Pratt example

[39555]

An example of the usage of the Pratt parser for parsing a weird cexpr.

The result of the parsing is a nicely formatted ast and the expression in prefix notation.

1 + 2 + 3:

ADD
  ADD
    VAL 1
    VAL 2
  VAL 3

(+ (+ 1 2) 3)

Parser Problems

• [x] Parsing any complex postfix operator a ? b : c, foo(), foo(1 + 2), a[1 + 2] cannot be done without parsing it inside the operand parser (which means breaking the prefix precedence). Maybe the api should provide the input inside the operator closures. But I'm having strange lifetime troubles with it for now

  EDIT: I made it by switching all callbacks to function pointers fn() ->. I had completely forgotten about it. Now the input is a first argument in closures.

• [x] Maybe closures should return an error instead of plain value to allow validation of the input e.g dereferencing the literal 1->foo or handling unbalanced delimiter in complex postfixes.

  EDIT: all closures now return PResult. Looks quite ugly..

• [x] Concept of neither associativity. I don't know yet how it works but the parser could potentially reject a == b == c somehow.

  EDIT: added Assoc::Neither and tests. This should fail: a == b == c, a < b < c.

[39555]

Well, the example is complete and all the features are there! I assume with #618 it would be the same. I may check it later.

[epage]

@39555 I have to say, I am thoroughly impressed with the dedication you have put to this investigation, having

• Implemented the initial recursive version
• Implemented an iterative version out of my concern for stackoverflows
• Implemented a full C expression parser with it to see what feature are missing, adding features that I'm not seeing in other libraries that provide a generic "pratt" parser