Add ParseCheck CLI

build.gradle

@@ -53,4 +53,12 @@ task parseExamples(type: JavaExec, dependsOn: testClasses) {
     mainClass = 'com.mpl.test.ParseExamples'
     classpath = sourceSets.test.runtimeClasspath
     args = ['examples']
+}
+
+// Grammar-check CLI (see ParseCheck.java): file paths or '-' for stdin via
+// --args passthrough, e.g. ./gradlew -q parseCheck --args='examples/01_hello_world.mpl'
+task parseCheck(type: JavaExec, dependsOn: classes) {
+    mainClass = 'com.mpl.tools.ParseCheck'
+    classpath = sourceSets.main.runtimeClasspath
+    standardInput = System.in
 }

src/main/java/com/mpl/tools/ParseCheck.java

@@ -0,0 +1,71 @@
+package com.mpl.tools;
+
+import com.mpl.parser.*;
+import org.antlr.v4.runtime.*;
+import java.io.IOException;
+import java.nio.file.*;
+
+/**
+ * Grammar-check CLI: the stable seam over the ANTLR parser for tooling
+ * (the conformance fuzzer today, the Java runtime in Stage 4).
+ *
+ * Arguments are file paths, or "-" for stdin. Exit 0 iff every input
+ * parses; otherwise exit 1, printing "<file>:<line>:<col>: <message>"
+ * for the first error of each failing input (col is 0-based, as ANTLR
+ * reports it).
+ */
+public final class ParseCheck {
+
+    public static void main(String[] args) throws IOException {
+        if (args.length == 0) {
+            System.err.println("Usage: ParseCheck <file.mpl | -> ...");
+            System.exit(2);
+        }
+        boolean allOk = true;
+        for (String arg : args) {
+            String name = arg.equals("-") ? "<stdin>" : arg;
+            CharStream input;
+            try {
+                // CharStreams works in Unicode code points; the deprecated
+                // ANTLRInputStream broke on supplementary-plane glyphs.
+                input = arg.equals("-")
+                        ? CharStreams.fromStream(System.in)
+                        : CharStreams.fromPath(Paths.get(arg));
+            } catch (IOException e) {
+                System.out.println(name + ":0:0: " + e.getMessage());
+                allOk = false;
+                continue;
+            }
+            String error = firstError(input);
+            if (error != null) {
+                System.out.println(name + ":" + error);
+                allOk = false;
+            }
+        }
+        System.exit(allOk ? 0 : 1);
+    }
+
+    /** Returns "line:col: message" for the first syntax error, or null. */
+    private static String firstError(CharStream input) {
+        final String[] first = {null};
+        var listener = new BaseErrorListener() {
+            @Override
+            public void syntaxError(Recognizer<?, ?> recognizer, Object offendingSymbol,
+                                    int line, int charPositionInLine, String msg,
+                                    RecognitionException e) {
+                if (first[0] == null) {
+                    first[0] = line + ":" + charPositionInLine + ": " + msg;
+                }
+            }
+        };
+        MPLLexer lexer = new MPLLexer(input);
+        lexer.removeErrorListeners();
+        lexer.addErrorListener(listener);
+        CommonTokenStream tokens = new CommonTokenStream(lexer);
+        MPLParser parser = new MPLParser(tokens);
+        parser.removeErrorListeners();
+        parser.addErrorListener(listener);
+        parser.program();
+        return first[0];
+    }
+}