Implement soft-mode for generated parsers with access to partial results on syntax errors

ref peggyjs#501

Author: frostburn

Diff for: CHANGELOG.md

@@ -9,6 +9,7 @@ Unreleased
 ### Breaking changes
 
 ### New features
+ - [#501](https://github.com/peggyjs/peggy/issues/501) Implement soft-mode for generated parsers with access to partial results on syntax errors.
 
 ### Bug fixes
 

Diff for: lib/compiler/passes/generate-js.js

@@ -1393,9 +1393,8 @@ function generateJS(ast, options) {
       "      peg$maxFailPos",
       "    });",
       "  }",
-      "  if (peg$result !== peg$FAILED && peg$currPos === input.length) {",
-      "    return peg$result;",
-      "  } else {",
+      "  var success = (peg$result !== peg$FAILED && peg$currPos === input.length);",
+      "  function fail() {",
       "    if (peg$result !== peg$FAILED && peg$currPos < input.length) {",
       "      peg$fail(peg$endExpectation());",
       "    }",
@@ -1408,6 +1407,14 @@ function generateJS(ast, options) {
       "        : peg$computeLocation(peg$maxFailPos, peg$maxFailPos)",
       "    );",
       "  }",
+      "  if (options.soft) {",
+      "     return {result: peg$result, success, fail};",
+      "  }",
+      "  if (success) {",
+      "    return peg$result;",
+      "  } else {",
+      "    fail();",
+      "  }",
       "}"
     );
 

Diff for: test/api/generated-parser-api.spec.js

@@ -14,12 +14,29 @@ describe("generated parser API", () => {
       expect(parser.parse("a")).to.equal("a");
     });
 
+    it("parses input in soft-mode", () => {
+      const parser = peg.generate("start = 'a'");
+
+      const result = parser.parse("a", {soft: true});
+      expect(result.result).to.equal("a");
+      expect(result.success).to.equal(true);
+    });
+
     it("throws an exception on syntax error", () => {
       const parser = peg.generate("start = 'a'");
 
       expect(() => { parser.parse("b"); }).to.throw();
     });
 
+    it("gives partial result on syntax error in soft-mode", () => {
+      const parser = peg.generate("start = 'a'+");
+
+      const result = parser.parse("aab", {soft: true});
+      expect(result.result).to.deep.equal(["a", "a"]);
+      expect(result.success).to.equal(false);
+      expect(result.fail).to.throw('Expected "a" or end of input but "b" found.');
+    });
+
     // Regression: https://github.com/peggyjs/peggy/pull/197
     it("correctly describe character class in syntax error", () => {
       const parser = peg.generate("start = [123-5]");

Diff for: test/cli/fixtures/imports_peggy.js

@@ -501,9 +501,8 @@ function peg$parse(input, options) {
       peg$maxFailPos
     });
   }
-  if (peg$result !== peg$FAILED && peg$currPos === input.length) {
-    return peg$result;
-  } else {
+  var success = (peg$result !== peg$FAILED && peg$currPos === input.length);
+  function fail() {
     if (peg$result !== peg$FAILED && peg$currPos < input.length) {
       peg$fail(peg$endExpectation());
     }
@@ -516,6 +515,14 @@ function peg$parse(input, options) {
         : peg$computeLocation(peg$maxFailPos, peg$maxFailPos)
     );
   }
+  if (options.soft) {
+     return {result: peg$result, success, fail};
+  }
+  if (success) {
+    return peg$result;
+  } else {
+    fail();
+  }
 }
 
 module.exports = {

Diff for: test/cli/fixtures/lib.js

@@ -471,9 +471,8 @@ function peg$parse(input, options) {
       peg$maxFailPos
     });
   }
-  if (peg$result !== peg$FAILED && peg$currPos === input.length) {
-    return peg$result;
-  } else {
+  var success = (peg$result !== peg$FAILED && peg$currPos === input.length);
+  function fail() {
     if (peg$result !== peg$FAILED && peg$currPos < input.length) {
       peg$fail(peg$endExpectation());
     }
@@ -486,6 +485,14 @@ function peg$parse(input, options) {
         : peg$computeLocation(peg$maxFailPos, peg$maxFailPos)
     );
   }
+  if (options.soft) {
+     return {result: peg$result, success, fail};
+  }
+  if (success) {
+    return peg$result;
+  } else {
+    fail();
+  }
 }
 
 module.exports = {