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anonymous /ParsingProblem.swim Secret
Created Aug 19, 2014

What would you like to do?
This problem involves parsing a *syntax* that defines a *block* of code
composed of two or more *statements*. The syntax supports 2 different ways to
define the block. The problem is how to determine when a statement ends.
Here is the example code written in Ruby:
def f(x, y)
a = x * y
a + 1
This is a function named `f` that has a block containing 2 statements.
In the problem syntax there are 2 main ways to write this:
* Indentation-based:
\f(x, y)
a = x * y
a + 1
* Bracket-based:
\f(x, y) { a = x * y; a + 1 }
There are many permutations of these. For example the bracket based one can be:
\f(x, y) { a
= x
* y
; a
+ 1
The driving principle here is that when you have braces, statements are
terminated by semicolons, or the closing brace. This gives you lots of
formatting freedom.
In indentation based scoping, statements are terminated by an EOL *after* a
complete construct. This is to say, that you can split a statement into 2
lines, if the first line cannot be valid on its own:
\f(x, y)
a = x *
a + 1
Let's look at Ruby, which is a typical syntax that tries to avoid semicolons
but is forced to support backslash continuation. Note: this is getting close to
the heart of this problem.
This is valid Ruby:
def f(x, y)
a = x *
a + 1
The first statement can be split because the first half is not complete.
This is invalid Ruby:
def f(x, y)
a = x
* y
a + 1
The first half is valid. The line ends, so the statement ends. The second part
is invalid, so we have a syntax error.
The fix here is a continuation marker:
def f(x, y)
a = x \
* y
a + 1
This makes sense, because Ruby is a *bracketed* syntax with no mandatory
statement terminator (semicolon).
Here's the *new* idea that is not supported anywhere yet (afaik). If you are
using indentation scoping, you can also use indentation for statement
\f(x, y)
a = x
* y
a + 1
The parser can look 1 token ahead for an indent, and take that to mean a
continuation, if indentation is not valid for some other reason. This is a
really nice addition to indentation based scoping. Note: I've suggested it to
the CoffeeScript project, but they've not used it yet.
OK, that's the groundwork. The problem that vexes me is how to express this
cleanly in a grammar.
= Strawman Solution #1
Indentation has 4 distinct tokens:
- indent :: start an indentation block
- ondent :: match current indentation on a line
- undent :: end an indentation block
- andent :: a single line indented more than current indentation
So `andent` is what we use for continuation. If we do that, we simply make
`andent` a part of whitespace (token separation) while in the
indentation-honoring scope.
So then we have 3 grammars (classes):
- The main grammar
- The grammar for `{}` blocking
- The grammar for indent blocking
with possibly different whitespace meaning in each. Here's the grammar. It's
stripped down to the problem at hand. (Note: using Pegex syntax here, but should
be easily understandable):
%grammar main
code: func-def*
func-def: '\' func-name arg-list? code-body
code-body: bracket-code-body | indent-code-body
assignment: symbol + '=' + expression
expression: symbol + binary-operator + symbol
%grammar indentation
indent-code-body: indent statement+ undent
statement: ( assignment | expression ) EOL
ws: comment | SPACE | EOL andent
%grammar bracketed
bracket-code-body: '{' ( statement* %% ';' ) '}'
statement: assignment | expression
ws: comment | SPACE | EOL
Pegex Note: `+` means `ws+` (1 or more whitespace matches). Also `-` means
`ws*` but that's not used in theis example.
The important thing is that you don't want to duplicate every rule under
something as high level as a code block. You also don't want to have `andent`
as a token in the rules because it could be almost anywhere. By mixing it into
whitespace matching (if this is really possible), we win, because we have to
account for that anyway and it has the minimal syntax helpers of (`-` and `+`).
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