WIP this probably works now?

My only concern is that state may not be correctly piped through all the combinator functions. This would require a lot of extra testing to validate. The good part is it wasn't much work to make the test suite pass, and I ran this version of the library against three of my programming languages, including Squiggle, and it appeared to function correctly still.

examples/python-ish.js

@@ -7,96 +7,46 @@ let P = require('..');
 
 ///////////////////////////////////////////////////////////////////////
 
-// LIMITATIONS: Python allows not only multiline blocks, but inline blocks too.
-//
-//   if x == y: print("nice")
-//
-// vs.
-//
-//   if x == y:
-//       print("nice")
-//
-// This parser only supports the multiline indented form.
-
-// NOTE: This is a hack and is not recommended. Maintaining state throughout
-// Parsimmon parsers is not reliable since backtracking may occur, leaving your
-// state inaccurate. See the relevant GitHub issue for discussion.
-//
-// https://github.com/jneen/parsimmon/issues/158
-//
-function indentPeek() {
-  return indentStack[indentStack.length - 1];
-}
-
-let indentStack = [0];
-
 let Pythonish = P.createLanguage({
-  // If this were actually Python, "Block" wouldn't be a statement on its own,
-  // but rather "If" and "While" would be statements that used "Block" inside.
-  Statement: r =>
-    P.alt(r.ExpressionStatement, r.Block),
+  Program: r =>
+    r.Statement.many().trim(r._).node('Program'),
 
-  // Just a simple `foo()` style function call.
-  FunctionCall: () =>
-    P.regexp(/[a-z]+/).skip(P.string('()'))
-      .node('FunctionCall')
-      .desc('a function call'),
+  Statement: r =>
+    P.alt(r.Call, r.Block),
 
-  // To make it a statement we just need a newline afterward.
-  ExpressionStatement: r =>
-    r.FunctionCall.skip(P.string('\n')),
+  Call: r =>
+    P.letter.skip(P.string('()')).skip(r.End).node('Call'),
 
-  // The general idea of this is to assume there's "block:" on its own line,
-  // then we capture the whitespace used to indent the first statement of the
-  // block, and require that every other statement has the same exact string of
-  // indentation in front of it.
   Block: r =>
     P.seqObj(
       P.string('block:'),
-      P.string('\n'),
-      ['indent', P.regexp(/[ ]+/)],
-      ['statement', r.Statement]
-    ).chain(args => {
-      // `.chain` is called after a parser succeeds. It returns the next parser
-      // to use for parsing. This allows subsequent parsing to be dependent on
-      // previous text.
-      let {indent, statement} = args;
-      let indentSize = indent.length;
-      let currentSize = indentPeek();
-      // Indentation must be deeper than the current block context. Otherwise
-      // you could indent *less* for a block and it would still work. This is
-      // not how any language I know of works.
-      if (indentSize <= currentSize) {
-        return P.fail('at least ' + currentSize + ' spaces');
-      }
-      indentStack.push(indentSize);
-      return P.string(indent)
-        .then(r.Statement)
-        .many()
-        .map(statements => {
-          indentStack.pop();
-          return [statement].concat(statements);
-        });
-    })
-    .node('Block'),
+      r.End,
+      P.indentMore,
+      ['first', r.Statement],
+      ['rest', P.indentSame.then(r.Statement).many()],
+      P.indentLess
+    ).map(args => {
+      let {first, rest} = args;
+      let statements = [first, ...rest];
+      return {statements};
+    }).node('Block'),
+
+  _: () => P.optWhitespace,
+  NL: () => P.string('\n'),
+  End: r => P.alt(r.NL, P.eof),
 });
 
 ///////////////////////////////////////////////////////////////////////
 
 let text = `\
+z()
 block:
-    a()
-    b()
+  a()
+  b()
+  block:
     c()
-    block:
-      d()
-      e()
-      f()
-    block:
-        g()
-        h()
-        i()
-        j()
+    d()
+e()
 `;
 
 function prettyPrint(x) {
@@ -105,5 +55,5 @@ function prettyPrint(x) {
   console.log(s);
 }
 
-let ast = Pythonish.Block.tryParse(text);
+let ast = Pythonish.Program.tryParse(text);
 prettyPrint(ast);