tree a93e66e793d1b480be8c75b25cb6e573e085bcc7
parent f1b74ba8eb6fcdd7faf6ecd330defb5add91117d
author Peter Kelly <peter@uxproductivity.com> 1441023323 +0700
committer Peter Kelly <peter@uxproductivity.com> 1441023323 +0700

Flat: Option to print expressions as trees

When printing out a grammar definition, add the ability to show the body
of each rule as an expression tree, with one node per line. This makes
it easier to understand the structure of a grammar. In the future, it
will also be useful for displaying information associated with each
node, such as when we implement type inference.

As an example, the following rule in the existing concrete format:

    Term    : $Add(Factor PLUS Term)
            | $Sub(Factor MINUS Term)
            | Factor;

look as follows when printed with the -e (or --exprtree) option:

    Term

        Choice
        |-- Label "Add"
        |   \-- Sequence
        |       |-- Ident "Factor"
        |       |-- Ident "PLUS"
        |       \-- Ident "Term"
        |-- Label "Sub"
        |   \-- Sequence
        |       |-- Ident "Factor"
        |       |-- Ident "MINUS"
        |       \-- Ident "Term"
        \-- Ident "Factor"