feat(parse): Consume try, catch, and end try lexemes

Throwing exceptions and catching them aren't yet supported in `brs`, so
executing only the `try` block seems to be a reasonable "bare minimum"
approach to try/catch support.  Handle `try`, `catch`, and `end try`
lexemes in the parser, emitting TryCatch statements for the interpreter
to execute naïvely.

see #554

src/coverage/FileCoverage.ts

@@ -255,6 +255,11 @@ export class FileCoverage implements Expr.Visitor<BrsType>, Stmt.Visitor<BrsType
         return BrsInvalid.Instance;
     }
 
+    visitTryCatch(statement: Stmt.TryCatch) {
+        // TODO: implement statement/expression coverage for try/catch
+        return BrsInvalid.Instance;
+    }
+
     visitFor(statement: Stmt.For) {
         this.execute(statement.counterDeclaration);
         this.evaluate(statement.counterDeclaration.value);

src/interpreter/index.ts

@@ -1014,6 +1014,11 @@ export class Interpreter implements Expr.Visitor<BrsType>, Stmt.Visitor<BrsType>
         }
     }
 
+    visitTryCatch(statement: Stmt.TryCatch): BrsInvalid {
+        this.visitBlock(statement.tryBlock);
+        return BrsInvalid.Instance;
+    }
+
     visitBlock(block: Stmt.Block): BrsType {
         block.statements.forEach((statement) => this.execute(statement));
         return BrsInvalid.Instance;

src/lexer/ReservedWords.ts

@@ -58,6 +58,7 @@ export const ReservedWords = new Set([
  */
 export const KeyWords: { [key: string]: L } = {
     and: L.And,
+    catch: L.Catch,
     dim: L.Dim,
     else: L.Else,
     elseif: L.ElseIf,
@@ -70,6 +71,7 @@ export const KeyWords: { [key: string]: L } = {
     "end if": L.EndIf,
     endsub: L.EndSub,
     "end sub": L.EndSub,
+    endtry: L.EndTry,
     "end try": L.EndTry, // note: 'endtry' (no space) is *not* a keyword
     endwhile: L.EndWhile,
     "end while": L.EndWhile,
@@ -96,6 +98,7 @@ export const KeyWords: { [key: string]: L } = {
     stop: L.Stop,
     sub: L.Sub,
     to: L.To,
+    try: L.Try,
     throw: L.Throw,
     true: L.True,
     while: L.While,

src/parser/Parser.ts

@@ -28,7 +28,9 @@ type BlockTerminator =
     | Lexeme.EndSub
     | Lexeme.EndFunction
     | Lexeme.Newline // possible only in a single-line `if` statement
-    | Lexeme.Eof; // possible only in a single-line `if` statement
+    | Lexeme.Eof // possible only in a single-line `if` statement
+    | Lexeme.Catch
+    | Lexeme.EndTry;
 
 /** The set of operators valid for use in assignment statements. */
 const assignmentOperators = [
@@ -90,7 +92,13 @@ const allowedProperties = [
 ];
 
 /** List of Lexeme that are allowed as local var identifiers. */
-const allowedIdentifiers = [Lexeme.EndFor, Lexeme.ExitFor, Lexeme.ForEach];
+const allowedIdentifiers = [
+    Lexeme.EndFor,
+    Lexeme.ExitFor,
+    Lexeme.ForEach,
+    Lexeme.Try,
+    Lexeme.Catch,
+];
 
 /**
  * List of string versions of Lexeme that are NOT allowed as local var identifiers.
@@ -576,6 +584,10 @@ export class Parser {
                 return stopStatement();
             }
 
+            if (check(Lexeme.Try)) {
+                return tryCatch();
+            }
+
             if (check(Lexeme.If)) {
                 return ifStatement();
             }
@@ -629,6 +641,32 @@ export class Parser {
             return setStatement(...additionalterminators);
         }
 
+        function tryCatch(): Stmt.TryCatch {
+            let tryKeyword = advance();
+            let tryBlock = block(Lexeme.Catch);
+            if (!tryBlock) {
+                throw addError(peek(), "Expected 'catch' to terminate try block");
+            }
+
+            if (!check(Lexeme.Identifier)) {
+                // defer this error so we can parse the `catch` block.
+                // it'll be thrown if the catch block parses successfully otherwise.
+                throw addError(peek(), "Expected variable name for caught error after 'catch'");
+            }
+
+            let caughtVariable = new Expr.Variable(advance() as Identifier);
+            let catchBlock = block(Lexeme.EndTry);
+            if (!catchBlock) {
+                throw addError(peek(), "Expected 'end try' or 'endtry' to terminate catch block");
+            }
+
+            return new Stmt.TryCatch(tryBlock.body, catchBlock.body, caughtVariable, {
+                try: tryKeyword,
+                catch: tryBlock.closingToken,
+                endtry: catchBlock.closingToken,
+            });
+        }
+
         function whileStatement(): Stmt.While {
             const whileKeyword = advance();
             const condition = expression();

src/parser/Statement.ts

@@ -1,7 +1,6 @@
 import * as Expr from "./Expression";
 import { Token, Identifier, Location, Lexeme } from "../lexer";
 import { BrsType, BrsInvalid } from "../brsTypes";
-import { InvalidZone } from "luxon";
 import { AstNode } from "./AstNode";
 
 /** A set of reasons why a `Block` stopped executing. */
@@ -25,6 +24,7 @@ export interface Visitor<T> {
     visitIndexedSet(statement: IndexedSet): BrsType;
     visitIncrement(expression: Increment): BrsInvalid;
     visitLibrary(statement: Library): BrsInvalid;
+    visitTryCatch(statement: TryCatch): BrsInvalid;
 }
 
 let statementTypes = new Set<string>([
@@ -576,3 +576,30 @@ export class Library extends AstNode implements Statement {
         };
     }
 }
+
+export class TryCatch extends AstNode implements Statement {
+    constructor(
+        readonly tryBlock: Block,
+        readonly catchBlock: Block,
+        readonly errorBinding: Expr.Variable,
+        readonly tokens: {
+            try: Token;
+            catch: Token;
+            endtry: Token;
+        }
+    ) {
+        super("TryCatch");
+    }
+
+    accept<R>(visitor: Visitor<R>): BrsType {
+        return visitor.visitTryCatch(this);
+    }
+
+    get location() {
+        return {
+            file: this.tokens.try.location.file,
+            start: this.tokens.endtry.location.start,
+            end: this.tokens.endtry.location.end,
+        };
+    }
+}

test/e2e/Syntax.test.js

@@ -212,4 +212,16 @@ describe("end to end syntax", () => {
             "14", // arr = [13]: arr[0]++
         ]);
     });
+
+    test("try-catch.brs", async () => {
+        await execute([resourceFile("try-catch.brs")], outputStreams);
+        expect(allArgs(outputStreams.stdout.write).filter((arg) => arg !== "\n")).toEqual([
+            "[pre_try] a = ",
+            "5",
+            "[in_try] a = ",
+            "10",
+            "[post_try] a = ",
+            "10",
+        ]);
+    });
 });

test/e2e/resources/try-catch.brs

@@ -0,0 +1,13 @@
+sub main()
+    a = 5
+
+    print "[pre_try] a = " a
+    try
+        a = a * 2
+        print "[in_try] a = " a
+    catch e
+        ' currently unimplemented
+    end try
+
+    print "[post_try] a = " a
+end sub

test/parser/ParserTests.js

@@ -65,3 +65,20 @@ exports.locationEqual = function (loc1, loc2) {
         loc1.end.column === loc2.end.column
     );
 };
+
+/**
+ * Removes least-common leading indentation from a string, effectively "unindenting" a multi-line
+ * template string.
+ * @param {string} str - the string to unindent
+ * @return {string} `str`, but reformatted so that at least one line starts at column 0
+ */
+exports.deindent = function deindent(str) {
+    let lines = str.split("\n");
+    let firstNonEmptyLine = lines.find((line) => line.trim() !== "");
+    if (firstNonEmptyLine == null) {
+        return str;
+    }
+
+    let baseIndent = firstNonEmptyLine.length - firstNonEmptyLine.trim().length;
+    return lines.map((line) => line.substring(baseIndent)).join("\n");
+};

test/parser/controlFlow/TryCatch.test.js

@@ -0,0 +1,153 @@
+const brs = require("brs");
+
+const { deindent } = require("../ParserTests");
+
+function scan(str) {
+    return brs.lexer.Lexer.scan(str).tokens;
+}
+
+describe("parser try/catch statements", () => {
+    let parser;
+
+    beforeEach(() => {
+        parser = new brs.parser.Parser();
+    });
+
+    it("requires catch to end try block", () => {
+        const { errors } = parser.parse(
+            scan(
+                deindent(`
+                    try
+                        print "in try"
+                    end try
+                `)
+            )
+        );
+
+        expect(errors).toEqual(
+            expect.arrayContaining([new Error("Found unexpected token 'end try'")])
+        );
+    });
+
+    it("requires variable binding for caught error", () => {
+        const { errors } = parser.parse(
+            scan(
+                deindent(`
+                    try
+                        print "in try"
+                    catch
+                        print "in catch"
+                    end try
+                `)
+            )
+        );
+
+        expect(errors).toEqual(
+            expect.arrayContaining([
+                new Error("Expected variable name for caught error after 'catch'"),
+            ])
+        );
+    });
+
+    it("requires end try or endtry to end catch block", () => {
+        const { errors } = parser.parse(
+            scan(
+                deindent(`
+                    try
+                        print "in try"
+                    catch e
+                        print "in catch"
+                    end if
+                `)
+            )
+        );
+
+        expect(errors).toEqual(
+            expect.arrayContaining([
+                new Error(
+                    "(At end of file) Expected 'end try' or 'endtry' to terminate catch block"
+                ),
+            ])
+        );
+    });
+
+    it("accepts try/catch/end try", () => {
+        const { statements, errors } = parser.parse(
+            scan(
+                deindent(`
+                    try
+                        print "in try"
+                    catch e
+                        print "in catch"
+                    end try
+                `)
+            )
+        );
+
+        expect(errors).toEqual([]);
+        expect(statements).toBeDefined();
+        expect(statements).not.toBeNull();
+        expect(statements).toMatchSnapshot();
+    });
+
+    it("accepts try/catch/endtry", () => {
+        const { statements, errors } = parser.parse(
+            scan(
+                deindent(`
+                sub main()
+                    try
+                        print "in try"
+                    catch e
+                        print "in catch"
+                    endtry
+                end sub
+                `)
+            )
+        );
+
+        expect(errors).toEqual([]);
+        expect(statements).toBeDefined();
+        expect(statements).not.toBeNull();
+        expect(statements).toMatchSnapshot();
+    });
+
+    it("allows try/catch to nest in try", () => {
+        const { statements, errors } = parser.parse(
+            scan(
+                deindent(`
+                    try
+                        print "outer try"
+                        try
+                            print "inner try
+                        catch e
+                            print "in upper catch"
+                        end try
+                    catch e
+                        print "in catch"
+                    endtry
+                `)
+            )
+        );
+
+        expect(errors).toEqual([]);
+        expect(statements).toBeDefined();
+        expect(statements).not.toBeNull();
+        expect(statements).toMatchSnapshot();
+    });
+
+    it("allows try and catch as variable names", () => {
+        const { statements, errors } = parser.parse(
+            scan(
+                deindent(`
+                    try = "attempt"
+                    catch = "whoops, dropped it"
+                `)
+            )
+        );
+
+        expect(errors).toEqual([]);
+        expect(statements).toBeDefined();
+        expect(statements).not.toBeNull();
+        expect(statements).toMatchSnapshot();
+    });
+});