PEG for Python style indentation - parsing

Question

How would you write a Parsing Expression Grammar in any of the following Parser Generators (PEG.js, Citrus, Treetop) which can handle Python/Haskell/CoffeScript style indentation:
Examples of a not-yet-existing programming language:
square x =
x * x
cube x =
x * square x
fib n =
if n <= 1
0
else
fib(n - 2) + fib(n - 1) # some cheating allowed here with brackets
Update:
Don't try to write an interpreter for the examples above. I'm only interested in the indentation problem. Another example might be parsing the following:
foo
bar = 1
baz = 2
tap
zap = 3
# should yield (ruby style hashmap):
# {:foo => { :bar => 1, :baz => 2}, :tap => { :zap => 3 } }

Answer 1

Pure PEG cannot parse indentation.
But peg.js can.
I did a quick-and-dirty experiment (being inspired by Ira Baxter's comment about cheating) and wrote a simple tokenizer.
For a more complete solution (a complete parser) please see this question: Parse indentation level with PEG.js
/* Initializations */
{
function start(first, tail) {
var done = [first[1]];
for (var i = 0; i < tail.length; i++) {
done = done.concat(tail[i][1][0])
done.push(tail[i][1][1]);
}
return done;
}
var depths = [0];
function indent(s) {
var depth = s.length;
if (depth == depths[0]) return [];
if (depth > depths[0]) {
depths.unshift(depth);
return ["INDENT"];
}
var dents = [];
while (depth < depths[0]) {
depths.shift();
dents.push("DEDENT");
}
if (depth != depths[0]) dents.push("BADDENT");
return dents;
}
}
/* The real grammar */
start = first:line tail:(newline line)* newline? { return start(first, tail) }
line = depth:indent s:text { return [depth, s] }
indent = s:" "* { return indent(s) }
text = c:[^\n]* { return c.join("") }
newline = "\n" {}
depths is a stack of indentations. indent() gives back an array of indentation tokens and start() unwraps the array to make the parser behave somewhat like a stream.
peg.js produces for the text:
alpha
beta
gamma
delta
epsilon
zeta
eta
theta
iota
these results:
[
"alpha",
"INDENT",
"beta",
"gamma",
"INDENT",
"delta",
"DEDENT",
"DEDENT",
"epsilon",
"INDENT",
"zeta",
"DEDENT",
"BADDENT",
"eta",
"theta",
"INDENT",
"iota",
"DEDENT",
"",
""
]
This tokenizer even catches bad indents.

Answer 2

I think an indentation-sensitive language like that is context-sensitive. I believe PEG can only do context-free langauges.
Note that, while nalply's answer is certainly correct that PEG.js can do it via external state (ie the dreaded global variables), it can be a dangerous path to walk down (worse than the usual problems with global variables). Some rules can initially match (and then run their actions) but parent rules can fail thus causing the action run to be invalid. If external state is changed in such an action, you can end up with invalid state. This is super awful, and could lead to tremors, vomiting, and death. Some issues and solutions to this are in the comments here: https://github.com/dmajda/pegjs/issues/45

Answer 3

So what we are really doing here with indentation is creating something like a C-style blocks which often have their own lexical scope. If I were writing a compiler for a language like that I think I would try and have the lexer keep track of the indentation. Every time the indentation increases it could insert a '{' token. Likewise every time it decreases it could inset an '}' token. Then writing an expression grammar with explicit curly braces to represent lexical scope becomes more straight forward.

Answer 4

You can do this in Treetop by using semantic predicates. In this case you need a semantic predicate that detects closing a white-space indented block due to the occurrence of another line that has the same or lesser indentation. The predicate must count the indentation from the opening line, and return true (block closed) if the current line's indentation has finished at the same or shorter length. Because the closing condition is context-dependent, it must not be memoized.
Here's the example code I'm about to add to Treetop's documentation. Note that I've overridden Treetop's SyntaxNode inspect method to make it easier to visualise the result.
grammar IndentedBlocks
rule top
# Initialise the indent stack with a sentinel:
&{|s| #indents = [-1] }
nested_blocks
{
def inspect
nested_blocks.inspect
end
}
end
rule nested_blocks
(
# Do not try to extract this semantic predicate into a new rule.
# It will be memo-ized incorrectly because #indents.last will change.
!{|s|
# Peek at the following indentation:
save = index; i = _nt_indentation; index = save
# We're closing if the indentation is less or the same as our enclosing block's:
closing = i.text_value.length <= #indents.last
}
block
)*
{
def inspect
elements.map{|e| e.block.inspect}*"\n"
end
}
end
rule block
indented_line # The block's opening line
&{|s| # Push the indent level to the stack
level = s[0].indentation.text_value.length
#indents << level
true
}
nested_blocks # Parse any nested blocks
&{|s| # Pop the indent stack
# Note that under no circumstances should "nested_blocks" fail, or the stack will be mis-aligned
#indents.pop
true
}
{
def inspect
indented_line.inspect +
(nested_blocks.elements.size > 0 ? (
"\n{\n" +
nested_blocks.elements.map { |content|
content.block.inspect+"\n"
}*'' +
"}"
)
: "")
end
}
end
rule indented_line
indentation text:((!"\n" .)*) "\n"
{
def inspect
text.text_value
end
}
end
rule indentation
' '*
end
end
Here's a little test driver program so you can try it easily:
require 'polyglot'
require 'treetop'
require 'indented_blocks'
parser = IndentedBlocksParser.new
input = <<END
def foo
here is some indented text
here it's further indented
and here the same
but here it's further again
and some more like that
before going back to here
down again
back twice
and start from the beginning again
with only a small block this time
END
parse_tree = parser.parse input
p parse_tree

Answer 5

I know this is an old thread, but I just wanted to add some PEGjs code to the answers. This code will parse a piece of text and "nest" it into a sort of "AST-ish" structure. It only goes one deep and it looks ugly, furthermore it does not really use the return values to create the right structure but keeps an in-memory tree of your syntax and it will return that at the end. This might well become unwieldy and cause some performance issues, but at least it does what it's supposed to.
Note: Make sure you have tabs instead of spaces!
{
var indentStack = [],
rootScope = {
value: "PROGRAM",
values: [],
scopes: []
};
function addToRootScope(text) {
// Here we wiggle with the form and append the new
// scope to the rootScope.
if (!text) return;
if (indentStack.length === 0) {
rootScope.scopes.unshift({
text: text,
statements: []
});
}
else {
rootScope.scopes[0].statements.push(text);
}
}
}
/* Add some grammar */
start
= lines: (line EOL+)*
{
return rootScope;
}
line
= line: (samedent t:text { addToRootScope(t); }) &EOL
/ line: (indent t:text { addToRootScope(t); }) &EOL
/ line: (dedent t:text { addToRootScope(t); }) &EOL
/ line: [ \t]* &EOL
/ EOF
samedent
= i:[\t]* &{ return i.length === indentStack.length; }
{
console.log("s:", i.length, " level:", indentStack.length);
}
indent
= i:[\t]+ &{ return i.length > indentStack.length; }
{
indentStack.push("");
console.log("i:", i.length, " level:", indentStack.length);
}
dedent
= i:[\t]* &{ return i.length < indentStack.length; }
{
for (var j = 0; j < i.length + 1; j++) {
indentStack.pop();
}
console.log("d:", i.length + 1, " level:", indentStack.length);
}
text
= numbers: number+ { return numbers.join(""); }
/ txt: character+ { return txt.join(""); }
number
= $[0-9]
character
= $[ a-zA-Z->+]
__
= [ ]+
_
= [ ]*
EOF
= !.
EOL
= "\r\n"
/ "\n"
/ "\r"