliberatly used classes to construct the parsers, for I wanted an
extended class template example in a big tutorial on templates I'm writing.
For now, the resulting grammars are space-insensitive, because I grew tired
of always inserting Spaces parsers everywhere.

The parse tree is done with a tree struct. Since it holds strings
(captures), it can be manipulated at CT to recreate new code. Any
tree-walking function can collect the captures to build a D code string
which can then be mixed in.

For exampe, last week, I created a template constraints parser, to then
test the resulting tree with template parameters. It tests the entire
constraint with passed parameters and, if it fails, it recursively tests
the sub-constraints to find ones that return false.
So, given "if (isIntegral!T && !(isFloatingPoint!(U) || is(U : W)))", it
will test "isIntegral!T" and so on.

All in all, it's quite fun and works OK, it just slows down compilation a
bit. What was just an example in a tutorial is slowly becoming its own
project. I think I'll put it on Github in a week or two.

Philippe
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